PROCESSING-STRUCTURE-PROPERTY RELATIONSHIPS OF A POLYMER-TEMPLATED CHOLESTERIC LIQUID CRYSTAL EXHIBITING DYNAMIC SELECTIVE REFLECTION.

Transcription

1 PROCESSING-STRUCTURE-PROPERTY RELATIONSHIPS OF A POLYMER-TEMPLATED CHOLESTERIC LIQUID CRYSTAL EXHIBITING DYNAMIC SELECTIVE REFLECTION Thesis Submitted to The School of Engineering of the UNIVERSITY OF DAYTON In Partial Fulfillment of the Requirements for The Degree Master of Science in Materials Engineering By Madeline Marie Duning Dayton, Ohio December, 2012

2 PROCESSING-STRUCTURE-PROPERTY RELATIONSHIPS OF A POLYMER-TEMPLATED CHOLESTERIC LIQUID CRYSTAL EXHIBITING DYNAMIC SELECTIVE REFLECTION Name: Duning, Madeline Marie APPROVED BY: Charles E. Browning, Ph.D. Committee Chairperson Professor and Chairperson Chemical and Materials Engineering Timothy J. Bunning, Ph.D. Technical Advisor / Committee Member Chief, Functional Materials Division Air Force Research Laboratory / RXA Donald A. Klosterman, Ph.D. Committee Member Associate Professor Chemical and Materials Engineering John G. Weber, Ph.D. Associate Dean School of Engineering Tony E. Saliba, Ph.D. Dean, School of Engineering & Wilke Distinguished Professor ii

3 ABSTRACT PROCESSING-STRUCTURE-PROPERTY RELATIONSHIPS OF A POLYMER-TEMPLATED CHOLESTERIC LIQUID CRYSTAL EXHIBITING DYNAMIC SELECTIVE REFLECTION Name: Duning, Madeline Marie University of Dayton Advisor: Dr. Charles E. Browning Technical Advisor: Dr. Timothy J. Bunning Cholesteric liquid crystals (CLCs) are chiral-structured materials that exhibit selective reflection color. While methods to switch off this color with external stimuli have been established, the use of external stimuli to change the reflection color is currently under development. This work explores processing-structure-property relationships of a polymer-templated CLC system that exhibits large-scale reflection color changes with temperature. The effects of heating rate, curing intensity, and cell geometry on the magnitude of these color changes were investigated by collecting transmission spectra of the system during heating and cooling. The system exhibited a constant color change of nm at heating rates up to 10 C/min. Samples cured at intensities below 1.00 mw/cm 2 displayed larger color changes. The cell geometry was largely unaffected by the cell thickness. However, for cell thicknesses between 10 and 50 µm, as cell thickness increased, the magnitude of the color change decreased. The iii

4 potential impact of attaching, or tethering, the chiral-structured polymer/liquid crystal gel to the cell surface was also considered. Both tethered and untethered systems displayed large-scale color changes with temperature. iv

5 ACKNOWLEDGEMENTS I would like to thank my technical advisor, Dr. Timothy Bunning, and the Southwestern Ohio Council for Higher Education for giving me the opportunity to work at the Air Force Research Laboratory as both an undergraduate and graduate student. From conducting research in state-of-the-art labs to presenting at international conferences, my experiences at AFRL have been invaluable. I would also like to thank Dr. Bunning for his guidance on this project and for always pushing me to view research as telling a story. I would like to thank Dr. Lalgudi Natarajan and Vincent Tondiglia for teaching me the fundamentals of the research process. As I evolved from preparing cells to analyzing and interpreting data, they were there every step of the way to offer their time and patience. Dr. Michael McConney deserves thanks for working closely with me on the experimental details of this project and for his guidance in data interpretation. Thanks go to Anastasia Voevodin and Chad Keister for their assistance in sample preparation and data collection. I would also like to thank Dr. Timothy White for useful discussions. I would like to thank Dr. Charles Browning and the School of Engineering for the tuition assistance which helped make my master s degree possible. I would also like to thank Dr. Browning and Dr. Donald Klosterman for taking time out of their busy schedules to serve on my thesis committee. v

6 Finally, I would like to thank my parents, Tim and Lorraine Duning, and my husband, Eric Harper, for their moral support and continual mantra of you can do it! throughout this thesis experience. vi

7 TABLE OF CONTENTS ABSTRACT... iii ACKNOWLEDGEMENTS... v TABLE OF CONTENTS... vii LIST OF FIGURES... ix CHAPTER I INTRODUCTION Project Foundation Project Objectives Background Liquid Crystals Optical Characteristics of Cholesteric Liquid Crystals CLC Responses to Stimuli Polymer Stabilization of CLCs Dynamic Optical Response of a CLC Gel Project Synopsis CHAPTER II EXPERIMENTAL PROCEDURE Cell Preparation vii

8 2.2 Polymer/LC Gel Preparation Transmission Measurements Gel Thickness Measurements CHAPTER III RESULTS AND DISCUSSION Thermal Tuning Behavior and Analysis Heating Rate Study Curing Intensity Study Cell Geometry Study Cell Thickness Surface Attachment of Polymer Network CHAPTER IV CONCLUSIONS CHAPTER V FUTURE RECOMMENDATIONS REFERENCES APPENDIX A HEATING RATE STUDY RAW DATA APPENDIX B CURING INTENSITY STUDY RAW DATA APPENDIX C CELL GEOMETRY STUDY RAW DATA C.1 Polymer Film Depth Profiles C.2 Cell Thickness Heating Cycles C.3 Untethered Gel Raw Data APPENDIX D RELATED WORKS viii

9 LIST OF FIGURES Figure 1: CLC Structure and Light Reflection Figure 2: CLC Interaction with Unpolarized Light Figure 3: Thermally Responsive, Highly Reflective CLC Cell Figure 4: LC Mesophases Figure 5: CLC Selective Reflection Figure 6: Dynamic Selective Reflection Figure 7: Thermal Notch Tuning of CLC Gel Figure 8: Volume Phase Transitions of CLC Gel with Temperature Figure 9: Irgacure Figure 10: Components of Nematic LC E Figure 11: Components of Right-Handed LC Monomer Mixture RMM Figure 12: Experimental Setup Figure 13: Schematic of Sample Prepared for Gel Thickness Measurement Figure 14: Optical Profilometry Image of Polymer Film Figure 15: Typical Thermal Tuning Transmission Spectra Figure 16: Notch Shift with Temperature Figure 17: Maximum Relative Tuning Range as Function of Heating/Cooling Rate Figure 18: Reflection Notch Distortion at High Heating/Cooling Rates Figure 19: Tuning Response to Temperature Step ix

10 Figure 20: Dependence of Transition Temperatures on Heating/Cooling Rate Figure 21: Maximum Relative Tuning Range with Curing Intensity Figure 22: Reduction of Notch Tuning at High Crosslink Densities Figure 23: Proposed Impact of Cell Thickness on Cell Geometry Figure 24: Polymer Thickness Fraction of Total Cell Thickness Figure 25: Notch Position as Indication of Swollen Polymer Thickness Figure 26: Maximum Relative Tuning Range with Cell Thickness Figure 27: CLC Transmission Spectra Before Polymerization and After Reswelling Figure A 1: Notch Position During Heating and Cooling, 0.5 C/min Figure A 2: Notch Position During Heating and Cooling, 4 C/min Figure B 1: Transmission Spectra with Temperature, 0.09 mw/cm 2 Curing Intensity Figure B 2: Transmission Spectra with Temperature, 1.00 mw/cm 2 Curing Intensity Figure B 3: Notch Position with Temperature, Representative Curing Intensities Figure B 4: Notch Shift with Temperature, Representative Curing Intensities Figure C 1: Dry Polymer Film Thickness, 9.1 μm Cell Figure C 2: Swollen Polymer Film Thickness, 9.1 μm Cell Figure C 3: Dry Polymer Film Thickness, 49.0 μm Cell Figure C 4: Swollen Polymer Film Thickness, 49.0 μm Cell Figure C 5: Transmission Spectra with Temperature, ~10 μm Cell Figure C 6: Transmission Spectra with Temperature, ~50 μm Cell Figure C 7: Notch Position with Temperature, Representative Cell Thicknesses Figure C 8: Notch Shift with Temperature, Representative Cell Thicknesses Figure C 9: Transmission Spectra with Temperature, Untethered Gel x

11 Figure C 10: Notch Position with Temperature, Untethered Gel Figure C 11: Notch Shift with Temperature, Untethered Gel xi

12 CHAPTER I INTRODUCTION 1.1 Project Foundation Cholesteric liquid crystals (CLCs) are chiral materials that are widely used in photonic applications for their ability to selectively reflect circularly polarized light and respond to external stimuli. The periodic, helical structure of CLCs results in the reflection of a specific band of the optical spectrum and the transmission of the remaining wavelengths of light (Figure 1). This reflection band is commonly referred to as a notch. Figure 1: CLC Structure and Light Reflection. The molecules in a CLC form a helical structure through space (a). CLCs reflect specific wavelengths of light due to their periodic structures. In Figure 1b, green light is reflected by the CLC while the other colors of light are transmitted. 1

13 External stimuli such as heat, light, and electric field can be used to manipulate the CLC structure, thereby generating a change in the reflection properties of the material. The use of external stimuli to switch the reflection notch off to a clear, transmitting state has been well-documented [1]. A CLC device containing multiple CLC films can alternately reflect different colors of the optical spectrum through this switching action. External stimuli can also be used to change or tune the position of the reflection notch within the optical spectrum. CLC devices that change the reflected color by a tuning mechanism are simpler than devices that change the color by a switching mechanism because tuning enables a single CLC film to reflect a range of colors. Consequently, fewer CLC films are required to cover the entire optical spectrum. However, large-scale tuning of the reflection notch is a relatively difficult task to achieve compared to binary switching. Methods to tune the CLC reflection notch through a large color range are still under development [2,3,4]. Continued advances in CLC notch tuning would make CLC systems very attractive for dynamic filter applications. While the dynamic nature of cholesteric liquid crystals is a primary advantage of using CLCs in photonic applications, a large drawback of CLCs is that the reflection from a single cholesteric film is limited to 50% of the incident light intensity (when the light is unpolarized). Because CLCs have an inherent handedness, CLCs will only reflect circularly polarized light of the same handedness as the CLC helices. The left-handed CLC helices in Figure 2 reflect the left-handed circularly polarized component of unpolarized light, and transmit the right-handed circularly polarized component. 2

14 Figure 2: CLC Interaction with Unpolarized Light. When unpolarized light is incident upon a left-handed CLC, left-handed circularly polarized light (LHCPL) will be reflected but right-handed circularly polarized light (RHCPL) will be transmitted. In order for a CLC system to reflect 100% of unpolarized light, helices of both handedness would have to be present in the system. However, CLCs are fluids, and mixing two CLCs of opposite handedness would result in a racemic mixture with no net chirality. To overcome the reflectivity limitation of CLCs in devices, multiple cells containing different CLC mixtures can be stacked. When a cell containing a right-handed CLC is stacked with a cell containing a left-handed CLC, each cell reflects circularly polarized light of the corresponding handedness, resulting in a net reflection intensity close to 100%. Unfortunately, this solution is less than ideal because the additional cells add complexity to the device, leading to increased weight, cost, and power consumption. The extra cells also add interfaces that the light must traverse, increasing potential for scattering and absorptive losses [5]. The ultimate CLC device would consist of a single film contained in a single cell that reflected both right-handed and left-handed circularly polarized light. 3

15 In recent years, single CLC films that exhibit selective reflection with near 100% intensity have been achieved through the use of polymer stabilization [6,7,8,9,10,11]. In a polymer-stabilized liquid crystal, liquid crystalline monomers are polymerized in the presence of a liquid crystal fluid. The order of the liquid crystal (LC) is transferred to the resulting polymer network. The polymer network is distributed throughout the bulk LC fluid and acts as a stabilizing surface that helps maintain the original liquid crystal order. If the liquid crystal fluid is removed from the polymer and exchanged with a different LC solvent, the polymer remembers the original, as-polymerized LC order and serves as a template for the new solvent. Guo et al. utilized polymer stabilization to fabricate a single CLC film that simultaneously contained both right- and left-handed CLC helices [8,9]. A left-handed chiral-structured polymer network was formed by polymerizing in the presence of a left-handed CLC. The left-handed CLC solvent was subsequently removed and exchanged with a right-handed CLC solvent. In the porous regions between polymer strands, the CLC retained its right-handed chirality. However, in the vicinity of the polymer strands, anchoring interactions between the polymer and the liquid crystal molecules induced the formation of left-handed CLC helices. Consequently, both rightand left-handed CLC helices coexisted in the single film, causing both right- and lefthanded circularly polarized light to be reflected. McConney et al. recently reported a variation on the polymer stabilization method as a means to fabricate high-reflectivity CLC films [12]. Instead of a single polymer template, both right- and left-handed chiral-structured polymer templates were included in a single cell. The polymer templates were composed of chiral mixtures of liquid crystalline monoacrylate and diacrylate monomers. In order to separate the polymer 4

16 templates from each other within the cell, the polymer networks were attached, or tethered, to opposite cell surfaces by embedding photoinitiator in the substrate anchoring layers prior to photopolymerization. After polymerization, the CLC template fluids were removed and exchanged with a non-chiral liquid crystal solvent composed of n-cyanobiphenyls. Although the LC solvent had no intrinsic handedness, the chiralstructured polymer templates induced the formation of both right- and left-handed CLC helices. Since the polymer networks occupied opposite sides of the cell, distinct, homogeneous regions of opposite handedness existed through the film thickness (Figure 3a). Figure 3: Thermally Responsive, Highly Reflective CLC Cell. Chiral-structured polymer templates of opposite handedness were tethered to opposite sides of the cell, resulting in distinct right- and left-handed CLC regions through the cell thickness (a). Upon heating the cell, the selective reflection notch blue-tuned hundreds of nanometers and then red-tuned back to the original notch position (b). (McConney et al., 2012) [12] While this chiral-structured polymer/lc system successfully reflected both right- and left-handed circularly polarized light, resulting in a reflection intensity near 100%, a particularly interesting observation of this system was that upon heating, the reflection 5

17 notch tuned over a large range of wavelengths. The notch first blue-tuned hundreds of nanometers, and then at higher temperatures, red-tuned back to the original notch position (Figure 3b). The thermal tuning was attributed to a deswelling/reswelling mechanism driven by an order/disorder transition seen in polymer/liquid crystal gels [12]. Previous studies had reported this deswelling/reswelling mechanism in non-chiral polymer/lc gels [13,14], but this was the first demonstration of the mechanism in chiral-structured polymer/lc gels. 1.2 Project Objectives The thermally tunable, chiral-structured polymer/lc gel reported by McConney et al. shows potential for photonic applications. Through its deswelling/reswelling mechanism, this system offers a new method by which to dynamically tune the selective reflection notch of CLCs through a range of hundreds of nanometers. In addition, tethering the chiral-structured polymer network to a cell surface allows for a heterogeneous cell geometry, which can be leveraged to fabricate highly-reflective single CLC films. While the initial investigations of this system are intriguing, the structureproperty relationships of the chiral-structured polymer/lc system have not been fully explored. The goal of this thesis is to evaluate the impact of several processing and structure variables on the dynamic optical response of the chiral-structured polymer/lc system with temperature. This thesis will focus on the same system studied by McConney et al., namely, a CLC gel composed of an acrylate-based LC polymer and a non-chiral LC solvent mixture of n-cyanobiphenyls. Understanding some of the processing-structure- 6

18 property relationships of this system will help highlight the potential advantages and limitations of utilizing this system in applications. The preliminary experiments tracked the optical response of the system with a temperature ramp of 1 C/min. For most devices, 1 C/min would be an unacceptably slow heating rate. In order to determine the reasonable heating rate limits for applications, the effects of heating/cooling rate on the magnitude and quality of the optical response were studied. Because the thermal notch tuning is the result of a gel deswelling/reswelling mechanism, the chiral-structured polymer network directly impacts the dynamic optical behavior of the system. Therefore, altering the polymerization conditions would be expected to affect the optical response of the system to temperature. The curing intensity used to fabricate the chiral-structured polymer networks was varied in order to examine the appropriate polymerization conditions for producing thermally-tunable CLC gels. Finally, the preliminary experiments were conducted in cells with a specific thickness and geometry. The polymer network was grown from one side of the cell and the swollen network didn t extend through the entire cell thickness, creating a heterogeneous cell design. The potential contributions of this heterogeneous cell design to the optical behavior were examined by varying the cell thickness and considering the spatial aspects of the polymer network within the cell. Initial results from these thesis studies were reported in a previous publication [15]. 7

19 1.3 Background Liquid Crystals A liquid crystal is a phase of matter that exhibits properties intermediate between a crystalline solid and a liquid. The discovery of liquid crystals is typically attributed to Friedrich Reinitzer, who observed in 1888 that a cholesterol derivative appeared to have two melting points [16]. The first point was a transition from a solid to a cloudy liquid that affected polarized light, a characteristic typically reserved for crystalline solids. At the second, higher temperature, the cloudy liquid transitioned to a clear, isotropic liquid. Because this intermediate phase had the fluid nature of a liquid but the optical properties of a crystalline solid, the phase was termed liquid crystal. A substance that exhibits liquid crystalline phases within specific temperature ranges is known as a thermotropic liquid crystal. Thermotropic LCs form the liquid crystalline phase at temperatures between the crystalline solid state and the isotropic liquid state. Liquid crystals that form within a certain concentration range of a component in a mixture are known as lyotropic liquid crystals. Examples of lyotropic LCs are soap and phospholipid cell membranes [16]. Lyotropic LCs are biologically important but are not often used in devices. The molecular unit responsible for inducing a liquid crystal phase in a substance is referred to as a mesogen. Liquid crystals can be further classified by the shape of the mesogens. Mesogens that are rod-like form calamitic liquid crystals, while mesogens that are disc-like form discotic liquid crystals [16]. The key aspect of both types of molecules is that one axis is significantly longer than the other. Molecules that form calamitic LCs typically have rigid cores and flexible tails. The rigid cores help the mesogens align with 8

20 each other along the long axis while the flexible tails aid in the fluid movement of the mesogens. This mesogenic alignment is the reason why liquid crystals affect polarized light like solid crystals. Liquid crystals can possess both orientational and positional molecular order. The specific degree of molecular ordering between LC mesogens defines several liquid crystal mesophases. Nematic liquid crystals have long-range orientational order but no positional order (Figure 4). The orientational order is described in terms of the director, which is the average preferred orientation of the long axes of the molecules. Like the nematic phase, the chiral nematic phase has orientational order and lacks positional order. However, the molecules in a chiral nematic LC tend to align at a slight angle to each other so that the director rotates through space. The resulting structure is a helix with a specific pitch, or the distance for the director to make one full 360 turn. Chiral nematic liquid crystals are frequently referred to as cholesteric liquid crystals (CLCs) due to the fact that many common chiral nematic LCs are cholesterol derivatives [16]. 9

21 Figure 4: LC Mesophases. In the nematic phase (a) the long axis of the molecules is aligned in an average direction, known as the director (represented by an arrow). The director rotates in the cholesteric phase (b). In Figure 4b, the director has rotated 180, forming one half-pitch (p/2). The cholesteric phase can be formed if the LC molecules possess intrinsic chirality or if a chiral molecule, known as a chiral dopant, is added to a non-chiral LC [17]. The concentration of the chiral dopant influences the twist between the LC molecules, which determines the pitch of the CLC [1] Optical Characteristics of Cholesteric Liquid Crystals Due to their elongated, anisotropic shape, the properties of LC mesogens depend on the measurement direction. For instance, the index of refraction measured parallel to the director (known as the extraordinary index of refraction, n e ) differs from the index of refraction measured perpendicular to the director (ordinary index of refraction, n o ). Because liquid crystals possess molecular order, this optical anisotropy translates to the bulk material, causing liquid crystals to affect polarized light. Cholesteric liquid crystals 10

22 in particular exhibit interesting optical effects due to their periodic structure. To fully understand these effects, a brief discussion of light will follow. Light is described as a collection of electromagnetic waves with oscillating magnetic and electric fields. The relative directions of the electric fields define the polarization of the light. In circularly polarized light, the direction of the electric fields continuously rotates. Circularly polarized light in which the electric fields rotate counterclockwise is known as right-handed, while electric fields rotating clockwise give left-handed circularly polarized light. Unpolarized light consists of waves with electric fields in all directions. However, unpolarized light can be represented by light with half of the intensity right-handed circularly polarized, and half of the intensity left-handed circularly polarized. This model is verified by the fact that the light intensity of unpolarized light passing through a polarizer is cut in half [17]. As light passes through a material, the oscillating electric and magnetic fields of the light will cause the material to produce its own electric and magnetic fields. Typically, these induced electromagnetic waves will cancel each other out in all directions except the forward direction of the propagating light wave. If the optical properties of the material repeat periodically over a distance equal to half the wavelength of the incident light, another phenomenon known as constructive interference occurs. Light emitted in the backward direction by one layer of the material will be in phase with the light emitted in the backward direction by identical layers because the layers are separated by a distance of one half-wavelength. Because these light waves are in phase, they add together, resulting in a strong reflection in the reverse direction of the incident light beam. 11

23 Constructive interference can occur in cholesteric liquid crystals due to their periodic structure. The director in a CLC rotates around a helical axis, causing the index of refraction seen by light traveling along the helical axis to vary periodically. Constructive interference occurs when the pitch of the CLC structure is equal to the wavelength of the incident light [17]. Recall that the pitch of a CLC is defined as the distance required for the director to rotate 360. However, since the director is equivalent in opposite directions, the structure is repeated every 180, or every half-pitch. Therefore, when the pitch is equal to the wavelength of the incident light, the structure repeats every half-wavelength, creating constructive interference. The central wavelength of the light reflected by a CLC is given by the Bragg condition (Equation 1): (1) where λ b is the Bragg reflection, is the average index of refraction (the average of the extraordinary and ordinary indices of refraction), and p is the pitch [1]. Due to the variation in the index of refraction as the mesogens rotate, a band of wavelengths surrounding the central Bragg reflection will also be reflected by the CLC [17]. The bandwidth, Δλ, is the product of the birefringence of the LC (Δn) and the pitch (Equation 2): (2) where the birefringence of the LC is the difference between the extraordinary and ordinary indices of refraction (Δn = n e n o ), and the pitch is assumed to be constant [1]. This reflection of a narrow range of wavelengths by a CLC is termed selective 12

24 reflection. The selective reflection is often casually referred to as a notch, owing to the appearance of the selective reflection when examining in transmission mode (Figure 5). Due to the helical structure of cholesteric liquid crystals, circularly polarized light will only be reflected by a CLC when the handedness of the circularly polarized light matches the handedness of the cholesteric helix [16]. A right-handed CLC will reflect right-handed circularly polarized light, but will transmit left-handed circularly polarized light. As discussed previously, unpolarized light can be thought of as equal parts rightand left-handed circularly polarized light. Therefore, when unpolarized light is incident upon a CLC, a maximum of 50% of the light intensity will be selectively reflected. Figure 5 is the selective reflection resulting from unpolarized light incident upon a CLC, as evidenced by the notch depth of nearly 50%. Figure 5: CLC Selective Reflection. The notch position, λ b, of this CLC selective reflection is around 570 nm, and the bandwidth, Δλ, is about 50 nm. Unpolarized light was used to take the spectrum, resulting in a notch depth around 50%. 13

25 The intensity of light transmitted through a cholesteric film is often measured instead of the intensity of light reflected from the film. The reflection measurement requires a more complicated optical setup and the intensity of the reflection signal is often lowered by scattering. The transmission spectrum is considered to be approximately the inverse of the reflection spectrum when the cholesteric film is uniform through the thickness and there is no scattering present CLC Responses to Stimuli Due to their stimuli-responsive nature, liquid crystals are particularly advantageous over other materials when dynamic filters and displays are desired. Stimuli such as heat, light, and electric field can change the helical structure and alignment of a CLC, thereby changing the selective reflection notch. When stimuli disrupt the helical structure of a CLC, the reflection notch can be switched off (Figure 6a). Stimuli can also tune the notch position to longer or shorter wavelengths by expanding or contracting the pitch, respectively (Figure 6b). 14

26 Figure 6: Dynamic Selective Reflection. Stimuli can be used to switch the selective reflection notch of a CLC off and on (a) or tune the position of the notch within the optical spectrum (b). The sensitivity of the cholesteric pitch to temperature has been utilized in the creation of mood rings and aquarium thermometers [16]. As the temperature of the environment changes, these devices change color. The pitch change that produces these color changes is due to the liquid crystal transitioning between a smectic mesophase, which can be considered to have an infinite pitch, and the cholesteric/chiral nematic mesophase, which has a finite pitch. The use of the smectic to cholesteric phase transition to thermally tune the selective reflection notch has been well-studied [1]. A particularly large tuning range in a CLC mixture with an underlying smectic phase was reported by Natarajan et al [2]. The temperature of the mixture was increased from 23 to 55 C through a Joule heating method, and the reflection notch tuned from 2300 to 500 nm. 15

27 Just as LC mesogens have different indices of refraction parallel and perpendicular to the director, most mesogens have different electric dipole strengths parallel and perpendicular to the director. These dipoles can be either permanent or induced in the presence of an electric field [16]. The difference between the dipole strengths is known as the dielectric anisotropy. When an electric field is applied to LC mesogens, the strongest dipole will tend to orient parallel with the field. Consequently, depending on the field direction and dielectric anisotropy of the liquid crystal, electric fields can be used to reorient the mesogens of an LC film so that the original ordered structure is changed. An electric field can completely reorient the mesogens in a CLC film so that the directors are parallel with the electric field, eliminating the director rotation that creates the chiral nematic phase. Because there is no longer a periodic variation in the index of refraction, the selective reflection notch is cleared or switched and light is transmitted through the film. The development of electrical switching of the CLC reflection notch has been relatively successful [1]. Light can also be used to switch or tune CLC notches through photochemistry [1]. Many light-responsive cholesteric systems are based on azobenzene. In some photoresponsive CLCs, the liquid crystal mesogens themselves contain azobenzene groups. An optical response is generated when light induces the trans-cis isomerization of the azobenzene groups. In other CLCs, the chiral dopant is azobenzene-based. When the CLC is exposed to light, the chiral dopant s helical twisting power (HTP) changes, causing the pitch of the CLC to change and the selective reflection notch to tune. White et al. reported such a CLC system with an azobenzene-based chiral dopant in which the reflection notch red-tuned over 2000 nm upon exposure to UV light [4]. 16

28 1.3.4 Polymer Stabilization of CLCs While stimuli can easily alter the cholesteric liquid crystal structure, regaining the original CLC structure quickly upon removal of a stimulus is the more difficult task. Polymer networks dispersed within a liquid crystal matrix have been shown to effectively stabilize the liquid crystal phase. The polymer network remembers the original LC structure so that when a stimulus is turned off, elastic interactions between the polymer network and the low molar mass LCs return the mesogens to their initial alignment. In these polymer-stabilized liquid crystals (PSLCs), the concentration of the polymer is small (often < 10 wt %) so that the liquid crystal is the continuous phase [18,19]. The monomers used to form the polymer networks in PSLCs are typically liquid crystalline [18]. LC monomers are generally composed of a mesogenic core, alkyl spacers, and reactive end groups (often acrylates). Depending on the monomer configuration, upon polymerization the mesogens are either incorporated into the polymer main chain or exist as side groups off the main chain [17]. When LC monomers are dispersed inside a liquid crystal phase, the mesogen units align with the surrounding non-reactive LC molecules. During polymerization, the mesogen units maintain the orientation of the surrounding phase so that the resulting polymer is imprinted with the order of the low molar mass LC. After polymerization, the polymer network acts as a stabilizing surface dispersed throughout the bulk liquid crystal. Because liquid crystals are very sensitive to the influence of surfaces, the director of the low molar mass mesogens couples to the polymer fibrils through anchoring interactions. The polymer network imposes elastic torques on the mesogens that help the mesogens return to their original orientation after 17

29 the removal of a stimulus [20]. In order for the liquid crystalline polymer to effectively stabilize the low molar mass LC, the polymer must have some degree of crosslinking [21]. Crosslinking enables the imprinted LC order to withstand temperature changes, exposure to solvents, and mechanical deformations [22]. As discussed previously, a liquid crystalline polymer network will not only stabilize the LC mesophase when dispersed in a low molar mass liquid crystal, but can also transfer the original LC order to a different LC mixture that is introduced to the network. If a crosslinked LC polymer is formed in the presence of a CLC, the resulting polymer network will exhibit a macroscopic chirality matching the template CLC. If the low molar mass CLC is subsequently removed from the polymer and replaced with a non-chiral nematic LC, the chiral-structured polymer will template the nematic LC into a chiral arrangement with the same handedness and periodicity as the original CLC. Several groups have leveraged the ability of LC polymer networks to serve as CLC templates in order to create multiple CLC structures in one film. Traditional chiral-dopant based CLCs can only exhibit one selective reflection notch at a time due to diffusional limitations. Chiral-structured materials like crosslinked LC polymers are able to overcome these limitations because they induce the CLC phase through surface interactions with low molar mass LCs. If multiple polymer templates are formed in one film, the low molar mass LC molecules near each structure will be oriented into CLC helices with the handedness and periodicity of the corresponding template. Consequently, multiple CLC structures can be present in a single film, enabling the simultaneous reflection of multiple photonic bandgaps [23,24] or the reflection of both right- and lefthanded circularly polarized light [6,7,8,9,10,11] 18

30 This thesis project examines such a CLC system in which the cholesteric phase is induced by a polymer template. The chiral template is first formed by polymerizing liquid crystalline acrylate monomers in the presence of a CLC. Photoinitiator is dispersed within a polyimide surface substrate in order to attach the polymer network to one side of the cell. After polymerization, the remaining low molar mass mixture is removed through solvent exchange, and a nematic liquid crystal mixture of n-cyanobiphenyls is backfilled into the polymer structure. The selective reflection that arises from this polymer/liquid crystal gel is nearly the same as the reflection from the as-polymerized system. Because no chiral dopant is present, the selective reflection can be entirely attributed to the interactions between the chiral-structured polymer and the liquid crystal Dynamic Optical Response of a CLC Gel The focus of this thesis is the dynamic optical response of the chiral-structured polymer/lc system to a thermal stimulus first reported by McConney et al. [12]. Figure 7 illustrates the thermal notch tuning behavior of this CLC gel. 19

31 Figure 7: Thermal Notch Tuning of CLC Gel. (McConney et al., 2012) [12] At temperatures below ~60 C, the selective reflection notch exhibited by the structure is stable. From C, the notch blue-tunes (shifts toward shorter wavelengths). The net change in the notch position is several hundred nanometers, shifting from the nearinfrared regime to the visible regime. Then above 72 C, the notch red-tunes back to the original, room temperature notch position. However, during the course of the red tuning, the notch depth decreases. Above ~90 C, the notch is no longer distinguishable and the transmission spectrum is clear. As the polymer/lc system is cooled, the optical behavior is reversible so that the selective reflection notch returns to its original notch position and notch depth. The dynamic optical response to temperature can be attributed to volume phase transitions observed in liquid crystalline gels swollen in liquid crystal solvents [12]. Urayama et al. first demonstrated that changes in orientational ordering could cause a volume phase transition in a nematic liquid crystalline gel swollen in a nematic LC 20

32 solvent [13,14]. The nematic-isotropic transition temperature (T NI ) of the free nematic solvent was lower than the T NI of the LC polymer network swollen in the solvent. In the temperature range between the gel T NI and the solvent T NI, the solvent-gel system experienced a mismatch in orientational order due to the gel and the solvent transitioning between the nematic and isotropic phases at different temperatures. Because the orientational orders of the gel and solvent were not the same within this temperature range, the gel and solvent had an unfavorable interaction energy and solvent was expelled from the gel. At temperatures either below the solvent T NI or above the gel T NI, both the solvent and gel possessed the same orientational ordering, and the gel reswelled with the solvent. Similar to the volume phase transition reported by Urayama et al. for a nematic system, the chiral nematic system studied here also experiences volume phase transitions due to changes in orientational ordering. These volume phase transitions are responsible for the color changes exhibited by the chiral-structured polymer/lc gel with temperature. The cholesteric gel is composed of a chiral-structured LC acrylate polymer swollen in a nematic LC solvent mixture of n-cyanobiphenyls. As in the system studied by Urayama et al., the swollen polymer and pure solvent have different nematic-isotropic transition temperatures. Figure 8 depicts the volume phase transitions that occur in the CLC gel with increasing temperature. 21

33 Figure 8: Volume Phase Transitions of CLC Gel with Temperature. As the temperature of a chiral-structured polymer/lc gel (a) is increased, at the T NI of the nematic LC solvent, the solvent transitions to the isotropic phase and is expelled from the gel (b). The volume of the gel decreases as more solvent leaves the gel upon further heating. At the gel T NI, the gel transitions to the isotropic phase and solvent re-enters the gel (d). (McConney et al., 2012) [12] Below the T NI of the pure nematic solvent, which is around 60 C, both the swollen chiral polymer and nematic solvent exist as liquid crystal phases, and thus have a favorable interaction energy (Figure 8a). The low molar mass LC molecules inside the polymer structure are oriented into a helical fluid, so that a selective reflection is exhibited in the near-infrared at the template notch position. When the temperature is raised to the T NI of the solvent, any solvent not constrained by the polymer network transitions from the nematic phase to the disordered isotropic phase. Within the polymer network, the low molar mass LC molecules are prevented from transitioning to the isotropic phase due to the stabilizing effects of the polymer, which is still ordered. To 22

34 compensate for this unfavorable interaction, the low molar mass LC molecules are expelled from the gel, where they are free to rotate and form the energetically favorable isotropic phase (Figure 8b). Because solvent leaves the gel, the volume of the gel decreases, causing the pitch of the chiral-structured polymer to collapse (Figure 8c). Since the polymer controls the periodicity of the CLC helices formed by the solvent molecules within the network, the pitch of the CLC decreases, causing a blue-shift of the selective reflection notch. Upon further heating, the system reaches the T NI of the gel. At this point, the mesogenic parts of the polymer network begin to lose their orientational ordering and transition to the isotropic phase. Now, the orientational order of the isotropic polymer matches the order of the isotropic solvent. The interaction energy between the gel and solvent is favorable and the entropy of mixing drives the solvent back into the polymer network (Figure 8d). As the solvent re-enters the gel, the volume of the gel increases and the pitch of the chiral-structured polymer increases again. This results in a red-shift of the selective reflection notch. However, the polymer network is simultaneously expanding and becoming disordered, which causes its optical anisotropy (birefringence) to decrease. Because the birefringence of the CLC gel decreases, the selective reflection notch depth decreases. At a high enough temperature, the entire polymer/liquid crystal system is isotropic and the selective reflection disappears. Upon cooling the chiral-structured polymer/lc system from the isotropic state, the volume transitions and notch tuning are reversed. The selective reflection reappears and returns to its original notch position and notch depth. 23

35 1.4 Project Synopsis Chiral-structured polymer/lc gels were fabricated by polymerizing a chiral mixture of LC acrylate monomers in the presence of a cholesteric liquid crystal. The polymer network was tethered to one cell surface by embedding photoinitiator in one polyimide surface substrate. When a non-chiral LC solvent was backfilled into the chiralstructured polymer, a polymer-templated cholesteric liquid crystal was formed. Through surface interactions with the polymer fibrils, the low molar mass LC mesogens formed CLC helices with the same handedness and periodicity as the original CLC. The selective reflection notch exhibited by these CLC gels was tuned using a thermal stimulus. Upon heating, the gels underwent volume phase transitions as the nematic LC solvent and CLC gel transitioned from the nematic to isotropic phases at different temperatures. Deswelling of the CLC gels translated to blue-tuning of the selective reflection notch, while reswelling of the gels translated to red-tuning of the notch. While the dynamic optical response of this system to a thermal stimulus is intriguing, previous work has failed to explore the impact of processing and structure variables on the optical behavior. This thesis examines the effects of heating rate, curing intensity, and cell geometry on the thermal notch tuning behavior of the chiral-structured polymer/lc system. The CLC gels were subjected to heating and cooling cycles at rates ranging from C/min. The curing intensity used to form the chiral-structured polymer networks was varied from mw/cm 2. The potential contributions of the cell geometry to the dynamic optical behavior were explored by varying the cell thickness and by comparing the surface-tethered system to an untethered system. 24

36 This project originally stemmed from a system which contained two chiralstructured LC polymer networks tethered to opposite surfaces of a single cell. Tethering the polymer scaffolds to opposite surfaces enabled the simple fabrication of a hyperreflective CLC cell that reflected both right- and left-handed circularly polarized light. However, because the focus of this project is the thermal notch tuning behavior, the cells were fabricated with a single chiral-structured polymer template for simplicity. Results from the study of the effects of processing variables on a single chiral-structured polymer/lc system can then be applied to variations on this system (e.g. a hyperreflective cell). 25

37 CHAPTER II EXPERIMENTAL PROCEDURE 2.1 Cell Preparation Liquid crystal films are often prepared inside thin glass cells, which serve as optically transparent vessels for the fluid mixtures. The cells used for experimentation in this project were constructed according to the following procedure. Glass slides were dipped into an acetone wash followed by a methanol wash to remove dust and dirt from the slide surfaces. The slides were plasma cleaned for 5 minutes to remove any additional organic residue. Polyimide films were deposited on the slide surfaces to serve as alignment layers. The pre-polyimide solution was prepared by mixing 8 ml of PI-2555 polyimide precursor (HD MicroSystems) with 32.5 ml of N-Methyl-2-pyrrolidone and 9.1 ml of 1-Methoxy-2-propanol. Glass slides were mounted individually in a SPIN 150 wafer spinner. The solution was filtered through a syringe filter with a 0.45 µm GHP membrane (Pall Acrodisc) to remove particulate impurities. The filtered solution was applied to the surface of the glass slide and a spin coating cycle of 15 seconds at 1500 rpm followed by 60 seconds at 3000 rpm was used to coat the slide with the prepolyimide. Half of the glass slides were coated with the pre-polyimide solution described above, and half of the slides were coated with a pre-polyimide solution containing 1% by weight photoinitiator Irgacure 369 (Ciba, of BASF). The photoinitiator was chosen to 26

38 initiate the photopolymerization of the LC acrylate monomers. Figure 9 depicts the Irgacure 369 photoinitiator. The same glass coating procedure was used for the solution containing initiator. 2-Benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone Figure 9: Irgacure 369 Adding photoinitiator to the pre-polyimide solution served to embed photoinitiator in the polyimide alignment layer. Previous work performed by McConney et al. has demonstrated that embedding photoinitiator in the polyimide alignment layer causes the LC polymer network to attach (or become tethered ) to the polyimide layer during the polymerization process [10,11,12]. The specific nature of this attachment (e.g. chemical bonds, chain entanglement) is currently the focus of related research. However, the attachment of the LC polymer network to the polyimide film appears to be strong. A tape test performed on the dried polymer film caused the LC polymer network and underlying polyimide layer to delaminate from the glass slide together [10]. By adding photoinitiator to only one of the two polyimide films in each cell, the chiral-structured polymer network becomes tethered to only one side of the cell. This one-sided tethering condition gives this system a heterogeneous cell design. After the glass slides were coated with either of the pre-polyimide solutions, the slides were baked on a hot plate at 80 C for 30 minutes. The baking step served to 27

39 partially cure the polyimide precursor and drive off the solvents. The relatively low temperature of 80 C was chosen to prevent potential thermal initiation of the photoinitiator contained within the polyimide solution. The recommended cure temperature to produce complete imidization of the polyimide is 200 C (HD MicroSystems). Consequently, the post-baked films likely contained a large portion of non-imidized polyamic acid. However, complete imidization of these films is unnecessary for the purpose of creating alignment layers. For simplicity, the post-baked films will still be referred to as polyimide films. The glass slides coated with the polyimide films were hand-rubbed 30 times with a weighted piece of velvet. Rubbing is a common technique used to turn polyimide films into alignment surfaces. Without the presence of alignment surfaces in a cell, the directions of the CLC helices would be almost randomly distributed, causing the CLC film to be optically opaque. Alignment surfaces give LC mesogens at the interface a preferred orientation, causing the majority of CLC helices within the cell to point in the same direction. Because velvet has a grooved texture, rubbing with velvet may introduce microgrooves to the surface of the polyimide. The shear mechanical force of the rubbing may also force the polymer chains to align along the rubbing direction. Whether the LC mesogens at the glass surface fit inside polyimide microgrooves or simply align along the polymer chains, rubbing causes the director of the LC mesogens at the surfaces to align parallel to the glass slides. Consequently, the CLC helical axis is oriented normal to the glass slides, which is known as homogeneous alignment. Cells were constructed by gluing together one glass slide coated with polyimide and one slide coated with the photoinitiator-doped polyimide. The glass slides were 28

40 oriented so that the polyimide layers faced each other and the rubbing directions were anti-parallel. Anti-parallel construction helps compensate for possible gradations in the polyimide films introduced by hand-rubbing. Norland optical adhesive NOA65 was applied along two of the edges of each slide to glue the slides together. Glass rod spacers with a diameter of 30 µm were mixed in the glue to control the gap size between the two glass slides, which is referred to as the cell thickness. For the cell thickness study, spacers with diameters ranging from 10 to 50 μm were chosen to control the cell thickness. The slides were glued together with an offset to help facilitate capillary filling of the cell, and the cell was placed under a UV lamp for 5 minutes to cure the optical adhesive. The gap thicknesses of the finished, unfilled cells were measured optically with a spectrometer. A white light source was shone through the cell, normal to the glass face. The resulting interference fringes were captured by a spectrometer. For empty cells filled only with air and incident light normal to the cell face, the interference fringes can be related to cell thickness through Equation 3: ( ) ( ) (3) where t is the cell thickness, λ 1 and λ 2 are the wavelengths measured at fringe peaks (where λ 1 > λ 2), and m is the number of fringes between λ 1 and λ 2 [25]. 2.2 Polymer/LC Gel Preparation The syrup used to create the chiral-structured polymer template was composed of 80% nematic E7 (Merck) and 20% chiral monomer RMM691 (Merck). This formulation was consistent with the baseline formulation previously studied by McConney et al. [12]. 29

41 E7 is a 4-component mixture that is a nematic liquid crystal at room temperature. Figure 10 illustrates the components of E7. 4-pentyl-4 -cyanobiphenyl 4-heptyl-4 -cyanobiphenyl 4-octyloxy-4'-cyanobiphenyl 4-pentyl-4 -cyanoterphenyl Figure 10: Components of Nematic LC E7 RMM691 is a right-handed mixture of liquid-crystalline monoacrylates and diacrylates. The relative ratios of the components of RMM691 are proprietary. Figure 11 illustrates the components of RMM

42 4-(3-acryloyloxypropyloxy)-benzoic acid 2-methyl-1,4-phenylene ester 4-(6-acryloyloxyhexyloxy)-benzoic acid-(4-methoxyphenylester) 2-methyl-1,4-phenylene-bis[4-(6-acryloyloxyhexyloxy)benzoate] 4-(6-acryloyloxyhexyloxy)benzoic acid (4-(trans-4-propylcyclohexyl) phenyl ester) 4-(6-acryloyloxyhexyloxy)-benzoic acid (4-cyanophenyl ester) Figure 11: Components of Right-Handed LC Monomer Mixture RMM691 31

43 When E7 is mixed with RMM691, the chirality of the RMM691 causes a righthanded cholesteric phase to form. Consequently RMM691 serves as both a chiral dopant and a reactive monomer mixture. Due to variations in the component ratios of RMM691, a concentration of 20% RMM691 in E7 forms a notch around nm. A notch in the infrared regime was desired to maximize the possible range of blue-tuning observable within spectrometer limits (the minimum resolvable wavelength was around 400 nm). At RMM691 concentrations below 15%, the formation of the cholesteric template was not as successful due to the lower concentration of the crosslinking components resulting from the lower overall monomer concentration. Therefore, the RMM691 concentration of 20% was within the appropriate window to achieve the desired polymer template. The empty cells were capillary filled with syrup that was heated above the clearing temperature (around 70 C). As the cells cooled to room temperature, the outside of the cells were gently hand-rubbed to provide shear alignment to the CLC fluid within the cells. The cells were exposed to UV light to polymerize the monomers in the mixture. The cells were mounted with the photoinitiator-doped side closest to the UV light. An Exfo Omnicure S1000 curing system with a mercury short arc lamp and 365 nm filter was used as the light source. Most of the cells were exposed for 30 minutes at an intensity of 2.2 mw/cm 2. For the curing intensity study, the intensity was controlled by adjusting the power setting on the Exfo system. The intensity of the UV light at the cell surface was measured with a Thorlabs PM100A optical power meter equipped with a S302C thermal power sensor. After curing, the nematic LC E7 and unreacted monomer were removed from the cells through solvent exchange. The cells were immersed in a 50 C bath of cyclohexane, 32

44 which is a poor solvent for this LC polymer. After several days, when visual inspection confirmed that only polymer remained, the cells were removed from the solvent. The remaining polymer was evidenced by a thin film with a slight bluish tint adhered to the side of the cell with the initiator-doped polyimide layer. No polymer was visible on the opposite glass substrate. The cells dried in the open air for a few hours. Any residual cyclohexane was removed by placing the cells in a vacuum oven at 50 C for 20 minutes. The dry, collapsed polymer networks within the cells were swollen with pure nematic E7 to form the polymer/lc gel. E7 was capillary filled back into the cells on a hot plate at 50 C. The polymer was allowed to swell at 50 C until the gel was visibly clear (indicating a reflection notch in the infrared regime). 2.3 Transmission Measurements The optical response of the samples to temperature was measured by collecting transmission spectra while the samples were heated or cooled. A sample was mounted on a thermoelectric cooler (TEC) with Dow Corning 340 silicone heat sink compound to provide good thermal contact between the TEC and the sample. An aluminum block was used as a heat sink for the TEC. The TEC was attached to the aluminum block with the Dow Corning 340 silicone heat sink compound. Both the aluminum heat sink and thermoelectric cooler had a hole through the center to make transmission measurements through the sample possible. A Keithley 2510 TEC controller was used to control the temperature of the thermoelectric cooler. The actual temperature at the surface of the TEC was measured by a thermistor mounted on the surface of the TEC. The temperature 33

45 of the TEC was assumed to be approximately equal to the average temperature of the polymer/liquid crystal gel inside the cell, given enough time to equilibrate. An Ocean Optics tungsten halogen broadband light source was used to generate the unpolarized light beam for the transmission measurements. The light was directed through an optical fiber, which was positioned so that the beam would pass through both the sample and the hole in the TEC apparatus. The light transmitted through the sample was focused into an optical fiber on the opposite side of the TEC. A bifricated optical fiber cable was used to direct light into separate Ocean Optics spectrometers. The fiber that transmitted visible light was connected to the visible spectrometer and the fiber that transmitted near-infrared (NIR) light was connected to the NIR spectrometer. Figure 12 illustrates the experimental setup. 34

46 Figure 12: Experimental Setup The optical response of the samples was primarily measured by collecting transmission spectra as the samples were heated from C and then cooled from C. The temperature was stepped up or down after every 60 seconds and then held. The heating/cooling rate was defined by the size of the temperature step. For example, a sample heated at 2 C/minute would be held at 50 C for 60 seconds, then stepped up to 52 C and held for 60 seconds, etc. A LabVIEW program was used to collect data and control the TEC controller. For the majority of the experiments, data was collected approximately every 10 seconds. 35

47 2.4 Gel Thickness Measurements In order to explore how the heterogeneous cell geometry of this system might change as a function of cell thickness, the thicknesses of the swollen polymer/lc gels were measured for cell thicknesses between 10 and 50 μm. The thicknesses of the dried, unswollen polymer films were also measured and compared to the swollen gel thicknesses. To prepare the samples for the thickness measurements, cells that had been dried through solvent exchange with cyclohexane were first physically cracked open. By visual inspection, all of the dried polymer appeared to be adhered to the glass slide that had been coated with the photoinitiator-doped polyimide. A razor was used to remove glue and dried polymer near the edges of this glass slide so that the glass was exposed to serve as a reference for the thickness measurements (region a in Figure 13). The glass slide with polymer was heated on a hot plate to 50 C to aid in the reswelling process. Small drops of nematic LC E7 were placed on the polymer film with a pipette (region c in Figure 13). The LC drops did not cover the entire surface of the polymer, leaving some dried polymer regions exposed for the dry thickness measurements (region b in Figure 13). Figure 13: Schematic of Sample Prepared for Gel Thickness Measurement Representation of the profile of a sample prepared for both dried polymer and swollen polymer/lc gel thickness measurements. Region a is the reference glass, b is a dried polymer region, and c is a region swollen with LC solvent. 36

48 In the drop regions, the polymer swelled with the LC solvent for several minutes until the swollen regions were visibly clear, suggesting that the selective reflection exhibited by the gel was in the near-infrared regime and that the gel had fully reswollen. An air gun was used to blow off excess LC from the gel surface. Transmission spectra were taken of the swollen regions to confirm that the selective reflection from the gel was close to the templated notch position. The closer the selective reflection was to the original notch position, the closer the gel was to its maximum swelling capacity. A Veeco Wyko optical profilometer was used to measure the thicknesses of the swollen polymer regions and the dried polymer regions. For each region that was measured, the glass surface was included in the field of view to serve as a reference. In general, the profilometer was focused in the middle of the sample depth and then scanned above and below to ensure the capture of the entire sample. Figure 14 is a topographical image of a region containing swollen polymer (c), dried polymer (b), and the glass surface (a). 37

49 Figure 14: Optical Profilometry Image of Polymer Film. Topographical image of a chiral-structured LC polymer film swollen with nematic LC E7. Region a is the glass surface, b is the dried polymer, and c is the polymer swollen with solvent. The Veeco Vision software was used to generate histogram plots of the depth profiles of the samples. The thicknesses of the dried polymer and swollen polymer were calculated by taking the differences between the glass surface depth and the dried or swollen polymer depths, respectively. Due to variations in the polymer film thickness, the depth with the largest count (most number of pixels occupying that depth) was used to estimate the average depth of the polymer region. Selected histogram plots are provided in Appendix C. 38

50 CHAPTER III RESULTS AND DISCUSSION 3.1 Thermal Tuning Behavior and Analysis The chiral-structured polymer/lc system examined in the present study demonstrated large-scale reflection notch tuning with temperature that was comparable to the tuning reported previously by McConney et al [12]. At lower temperatures, the selective reflection notch arising from the system was stable in the near-infrared. Around 60 C, the notch began to blue-shift to smaller wavelengths. Blue-tuning continued through about 70 C, at which point the notch was in the visible regime. Above 70 C, the notch red-tuned back to the original notch position while simultaneously losing contrast. Above 90 C, the reflection notch was no longer discernible and the transmission spectrum was clear. Figure 15 shows typical transmission spectra obtained while heating the chiral-structured polymer/lc gel. 39

51 Figure 15: Typical Thermal Tuning Transmission Spectra In order to quantify the thermal tuning behavior, the notch position was plotted as a function of temperature. A LabVIEW program was used to track the notch position with time and temperature. The notch position was defined as the center of the full-width half maximum (FWHM) of the notch. The full-width half maximum of a notch is the width of the notch at half of the notch depth. To account for slight differences in the initial, room temperature notch position amongst the samples, the notch position λ was normalized to the initial notch position λ 0. Figure 16 shows the normalized notch position λ/λ 0, or notch shift, with temperature for a typical 1 C/min heating cycle. 40

52 Figure 16: Notch Shift with Temperature. The normalized notch position as a function of temperature for a 1 C/min heating cycle. The values of interest for the thermal tuning experiments were primarily the maximum tuning range and the nematic-isotropic transition temperatures for the nematic solvent and the gel. The larger the tuning range, the greater the portion of the optical spectrum that could be selectively reflected by the chiral-structured polymer/lc system. The transition temperatures determine the temperature range in which the system exhibits the dynamic optical behavior. The maximum tuning range Δλ max was defined as the difference between the initial notch position and the minimum notch position achieved during blue-tuning. In order to compensate for differences in the initial notch position, the tuning range was also normalized to the initial notch position λ 0. Because the blue- and red-tuning are attributed to the nematic-isotropic transitions of the nematic solvent and gel, respectively, the transition temperatures were estimated by examining the onset of blue- and red-tuning. The T NI of the nematic solvent was taken to occur at the 10% value of the total blue-tuning range upon heating. In Figure 16, the T NI LC is around 58 C. 41

53 Similarly, the T NI of the gel was taken to occur at the 10% value of the red-tuning range upon heating. The T NI gel of the sample in Figure 16 is around 73 C. 3.2 Heating Rate Study Devices that would utilize the chiral-structured polymer/lc system would operate on a temperature-ramping or temperature-step mechanism. Therefore, the sensitivity of the dynamic optical behavior of the system to heating/cooling rate is an important factor to explore. The optical response of the system was measured at heating/cooling rates ranging from 0.5 C/min to 10 C/min. The samples used in this study were prepared in ~30 μm cells and were cured at an intensity of 2.2 mw/cm 2. Figure 17 shows the maximum tuning range exhibited by the samples for the various heating and cooling rates. Figure 17: Maximum Relative Tuning Range as Function of Heating/Cooling Rate 42

54 Within the range of heating/cooling rates studied, the maximum amount of reflection notch tuning stayed approximately constant. The initial notch position was around nm and the notch tuned to around 900 nm, which was approximately a 39% change in notch position. The tuning range achieved was the same for both heating and cooling temperature ramps, which demonstrates the reversibility of the optical tuning behavior. Multiple trials were performed for each heating/cooling rate, and no deviation in optical response between the trials was observed. Tuning data for selected heating/cooling rates can be found in Appendix A. While the tuning range of the system was independent of the heating/cooling rate, the reflection notch shape was not maintained throughout the entire tuning range for temperature ramping rates greater than or equal to 3 C/min. During the blue-tuning regime, the notch broadened and developed multiple peaks. Above 7 C/min, the notch distortion during blue-tuning was such that the broadened notch encompassed the majority of the tuning range (Figure 18). Upon further heating/cooling into the red-tuning regime, the notch shape improved and the notch only exhibited one peak. 43

55 Figure 18: Reflection Notch Distortion at High Heating/Cooling Rates. Above 7 C/min, the selective reflection notch exhibited significant distortion throughout the blue-tuning regime. When the pitch is constant throughout a cholesteric liquid crystal, the bandwidth of the selective reflection notch is determined by the product of the liquid crystal birefringence and the pitch ( ). However, when the pitch varies throughout the CLC, the bandwidth is given by Equation 4 [26]: (4) The greater the difference between the maximum and minimum pitch present in the CLC sample, the larger the bandwidth of the reflection notch. The broadening of the reflection notch when using faster temperature ramping rates suggests that a pitch gradient develops throughout the chiral-structured polymer/lc gel during deswelling. This pitch gradient could partially be due to the nature of the experimental setup. One side of the cell was in contact with the thermoelectric cooler, and the other side was left open to the surrounding air, which would cause an inevitable temperature gradient through the cell thickness. At 44

56 higher heating and cooling rates, the system would have less time to reach thermal equilibrium and the temperature gradient through the cell thickness would increase. Since the system temperature is related to the degree of swelling of the chiral-structured polymer/lc gel, a temperature gradient through the cell thickness would be expected to cause a non-uniform degree of swelling through the gel, which would translate to a pitch gradient through the cell thickness. Further work examining the thermal and molecular diffusion processes occurring during the heating and cooling cycles would have to be done in order to fully explain the notch broadening behavior at higher heating/cooling rates. Creating a heat transfer model of the system could help determine whether a temperature gradient through the cell thickness would be significant enough to account for the notch distortion at higher heating rates. The potential temperature gradient could also be explored using a fairly simple experiment. A hyper-reflective cell containing two chiral-structured polymer templates of opposite handedness could be subjected to fast heating and cooling cycles. Due to the one-sided experimental setup and the cell design, one of the polymer templates would be closer to the thermoelectric cooler. The right- and left-handed components of the selective reflection could be collected separately from each other and then compared. If a significant thermal gradient developed through the cell, the selective reflection arising from the polymer-templated CLC furthest from the TEC should lag behind the polymer-templated CLC closest to the TEC. In order to further examine the response of the chiral-structured polymer/lc gel to rapid temperature changes, the temperature of the system was stepped from below the T LC NI to above the T LC NI. Over the course of 15 seconds, the temperature of the 45

57 thermoelectric cooler attached to the cell increased from 50 C to 65 C. After overshooting to 70 C, the temperature of the TEC equilibrated to around 65 C after 40 seconds. Figure 19 shows the tuning response of the system to this 15-degree temperature step. Figure 19: Tuning Response to Temperature Step. Upon a 15-degree temperature step from below the T NI LC to above the T NI LC, the reflection notch blue-tuned. The full tuning response spanned a period of more than 2 minutes. Approximately 40 seconds after the start of the temperature step, the selective reflection notch began to blue-tune. The notch tuned hundreds of nanometers over the course of the next 100 seconds before settling at the final, shorter wavelength. The total response time of the system was longer than 2 minutes. The tuning response of the system to a 15-degree cooling step was also measured. As in the case of the heating step, the response time of the system was greater than 2 minutes. Multiple heating and cooling steps were performed with no degradation in the tuning performance of the system. The ability of the chiral-structured polymer/lc system to exhibit repeatable, large-scale notch tuning in response to extreme temperature steps 46

58 demonstrates the robustness of the tuning mechanism. However, a response time on the order of 2 minutes would be extremely slow for applications. Further investigation into both the thermal and molecular diffusion processes occurring during heating/cooling would be required to determine if the response time of this system could be reduced. The response time of the system could be limited by the diffusion of the low molar mass LC solvent molecules out of and into the chiral-structured polymer network during deswelling and reswelling. Given the relatively thin cell thickness of 30 μm, thermal diffusion alone is likely insufficient to account for the amount of time the system takes to reach equilibrium in response to a temperature step. The nematic-isotropic transition temperatures of the nematic solvent and the gel were also examined as a function of heating/cooling rate. During heating ramps, the nematic-isotropic transition temperature of the solvent, T LC NI, corresponded with the onset of blue-tuning, and the nematic-isotropic transition temperature of the gel, T gel NI, corresponded with the onset of red-tuning. Figure 16 previously indicated these transition temperatures on the notch shift vs. temperature tuning curve. For the cooling cycles, the nematic-isotropic transition temperatures were considered to occur at the same points LC on the cooling tuning curves as on the heating tuning curves. Figure 20 shows the T NI and the T gel NI as functions of heating/cooling rate for both heating and cooling cycles. 47

59 a) b) Figure 20: Dependence of Transition Temperatures on Heating/Cooling Rate. The nematic-isotropic transition temperatures of the nematic solvent (a) and the CLC gel (b) as functions of heating/cooling rate. At temperature ramps below 2 C/min, the transition temperatures occurred at approximately the same temperatures for both heating and cooling. The T NI LC was ~60 C and the T NI gel was ~74 C. Above 2 C/min, the transition temperatures during heating deviated from the transition temperatures during cooling. At the fastest temperature 48

60 ramps, the difference between the transition temperatures during heating and cooling was as much as 14 C. This discrepancy could also be potentially explained by molecular diffusion limitations of the system and/or the one-sided heating setup. At higher heating/cooling rates, the bulk temperature of the chiral-structured gel would lag behind the temperature right at the cell interface. Since the temperature is only measured at the interface and not in the bulk, the nematic-isotropic transition temperatures of the solvent and gel would appear to occur at higher temperatures upon heating and lower temperatures upon cooling. Additional plots provided in Appendix A demonstrate the difference between heating and cooling cycles at higher heating/cooling rates. The study of the impact of heating/cooling rate on the dynamic optical behavior of the chiral-structured polymer/lc system demonstrates the robust nature of this system. At heating/cooling rates up to 10 C/min, the CLC gel exhibited a maximum tuning range of nm, which was approximately a 39% change in initial notch position. This relatively large tuning range encompassed both near-infrared and visible wavelengths, which would be attractive for dynamic filter applications. The fact that the tuning range was independent of the heating/cooling rate (within the range studied) and that the thermal tuning behavior was reversible and repeatable makes this thermally tunable CLC system potentially viable for applications. However, the broadening of the selective reflection notch during faster heating/cooling cycles does not allow for precise control over the reflection color during tuning. The system also had a tuning response time on the order of 2 minutes when it was subjected to a 15-degree temperature step. This response time would be unacceptably slow for dynamic filter applications. Further work could investigate whether a significant thermal gradient develops through the cell thickness at 49

61 higher heating/cooling rates, contributing to notch distortion during tuning and discrepancies in the measured nematic-isotropic transition temperatures. An improved, more uniform temperature control design could reduce potential temperature gradients through the cell thickness. Studying the molecular diffusion processes during gel deswelling and reswelling could also provide insight into possible ways the response time of the system could be reduced. 3.3 Curing Intensity Study The dynamic optical behavior exhibited by the chiral-structured polymer/lc system is directly related to the swellability of the polymer network. The blue-tuning of the selective reflection notch is due to deswelling of the polymer/lc gel caused by an order mismatch between the solvent and the gel. Conversely, red-tuning is due to the gel reswelling as the orientational orders of the solvent and gel again become compatible. Since the chiral-structured polymer network plays a crucial role in the thermal tuning behavior, altering the polymerization conditions during the formation of the CLC gel could be expected to impact the thermal tuning. The preliminary investigations of this system only examined chiral-structured polymer/lc gels cured at intensities around 1-2 mw/cm 2. In order to determine the range of curing intensities which would yield thermally-tunable CLC gels, the curing intensity was varied from 0.04 mw/cm 2 to mw/cm 2. The gels were subjected to heating/cooling cycles and the maximum tuning ranges exhibited by the gels were compared. The polymer/lc gels were all prepared in cells of ~30 μm thickness. 50

62 When the dried polymer network formed at 0.04 mw/cm 2 was reswelled with nematic liquid crystal solvent, no selective reflection notch was exhibited. The curing intensity of 0.04 mw/cm 2 was likely too low to yield sufficient crosslinks to template the cholesteric phase. The samples cured from mw/cm 2 did exhibit a selective reflection notch that tuned with temperature. Figure 21 shows the maximum change in relative notch position achieved by these samples. Figure 21: Maximum Relative Tuning Range with Curing Intensity The samples cured at lower intensities demonstrated the largest amount of tuning. The selective reflection notch arising from samples cured at an intensity of 0.09 mw/cm 2 tuned approximately 45% from the initial notch position. The tuning range exhibited by the samples decreased with increasing curing intensity until around 1.00 mw/cm 2. Above 1.00 mw/cm 2, the tuning range stayed approximately constant at ~33%. Additional raw tuning data for selected curing intensities is provided in Appendix B. 51

63 Previous work by Okuno et al. demonstrated that the swellability of a nematic gel in a nematic solvent was strongly dependent on the crosslink density of the LC polymer network [27]. The swellability of the nematic gel decreased with increasing crosslink density until at a high enough crosslink density, the nematic gel showed little change in volume in response to the nematic-isotropic transitions of the solvent and gel. The increased resistance of the gel to volume changes was attributed to the increased elastic modulus of the polymer network at higher crosslink densities. Similarly, preliminary investigations of the chiral-structured polymer/lc gel studied here demonstrated that at a certain crosslink density, the dynamic optical response of the system was nearly eliminated [15]. The crosslink density of the polymer network was increased by adding the LC diacrylate crosslinker RM257 to the unpolymerized syrup. As the highlycrosslinked gel was heated past the T NI of the nematic solvent, very little blue-tuning of the selective reflection notch occurred (Figure 22). The reduction in tuning was attributed to an increased network modulus that reduced the polymer network s capacity for deswelling/reswelling. 52

64 Figure 22: Reduction of Notch Tuning at High Crosslink Densities. As the crosslink density of the polymer network was increased through the addition of crosslinking monomer, the amount of notch tuning exhibited by the system decreased. At a crosslinker concentration of 5%, the notch tuning behavior was nearly eliminated. (McConney et al., 2012) [15] The larger tuning ranges achieved by the samples cured below 1.00 mw/cm 2 in this study could be due to decreased crosslink densities of the chiral-structured polymer networks. Polymer networks with lower crosslink densities would have smaller moduli, allowing for greater contraction of the polymer networks upon heating. At the maximum contracted state, the polymer networks with low crosslink densities would have a smaller pitch, translating to a bluer reflection notch and larger maximum blue-tuning magnitude. Simplistically, lower curing intensities can be related to lower crosslink densities. The light intensity during photopolymerization impacts the rate of formation of primary free radicals from the initiator ( ) through the following relation (Equation 5): [ ] (5) where I 0 is the incident light intensity, ϕ is the quantum yield, l is the path length, ε is the extinction coefficient of the initiator, and [S] is the concentration of the initiator [28]. As 53

65 the concentration of primary free radicals [R ] increases, the rate of chain initiation reactions increases (Equation 6): [ ][ ] (6) where k i is the rate constant and [M] is the monomer concentration [28]. The more chain initiation reactions that occur, the more chain radicals are available to participate in crosslinking reactions. The increase in both primary and chain radicals also increases the likelihood of chain termination through combination with other radicals, resulting in shorter polymer chains. Shorter polymer chains and greater number of crosslinks together increase the crosslink density of the polymer network. So by only considering these rough expressions, the CLC gels cured at the lowest intensities would be expected to have the lowest crosslink densities. The constant tuning range above a curing intensity of 1.00 mw/cm 2 suggests that the swellability of the polymer network did not significantly change at higher curing intensities. Further examination of the polymerization kinetics of this system would be required to fully explain the constant tuning magnitude above 1.00 mw/cm 2. At higher curing intensities, monomer diffusion may have become the rate-controlling step instead of the rate of formation of primary free radicals (diffusion-limited cure), causing the rate of polymerization to level off with increasing curing intensity [29]. The placement of the photoinitiator in the top polyimide substrate layer as opposed to evenly dispersed throughout the bulk monomer mixture should also be taken into account. As the curing intensity increased, the rate of primary radical formation would have increased, increasing the concentration of primary radicals near the surface exposed to UV light. At elevated concentrations, the primary radicals would have had a greater likelihood of 54

66 recombining with each other before they could escape from the cage to react with monomers. So, perhaps the increase in primary radical formation with higher curing intensities was negated by an increase in the rate of radical recombination, yielding little change in crosslink density. While the tuning range of the chiral-structured polymer/lc gel is related to the crosslink density of the polymer network, the tuning range is not the only indicator of crosslink density. The gel nematic-isotropic transition temperature of the system can also reflect changes in the crosslink density of the polymer network. Previous work has demonstrated that as the crosslink density increases, the gel T NI increases [15,27]. When the dry liquid crystalline polymer network is swollen with a nematic LC solvent with a T NI much lower than the polymer T NI, the solvent effectively lowers the T NI of the gel. At higher crosslink densities, the polymer network absorbs less solvent, making the volume fraction of the polymer in the gel larger. Consequently, the solvent contributes less to the overall gel T NI and the gel T NI is higher. In order to investigate whether the gel T NI could serve as an additional indication of the underlying crosslink density of the system studied here, the gel nematic-isotropic transition temperatures of the samples cured at varying light intensities were compared. However, the transition temperatures appeared to be inconsistent and scattered. Because examining the red-tuning onset temperature is a more indirect method of measuring the gel T NI, future work could utilize differential scanning calorimetry (DSC) to obtain more accurate measurements. Future work could also potentially use DMA to measure the moduli of the dried chiral-structured polymer films polymerized at different curing intensities. The modulus measurements could then be 55

67 used to make more definitive conclusions about the impact of the curing intensity on the network crosslink density. On a side note, the tuning range exhibited by the samples cured around 2 mw/cm 2 was ~33% in this study, whereas the samples cured at ~2 mw/cm 2 in the heating rate study exhibited ~39% tuning. Different syrup batches were used for the two studies, so the difference between the tuning ranges was likely due to variations in the composition of the RMM691 mixture. Variations in the concentrations of the crosslinking components in RMM691 would lead to variations in the crosslink density of the polymer network, which would impact the maximum tuning range displayed by the system. The results from the curing intensity study indicate that polymerizing the chiralstructured polymer networks at intensities below 1.00 mw/cm 2 yields CLC gels with the largest tuning ranges. In the system studied here, the minimum curing intensity to produce a thermally tunable CLC gel should be somewhere between 0.04 and 0.09 mw/cm 2, as the samples cured at 0.04 mw/cm 2 did not exhibit a selective reflection notch. In terms of applications, a dynamic filter employing this chiral-structured polymer/lc system would ideally have the capability to reflect a large range of optical wavelengths. Therefore, this study suggests that the CLC gels prepared with low curing intensities (i.e. around 0.09 mw/cm 2 ) would be the most desirable for dynamic filter applications. 56

68 3.4 Cell Geometry Study Cell Thickness The chiral-structured polymer/lc system first reported by McConney et al. was the first CLC system to demonstrate selective reflection notch tuning with temperature as a result of orientational order-induced volume transitions [12]. This system had a heterogeneous design through the cell thickness. The chiral-structured polymer network was tethered to one side of the cell by embedding photoinitiator in the polyimide layer on one glass surface. Preliminary studies utilizing this cell design indicated that upon refilling the cell with nematic LC after polymerizing and drying the polymer template, two distinct regions existed through the cell thickness [10,11,12]. In a cell with a thickness of 30 μm, the swollen polymer occupied approximately 2/3 of the cell thickness (about 20 μm), and pure nematic solvent occupied the remaining third of the cell (Figure 23a). The potential contributions of this heterogeneous cell design to the dynamic optical behavior of the system have not been previously examined. In this study, the cell thickness was varied in an attempt to alter the ratio between the size of the polymer/lc gel region and the size of the solvent-only region. Previous studies indicated that the maximum reswollen polymer thickness was ~20 μm (for a polymer network fabricated in a 30 μm cell). If the structure of the reswollen polymer network is assumed to be comparable to the as-polymerized network before draining, perhaps the growth of the polymer during polymerization was also limited to a thickness of 20 μm. Since the photoinitiator molecules were initially confined to one polyimide surface substrate, the opposite end of the cell may have been deficient in photoinitiator during polymerization, 57

69 causing the polymer network to terminate before reaching the opposite side. Therefore, if the cell thickness were changed, perhaps the polymer would still grow to a thickness of 20 μm and the thickness of the solvent-only region would change, as illustrated in Figure 23. Decreasing the cell thickness would decrease or even eliminate the solvent-only region (Figure 23b), while increasing the cell thickness would increase the size of the solvent-only region (Figure 23c). Figure 23: Proposed Impact of Cell Thickness on Cell Geometry. Figure 23a illustrates the cell geometry initially observed in 30 μm cells. The swollen polymer network occupied 2/3 of the total cell thickness (region A), leaving the rest of the cell for pure nematic solvent (region B). In Figure 23b, the cell thickness is decreased so that the polymer network occupies the full cell thickness and the B region is eliminated. In Figure 23c, the cell thickness is increased, which increases the thickness of the B region. Cells of thicknesses between 10 and 50 μm were considered in this study. In order to get a sense of the ratio of the polymer/lc gel region (region A) to the solvent-only region (region B) within the different cell thicknesses, the thicknesses of the gels were measured using optical profilometry according to the procedure described previously. The thicknesses of the dried polymer films containing no LC solvent were also measured. In order to compare the relative cell geometries of the different cell thicknesses, the polymer thickness values were taken as fractions of the initial cell thicknesses in which 58

70 the polymer templates were formed. Figure 24 shows the dried and swollen polymer thickness fractions for the various cell thicknesses. Figure 24: Polymer Thickness Fraction of Total Cell Thickness Contrary to the original hypothesis, the polymer/lc gel region did not retain the same thickness of 20 μm as the cell thickness was changed. Rather, the gel region appeared to occupy the same fraction of the total cell thickness regardless of the thickness of the cell. For cell thicknesses between 10 and 50 µm, the swollen polymer thickness was approximately 85% of the initial cell thickness (which is greater than the 2/3 ratio reported in previous studies) [10,11,12]. The dried polymer thickness was also directly proportional to the initial cell thickness. The thickness fraction of the dried polymer films was ~15% for all the cell thicknesses examined. While the initial hypothesis was that the polymer growth was limited to a thickness of 20 μm, the swollen polymer thicknesses measuring greater than 20 μm in thicker cells show otherwise. In addition, the polymer thickness measurements for cells 59

71 less than 20 μm indicate that the gel region did not extend through the entire cell thickness after reswelling as initially anticipated. Consequently, the cell geometries proposed in Figures 23b and 23c were never realized. Regardless of the cell thickness, the cell geometry resembled Figure 23a. Examination of the selective reflection notch exhibited by the swollen gels can provide additional insight into the two-region cell geometry observed in the μm cells. All of the swollen polymer films exhibited reflection notches that were approximately 100 nm bluer than the as-polymerized reflection notches before draining. Since the pitch of the chiral-structured polymer/lc system is dependent on the degree of swelling of the system [12,30], this bluer notch position suggests that the polymer films may not have fully reswollen back to the same degree as before draining. Incomplete swelling could account for the 15% discrepancy between the swollen polymer thickness and the total cell thickness. To further investigate this hypothesis, a polymer film was swollen to various degrees in multiple spots by controlling the time the spots were heated during solvent uptake. The thickness and notch position of each spot were measured. The polymer film was originally formed in a 50.9 μm cell that exhibited a reflection notch at 1216 nm before draining. After reswelling the polymer film with solvent, the spot swollen to the greatest degree had a thickness of 44.4 μm and showed a reflection notch at 1100 nm. As shown in Figure 25, the thickness of the swollen polymer and the reflection notch position have a linear relationship. If notch position is assumed to be an accurate indicator of the swollen polymer thickness, the trend in Figure 25 suggests that full reswelling to a notch position of 1216 nm would correspond with a swollen polymer thickness equivalent to the initial cell thickness of 50.9 μm. 60

72 Figure 25: Notch Position as Indication of Swollen Polymer Thickness. A chiralstructured polymer template was formed at 1216 nm in a 50.9 μm cell. The relationship between the thickness of the polymer reswelled with LC solvent and the position of the selective reflection notch exhibited suggests that full reswelling to 1216 nm would correspond with a polymer thickness equal to the cell thickness of 50.9 μm (marked with an X). The results illustrated in Figure 25 suggest that the chiral-structured polymer network grew to occupy the full cell thickness during polymerization instead of terminating at a length partially through the cell. This hypothesis could explain why the thicknesses of both the dry and swollen polymer films scaled with the cell thickness. Further experimentation using techniques other than optical profilometry would be required to confirm the estimation that the chiral-structured polymer grew to occupy the full cell thickness before draining. Although the polymer may have filled the entire cell thickness before draining, the polymer network consistently reswelled to only a fraction of the total cell thickness after having been dried, effectively creating a heterogeneous cell geometry. Because the ratio between the swollen polymer region and the solvent region remained the same for all cell thicknesses, the potential contribution of the solvent region to the dynamic optical behavior is still unclear. Additional work could explore this 61

73 issue by examining whether an improved reswelling technique could increase the maximum degree of swelling and effectively eliminate the solvent region. The realization that the polymer network likely did not terminate at a length partially through the cell during polymerization is a significant result of this thesis project. Previous work on similar surface-tethered systems reported heterogeneous cell geometries through the cell thickness, leading to the hypothesis that the uncommon technique of isolating photoinitiator in a single surface substrate to achieve surfacetethering was related to the heterogeneous geometry [10,11,12]. The results from the present study on the heterogeneous cell geometry alternatively suggest that the 2-region geometry is a product of incomplete reswelling of the CLC gel. While this study was unable to address the role that the heterogeneous cell geometry plays (if any) in the dynamic optical behavior of this system, this study does inform the assumptions for additional work on similar surface-tethered systems. Because the chiral-structured polymer network is responsible for locally inducing a CLC phase, understanding the spatial aspects of the polymer network is critical for tailoring the cell design, particularly for more complex designs like hyper-reflective cells. The second aspect of the cell thickness study was examining the dynamic optical response of cells between 10 and 50 μm to a thermal stimulus. As the cells were heated, cells of all thicknesses displayed the characteristic blue- and red-tuning observed in this chiral-structured polymer/lc system. Figure 26 shows the maximum normalized tuning range achieved in the different cell thicknesses. 62

74 Figure 26: Maximum Relative Tuning Range with Cell Thickness While the cell geometry proportions did not vary with cell thickness, the tuning range exhibited by the chiral-structured polymer/lc system did show a dependence on the cell thickness. As cell thickness increased, the tuning range decreased. Cells with a thickness around 10 μm exhibited near 49% tuning from the initial notch position while 50 μm cells only exhibited around 34% tuning. Additional raw tuning data for the thinnest and thickest cells (~10 and 50 μm, respectively) is provided in Appendix C. Given that the cell geometry did not change with cell thickness and that the monomer syrup composition and exposure conditions were kept the same for all cells, the decrease in tuning range with increasing cell thickness is an unexpected result. One parameter that may have been impacted by the change in cell thickness was the effective photoinitiator concentration. All cells were fabricated with an equal amount of polyimide substrate layer with 1% photoinitiator by weight, so cells of all thicknesses contained approximately the same mass of photoinitiator. However, changing the cell thickness also 63

75 changed the volume of monomer syrup contained in the cell. Consequently, the amount of photoinitiator per unit volume syrup changed with cell thickness. At the same time, since the photoinitiator was initially confined to the polyimide surface substrate, the effective photoinitiator concentration would only change with cell thickness if the photoinitiator diffused out of the substrate into the bulk monomer syrup. Further examination of the diffusion of the photoinitiator during polymerization would be required to determine whether the effective photoinitiator concentration indeed changed with the cell thickness. A more specific study on the effect of the photoinitiator concentration on the dynamic tuning behavior of the system could also help determine whether the photoinitiator contributed to the decrease in the tuning range with increasing cell thickness. The concentration of photoinitiator in the system impacts the polymerization kinetics, which would be expected to impact the formation of the chiralstructured polymer template. Perhaps differences in the structures of the polymer templates in the different cell thicknesses could account for the tuning range dependence on cell thickness. Comparing the selective reflection notches of the various cell thicknesses can start to provide some clues about the underlying structures of the chiral-structured polymer templates. Figure 27 shows the transmission spectra of the CLC templating fluid before polymerization and of the chiral-structured polymer/lc system after drying and reswelling for different cell thicknesses. 64

76 Figure 27: CLC Transmission Spectra Before Polymerization and After Reswelling The spectra of the selective reflection notches before polymerization were very similar for all the cell thicknesses. However, after polymerizing, drying, and reswelling the polymer templates, the reflection notches showed varying degrees of distortion from the pre-cure notches. The refilled notches of the thinnest cells were blue-shifted and had a broadened, distorted right edge. Meanwhile, the refilled notches of the thickest cells were closer to the original notch position and had less distortion on the right edge. The broadened notches exhibited by the thinner cells indicate that greater pitch variation was 65

77 present in the thinner cells. Additional experimentation could elucidate the possible relationship between the notch distortion and larger tuning range observed in thin cells. The pitch variation in thin cells could potentially be due to variations in the crosslink density through the cell thickness, which would also impact the tuning range of the system. The study of the impact of cell thickness on the thermal tuning behavior of the chiral-structured polymer/lc system shows a clear relationship between cell thickness and the maximum tuning range. Within the range of 10 to 50 μm, as the cell thickness increased, the maximum tuning range decreased. This result is unexpected and further experimentation is necessary to understand the relationship between cell thickness and maximum tuning range. Investigating the effect of the photoinitiator concentration on the thermal tuning behavior of the system may help explain the dependence of the tuning range on cell thickness. Since the impact of cell thickness on the thermal tuning behavior needs to be explored further, the ideal cell thicknesses for applications are yet unclear. While the large tuning ranges of the thinner cells are attractive for dynamic filter applications, the thinner cells also exhibited broadened, distorted selective reflection notches, which would limit the precision of a dynamic filter. As discussed in more detail in Appendix C, the notch depth of the 10 μm cells also decreased significantly throughout both the blue- and red-tuning regimes. Highly-reflective filters are desired for applications, so the notch depth decrease in the 10 μm cells would be a significant drawback. 66

78 3.4.2 Surface Attachment of Polymer Network Previous work has indicated that embedding photoinitiator in the polyimide surface substrate results in the strong attachment of the chiral-structured polymer network to the polyimide layer [10,11,12]. The attachment mechanism is the subject of current research, but preliminary results suggest that the chiral-structured polymer network may be adhered, or tethered, to the polyimide layer through chain entanglements. The initial motivation for tethering the LC polymer network to the polyimide surface substrate was to enable the simple fabrication of a cell that contained multiple polymer templates. Chiral-structured polymer templates of opposite handedness were tethered to opposite cell surfaces, which enabled the coexistence of two homogeneous CLC domains of opposite handedness within a single cell [12]. While this tethering technique was effective in allowing control over the cell design, the surface-tethered polymer/lc system also exhibited dynamic optical behavior with temperature that had been previously unreported. This optical behavior was due to volume change of the CLC gel through deswelling and reswelling. Because the LC polymer network was tethered to one surface, the dimensions of the network were constrained in the in-plane directions but not in the thickness direction [31]. Since the cholesteric liquid crystal helices were aligned in the thickness direction, deswelling or reswelling in the thickness direction corresponded with a pitch change in the CLC, which produced the selective reflection notch tuning behavior. Swelling in the thickness direction is responsible for the dynamic optical response of the CLC gel. While the one-sided surface attachment limits swelling of the polymer network to the thickness direction, the necessity of this limitation for producing large- 67

79 scale pitch changes in the CLC is unclear. In an effort to examine whether the chiralstructured polymer/lc system must be attached to a surface in order to exhibit the dynamic optical behavior with temperature, the optical response of an untethered system was studied. To fabricate the untethered system, 1% by weight photoinitiator was dispersed in the bulk monomer mixture instead of the polyimide layer. The cell thickness was ~30 μm and the curing intensity was 2.4 mw/cm 2. Upon heating, the untethered system exhibited the same characteristic tuning behavior as the surface-tethered system (refer to Appendix C for raw tuning data). From ~60-70 C, the reflection notch blue-tuned. Then above 72 C, the reflection notch redtuned back to the initial notch position. The maximum amount of notch tuning achieved by the untethered system was ~26%. These initial results suggest that one-sided surface attachment may not be required for large-scale pitch changes to occur in correlation with gel deswelling and reswelling. However, since the drying step in the preparation of the chiral-structured polymer/lc system results in the collapse of the polymer network, further control studies are needed to confirm that the system prepared in this study was actually untethered. Capillary forces during draining may have caused the polymer network to adhere to the cell surface, which could account for the similarity between the optical responses of this system and the tethered systems. Assuming that the chiral-structured polymer network was indeed untethered and free to swell in all directions, significant deswelling/reswelling in the thickness direction still had to occur in order to produce significant pitch changes. Isotropic polymer networks are expected to swell evenly in all directions, but due to their anisotropy, liquid crystalline polymer networks are expected to swell anisotropically [32]. Suto and Suzuki 68

80 observed anisotropic swelling in unattached hydroxypropyl cellulose films with cholesteric liquid crystalline order [30]. The degree of swelling in the thickness direction was greater than the degree of swelling in both the length and width directions. In addition, Suto and Suzuki observed pitch change in the CLC films, which was correlated with the degree of swelling in the thickness direction. Anisotropic swelling in the thickness direction could explain why the surface-tethered and untethered CLC systems studied here displayed similar optical behavior with temperature. In order to confirm the hypothesis that the untethered chiral-structured polymer/lc system experienced anisotropic swelling, the dimensions of the untethered CLC gels should be measured during deswelling/reswelling. The reflection notch tuning behavior was fundamentally the same for the surfacetethered and untethered CLC systems. However, the maximum tuning range achieved by the untethered system was 26%, which was much less than the ~39% tuning range exhibited by the surface-tethered system prepared with the same curing intensity and cell thickness. Further experimentation would be required to determine why the untethered system demonstrated less optical tuning. Because the photoinitiator was distributed in the bulk monomer syrup at a concentration of 1% by weight instead of the polyimide layer at a concentration of 1% by weight, the total mass of photoinitiator was likely higher for the untethered system. A greater amount of photoinitiator could increase the crosslink density of the polymer network, which would decrease both the swellability of the network and the maximum optical tuning range. The selective reflection notch tuning demonstrated by the supposedly untethered chiral-structured polymer/lc system suggests that the one-sided surface-tethering 69

81 condition may not be required for CLC gels to be capable of large-scale thermal notch tuning. Removing the surface-tethering constraint allows for greater versatility in cell design and fabrication. Depending on the application, dynamic filters could be constructed of either surface-tethered or untethered CLC gels. As discussed previously, surface-tethering is an effective technique for including multiple polymer templates in a single cell. At the same time, embedding the photoinitiator in the polyimide substrate in order to achieve surface-tethering could have ramifications on the polymerization process. If a tethered system is desired for applications, additional research should be done to understand the impact of embedding the photoinitiator in the polyimide layer. Otherwise, creating an untethered system by dispersing the photoinitiator in the bulk monomer mixture may remove unnecessary complications in the formation of the chiralstructured polymer template. 70

82 CHAPTER IV CONCLUSIONS The effects of several processing-structure-property variables on the dynamic optical response of a polymer-templated cholesteric liquid crystal to a thermal stimulus were investigated. The CLC system was composed of a surface-tethered, chiral-structured LC acrylate polymer template swollen with a nematic LC solvent mixture of n- cyanobiphenyls. Surface-tethering was accomplished by embedding photoinitiator in one polyimide surface substrate. As the CLC gels were heated, the selective reflection notch exhibited by the gels blue-tuned hundreds of nanometers and then red-tuned back to the original notch position. This large-scale tuning is attributed to a gel deswelling and reswelling mechanism driven by order/disorder transitions observed in liquid crystalline gels. The effects of heating/cooling rate, curing intensity, and cell geometry on the thermal tuning behavior were examined. All variations on the chiral-structured polymer/lc system demonstrated large-scale thermal tuning on the order of hundreds of nanometers. The chiral-structured polymer/lc system was subjected to heating and cooling cycles between 50 and 90 C at rates between 0.5 and 10 C/min. Regardless of the heating or cooling rate, the system demonstrated a maximum tuning range of nm, which was approximately a 39% change from the original notch position. Preliminary investigations of this system had only used a heating rate of 1 C/min to 71

83 thermally tune the selective reflection notch, which would be an impractical heating rate for dynamic filter applications. Consequently, the fact that the CLC gel was capable of tuning hundreds of nanometers at rates up to 10 C/min is a significant result of this thesis study. The tuning behavior was stable to multiple heating and cooling cycles, which also demonstrates the robust nature of this system. While the maximum tuning range was independent of the heating rate, above 3 C/min, the selective reflection notch broadened and became distorted during the bluetuning regime. At low heating/cooling rates, the nematic-isotropic transition temperatures of the LC solvent and CLC gel were consistent between heating and cooling cycles. The solvent T NI was ~60 C and the gel T NI was ~74 C. At higher heating/cooling rates, there was a larger discrepancy between the transition temperatures upon heating and cooling. When subjected to a 15 C temperature step, the system had a response time on the order of 2 minutes. The maximum tuning range of the system was impacted by the curing intensity used to form the chiral-structured polymer template. The tuning range was larger at lower curing intensities, likely due to lower crosslink densities of the polymer networks. The elastic moduli of the polymer networks would be decreased at lower crosslink densities, which would increase the swellability of the polymer networks. Above 1.00 mw/cm 2, the maximum tuning range stayed constant. Samples cured at 0.04 mw/cm 2 did not exhibit a selective reflection notch, meaning that the minimum curing intensity required to produce a thermally tunable CLC gel in this system is likely between 0.04 and 0.09 mw/cm 2. For the range of curing intensities studied here, 0.09 mw/cm 2 would be the most desirable for 72

84 dynamic filter applications, as the samples cured at 0.09 mw/cm 2 demonstrated the largest tuning ranges, ~45% from the original notch position. The cell geometry was not altered by changing the cell thickness. Regardless of cell thickness, the swollen polymer region occupied approximately 85% of the total cell thickness, leaving 15% for pure nematic solvent. The notch position of the reswollen gels were ~100 nm less than the templated notch position, suggesting that the gels had not completely reswollen to the as-polymerized state. The relationship between gel thickness and notch position further suggests that the polymer networks grew to occupy the full cell thickness during polymerization, contrary to the initial hypothesis. This information about the cell geometry is a particularly significant result of this thesis project, as it informs the assumptions of any future work on similar surface-tethered systems. The heterogeneous cell geometry in the chiral-structured polymer/lc system is likely a product of incomplete reswelling of the gel and not a result of polymer termination due to the isolation of the photoinitiator in a single surface substrate. Surprisingly, for cell thicknesses between 10 and 50 µm, as cell thickness increased, the maximum tuning range decreased. Cells with a thickness of 10 μm demonstrated a maximum tuning range of ~49%, while 50 μm cells only demonstrated ~34% tuning. However, the selective reflection notches of the thinner cells were broader and more distorted than the thicker cells, suggesting that larger pitch variation existed in the thinner cells. The relationship between the cell thickness and the maximum tuning range is not well understood at this point, so the ideal cell thicknesses for applications are yet unclear. 73

85 A potentially untethered chiral-structured polymer/lc system was created by dispersing photoinitiator in the bulk monomer mixture instead of the polyimide surface substrate. The system demonstrated the characteristic dynamic optical behavior with temperature, suggesting that surface-tethering may not be required to enable large-scale tuning in the chiral-structured polymer/lc gel. Since previous work had only reported the thermal tuning behavior in surface-tethered systems, this result is significant for furthering the understanding of the chiral-structured polymer/lc system. Without the surface-tethering constraint, additional cell designs are possible, increasing the versatility of this system for applications. Eliminating the need to isolate the photoinitiator in the polyimide surface substrate may also remove unnecessary complications in the formation of the chiral-structured polymer template. 74

86 CHAPTER V FUTURE RECOMMENDATIONS While this thesis project explored several processing-structure-property relationships of the chiral-structured polymer/lc system, additional research would be required before this system could be seriously considered for photonic applications. The results from the heating rate study indicate that the thermal and molecular diffusion processes occurring during the notch tuning regime should be examined more closely. The one-sided heating setup may have caused a thermal gradient to develop through the cell thickness at faster heating/cooling rates, which may have contributed to the notch broadening at rates above 3 C/min. To improve the heating setup, the CLC gels could be heated through a Joule heating method, as described by Natarajan et al [2]. The gels could be fabricated within glass cells coated with a transparent electrode like ITO. If an electric current were run through the electrodes, heat would be transferred to the gels from these resistive heating elements. The measured tuning response time on the order of 2 minutes would make this system undesirable for dynamic filter applications. Since the notch tuning behavior is the result of gel deswelling and reswelling, the diffusion of the low molar mass LC molecules into and out of the chiral-structured polymer network should be studied to determine whether molecular diffusion limits the response time of the system. If molecular diffusion is responsible for the slow response time, perhaps reducing the 75

87 crosslink density of the polymer network could be explored as an avenue to increase the solvent diffusion rate. Future work could expand upon the curing intensity study by using DMA to measure the moduli of the dried polymer networks cured at various intensities. The modulus measurements could then be used to make more definitive conclusions about the impact of curing intensity on the network crosslink density. The polymerization kinetics could also be studied in order to understand why the maximum tuning range remained constant above 1.00 mw/cm 2. The cure may have become diffusion-limited above 1.00 mw/cm 2, causing the rate of polymerization to level off. The placement of the photoinitiator in the polyimide alignment layer as opposed to evenly dispersed throughout the bulk monomer mixture also may have impacted the polymerization kinetics. Since the ratio between the gel and solvent regions was not actually varied in the cell geometry study, future work could look at different ways to examine whether the heterogeneous cell geometry contributes to the dynamic optical behavior of the system. Different swelling techniques could be investigated to try to fully reswell the CLC gel, which would effectively eliminate the solvent region at room temperature. The collapse of the chiral-structured polymer network during draining could also contribute to the gel s resistance to full reswelling, so additional techniques to remove unreacted monomer could also be considered. The inverse relationship between the cell thickness and maximum tuning range is one of the more puzzling results of this thesis project. Future work would be required to explain this relationship. A study of the effect of the photoinitiator concentration on the 76

88 dynamic tuning behavior could potentially help explain the decrease in tuning range with increasing cell thickness. Finally, the study of the impact of tethering the chiral-structured polymer network to one cell surface needs to be expanded. Additional work should confirm that the CLC gel fabricated by dispersing photoinitiator in the bulk monomer mixture instead of the polyimide layer was actually untethered and free to swell in all directions. Due to the draining process, capillary forces may have caused the polymer network to adhere to the cell surface. Both the surface-tethered and untethered gel dimensions could be measured during deswelling and reswelling to investigate whether the surface-tethering condition alters the degree of swelling in the pitch direction, which would impact the tuning range of the CLC gel. 77

89 REFERENCES [1] Timothy J. White, Michael E. McConney, and Timothy J. Bunning, "Dynamic color in stimuli-responsive cholesteric liquid crystals," Journal of Materials Chemistry, vol. 20, pp , [2] Lalgudi V. Natarajan et al., "Electro-thermal tuning in a negative dielectric cholesteric liquid crystal material," Journal of Applied Physics, vol. 103, p , [3] C. A. Bailey et al., "Electromechanical tuning of cholesteric liquid crystals," Journal of Applied Physics, vol. 107, p , [4] Timothy J. White et al., "Phototunable azobenzene cholesteric liquid crystals with 2000 nm range," Advanced Functional Materials, vol. 19, pp , [5] H. Philip Chen et al., "Glassy liquid-crystal films with opposite chirality as highperformance optical notch filters and reflectors," Advanced Materials, vol. 12, no. 17, pp , [6] Michel Mitov and Nathalie Dessaud, "Going beyond the reflectance limit of cholesteric liquid crystals," Nature Materials, vol. 5, pp , [7] A. C. Tasolamprou, M. Mitov, D. C. Zografopoulos, and E. E. Kriezis, "Theoretical and experimental studies of hyperrflective polymer-network cholesteric liquid crystal structures with helicity inversion," Optics Communications, vol. 282, pp. 78

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93 surface-attached polymer networks," Macromolecules, vol. 37, no. 3, pp , [32] Ryoichi Kishi, Masahiko Sisido, and Shigeo Tazuke, "Liquid-crystalline polymer gels: Anisotropic swelling of poly(γ-benzyl L-glutamate) gel cross-linked under a magnetic field," Macromolecules, vol. 23, no. 16, pp ,

94 APPENDIX A HEATING RATE STUDY RAW DATA Figure A 1: Notch Position During Heating and Cooling, 0.5 C/min For temperature ramps below 2 C/min, the heating and cooling curves closely resembled each other, as in Figure A 1. The tuning range achieved upon heating was approximately the same as the tuning range achieved upon cooling. The nematic-isotropic transition temperatures also occurred at nearly the same temperatures regardless of heating or cooling. 83

95 Figure A 2: Notch Position During Heating and Cooling, 4 C/min At faster heating/cooling rates, a clear discrepancy developed between the tuning curves. Although the tuning ranges were approximately the same for both heating and cooling cycles, the nematic-isotropic transition temperatures differed by several degrees. The tuning curves also became jagged in appearance as the change in notch position lagged behind the rapid change in temperature. The tuning curves for heating/cooling rates above 4 C/min are not pictured here due to their heavily distorted appearance. 84

96 APPENDIX B CURING INTENSITY STUDY RAW DATA Figures B 1 B 4 present the raw data for the curing intensities that resulted in the maximum and minimum amount of reflection notch tuning. Samples cured at 0.09 mw/cm 2 demonstrated the largest tuning range, while samples cured at and above 1.00 mw/cm 2 demonstrated the smallest tuning ranges. The tuning behavior for samples cured above 1.00 mw/cm 2 was very similar to the tuning behavior for samples cured at 1.00 mw/cm 2, so the data for 1.00 mw/cm 2 is representative of the higher curing intensities. Figure B 1: Transmission Spectra with Temperature, 0.09 mw/cm 2 Curing Intensity 85

97 Figure B 2: Transmission Spectra with Temperature, 1.00 mw/cm 2 Curing Intensity Figure B 3 depicts the tuning curves for samples cured at 0.09 and 1.00 mw/cm 2. Due to slight variations in sample preparation, the initial notch position varied between the samples. In order to examine the relative change in notch position exhibited by the different samples, the tuning data was normalized to the initial notch position, as shown in Figure B 4. 86

98 Figure B 3: Notch Position with Temperature, Representative Curing Intensities Figure B 4: Notch Shift with Temperature, Representative Curing Intensities 87

99 APPENDIX C CELL GEOMETRY STUDY RAW DATA C.1 Polymer Film Depth Profiles Representative dry and swollen polymer film depth profiles for the approximate smallest and largest cell thicknesses are provided in Figures C 1 C 4. These histogram plots were used to calculate the dry and swollen polymer film thicknesses. Figure C 1: Dry Polymer Film Thickness, 9.1 μm Cell. The reference glass is at a depth of -1.1 μm and the dry polymer film is at a depth of 0.4 μm, yielding a dry film thickness calculation of 1.5 μm. 88

100 Figure C 2: Swollen Polymer Film Thickness, 9.1 μm Cell. The reference glass is at a depth of -5.9 μm and the swollen polymer film is at a depth of 1.5 μm, yielding a swollen film thickness calculation of 7.4 μm. Figure C 3: Dry Polymer Film Thickness, 49.0 μm Cell. The reference glass is at a depth of -5.9 μm and the dry polymer film is at a depth of 2.1 μm, yielding a dry film thickness calculation of 8.0 μm. 89

101 Figure C 4: Swollen Polymer Film Thickness, 49.0 μm Cell. The reference glass is at a depth of μm and the swollen polymer film is at a depth of 5.4 μm, yielding a swollen film thickness calculation of 46.4 μm. 90

102 C.2 Cell Thickness Heating Cycles Representative tuning data for the smallest and largest cell thicknesses (~10 and 50 μm, respectively) is depicted in Figures C 5 C 8. Figure C 5: Transmission Spectra with Temperature, ~10 μm Cell 91

103 Figure C 6: Transmission Spectra with Temperature, ~50 μm Cell The transmission spectra clearly indicate that the ~10 μm cell blue-tuned to a shorter wavelength than the ~50 μm cell. However, the notch depth of the 10 μm cell decreased during blue-tuning while the notch depth of the 50 μm cell did not decrease during blue-tuning. Further investigation would be required to determine the exact cause of the notch depth decrease in the 10 μm cell. The notch depth of a CLC is generally impacted by the cell thickness, the liquid crystal birefringence, and the presence of any scattering [26]. The cell thickness determines the number of pitches of a certain wavelength that can exist within a cell. If the number of pitches in a cell falls below a minimum threshold, the intensity of the Bragg reflection will decrease [3]. The decrease in notch depth in the 10 μm cell could potentially be attributed to the relatively small number of pitches present within the thin cell. Figures C 7 and C 8 illustrate the tuning curves of the 10 and 50 μm cells. The 10 μm tuning curves are cut off above 78 C because accurate notch position measurements were not obtained above 78 C. The notch depth of the 10 μm cells decreased through 92

104 both the blue- and red-tuning regimes such that the notch became nearly indistinguishable at temperatures above ~78 C (refer to Figure C 5). Figure C 7: Notch Position with Temperature, Representative Cell Thicknesses Figure C 8: Notch Shift with Temperature, Representative Cell Thicknesses 93

105 C.3 Untethered Gel Raw Data Figures C 9 C 11 present the raw tuning data for the untethered chiral-structured polymer/lc system. As evidenced by the transmission spectra and tuning curves, the untethered system demonstrated the same characteristic thermal tuning behavior as the surface-tethered system. Figure C 9: Transmission Spectra with Temperature, Untethered Gel 94

106 Figure C 10: Notch Position with Temperature, Untethered Gel Figure C 11: Notch Shift with Temperature, Untethered Gel 95