CHAPTER 2 LIQUID CRYSTAL CHARACTERIZATION TECHNIQUES

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1 25 CHAPTER 2 LIQUID CRYSTAL CHARACTERIZATION TECHNIQUES Liquid crystalline mesogens are synthesized and characterized by various experimental methods such as spectral, thermal and electrical techniques. Spectral technique comprising of Fourier Transform Infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance Spectroscopy (NMR) which explores the existence of hydrogen bond in liquid crystal mesogen. Thermal characterization method includes the Polarizing Optical Microscope (POM) associated with sensitive temperature controller and Differential Scanning Calorimetry (DSC) which examines the existence mesophase and its textures, melt and clearing point of liquid crystalline material, crystallization kinetics, thermal span of the phases and its transition temperatures along with the enthalpy values. Electrical technique includes impedance analyzer which is employed to study the dielectric spectra of liquid crystalline material, relaxation frequency at a specific mesophase and crystallization kinetics of various materials. 2.1 INTRODUCTION Major equipments which have been employed to comprehend the physical characterization of liquid crystal materials are listed in table 2.1 and described in detail. Fourier Transform Infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance spectroscopy (NMR) are used to identify the existence of hydrogen bond and structural configuration of thermotropic liquid crystals.

2 26 POM associated with sensitive temperature controller has been employed to investigate the textural information and its thermal span of the liquid crystalline phases. Differential Scanning Calorimeter was employed to investigate the phase transition temperatures of mesophases, enthalpy values and order of the transitions (Patrick Navakd and Robert Cox 1984) respectively. Further DSC gives valuable information about the crystallization kinetics of the mesogen under investigation. Electrical characterization of liquid crystal is effectively done by impedance analyzer which renders the substantial information about the dielectric spectra of the liquid crystal sample and relaxation frequency at a particular mesophase. Further this electrical characterization is extremely powerful tool to elicit crystallization kinetics study in various compounds. Table 2.1 Instruments details Instruments Model and Specification Make Observation FTIR- ABB Bomen MB3000 Burlington, Canada Confirmation of Hydrogen bonding NMR Ultra Shield 300MHz Burker, USA Structural confirmation of molecules POM E600WPOL Tokyo, Japan Textural observation, optical tilt angle measurement Nikon CCD System DS-U1 Tokyo, Japan Record the textural image recording Instec Hot and Cold Stage HCS 402 STC 200 Instec, USA Temperature monitoring and control DSC DSC60, Shimadzu Tokyo, Japan Evaluation of transtion temperatures and enthalpy Impedance Analyzer LF4192A Agilent, USA Dielectric permittivity measurement and Relaxation studies XRD Rigaku Ultrax 18 HR6-112 HR6 112, Germany Structural determination

3 SALIENT FEATURE OF FTIR The ABB MB3000 is the most reliable FTIR laboratory analyzer available in the market combined with the intuitive Horizon MB TM FTIR software. The MB3000 will facilitate acquisition, processing and analysis of samples. Easy to use and maintenance free, it provides constant analysis results for years to come. The salient features of the FTIR laboratory analyzer are depicted below. a) Spectral range 485 to 8,500 cm -1 b) Resolution better than 0.7 cm -1 c) Maximum signal-to-noise ratio (root-mean-square, 60s, 4 cm - 1, at peak response): 50,000: 1 d) Signal sampling: 24-bit ADC e) Short-term stability: < 0.09 % f) Temperature stability: < 1 % / C g) Frequency repeatability (at 1918 cm -1 ): < cm -1 h) Frequency accuracy (at 1918 cm -1 ): < 0.06 cm Working Principle of FTIR Analyzer The basic components of an FTIR block diagram are shown schematically in Figure 2.1. The infrared source emits a broad band of different wavelength of infrared radiation. The IR source used in the Temet GASMET FTIR CR-series is a Silicon Carbide ceramic at a temperature of 1550 K. The IR radiation passes through an interferometer that modulates the infrared radiation. The interferometer performs an optical inverse Fourier transform on the entering IR radiation. The modulated IR beam passes through the sample (solid/liquid) where it is absorbed to various extents at

4 28 different wavelengths by the various molecules present. The sample is prepared as a circular pellet with help of KBr press (Model M-15, Techno Search Instruments). Finally, the intensity of the IR beam is detected by a detector, which is a liquid-nitrogen cooled MCT (Mercury-Cadmium- Telluride) detector. The detected signal is digitized and Fourier transformed by the computer to get the IR spectrum of the sample (solid/gas). IR Source Interferometer Sample Detector Data Processing and control Figure 2.1 Block diagram of FTIR The unique part of an FTIR spectrometer is the interferometer. Infrared radiation from the source is collected and collimated (made parallel) before it strikes the beam splitter. The beam splitter ideally transmits one half of the radiation, and reflects the other half. Both transmitted and reflected beams strike mirrors, which reflect the two beams back to the beam splitter. Thus, one half of the infrared radiation that finally goes to the sample (solid/gas) has first been reflected from the beam splitter to the moving mirror, and then back to the beam splitter. The other half of the infrared radiation going to the sample has first gone through the beam splitter and then reflected from the fixed mirror back to the beam splitter. When these two optical paths are reunited, interference occurs at the beam splitter because of the optical path difference caused by the scanning of the moving mirror. The optical path length difference between the two optical paths of a Michelson interferometer is two times the displacement of the moving mirror. The interference signal measured by the detector as a function of the optical path length difference is called the interferogram. A typical interferogram produced by the interferometer is nothing but the intensity of the infrared radiation as a function of the displacement of the moving mirror. At the peak position, the optical path length is exactly the same for the radiation that

5 29 comes from the moving mirror as it is for the radiation that comes from the fixed mirror. The spectrum can be computed from the interferogram by performing a Fourier transform. The Fourier transform is performed by the same computer that ultimately performs the quantitative analysis of the spectrum. The degree of absorption of infrared radiation at each wavelength is quantitatively related to the number of absorbing molecules in the sample (solid/gas). Since there is a linear relationship between the absorbance and the number of absorbing molecules, multi component quantitative analysis of solid/gas mixtures is feasible. 2.3 NMR SPECTROSCOPY Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful and theoretically complex analytical tool. Note worthy feature of NMR is that the experiments are being performed on the nuclei of atoms rather than the electrons. The chemical environment of specific nuclei is deduced from information obtained about the nuclei. The proposed structure of liquid crystalline complexes has been verified by 1 H NMR studies. The complexes are analyzed using Nuclear Magnetic Resonance Spectroscopy (NMR) Brucker international model ULTRA SHIELD of 300MHz. NMR spectrum of the complex is recorded in CDCl 3 with TMS as the internal standard Working Principle of NMR Spectrometer The nuclei of all elements carry a charge. When the spins of the protons and neutrons comprising these nuclei are not paired, the overall spin of the charged nucleus generates a magnetic dipole along the spin axis, and the intrinsic magnitude of this dipole is a fundamental nuclear property called the nuclear magnetic moment, µ. The symmetry of the charge distribution in

6 30 the nucleus is a function of its internal structure and if this is spherical (ie analogous to the symmetry of a 1s hydrogen orbital), it is said to have a corresponding spin angular momentum number of I=1/2, of which examples are 1 H, 13 C, 15 N, 19 F, 31 P etc Chemical Shift The magnetic field at the nucleus is not equal to the applied magnetic field; electrons around the nucleus shield it from the applied field. The difference between the applied magnetic field and the field at the nucleus is termed the nuclear shielding. In proton ( 1 H) NMR, p-orbitals play no part so a small range of chemical shift (10 ppm) is observed. We can easily see the effect of s-electrons on the chemical shift by looking at substituted methane, CH 3 X. As X becomes increasingly electronegative, so the electron density around the protons decreases, and they resonate at lower field strengths (increasing d H values). Chemical shift is defined as nuclear shielding / applied magnetic field. Chemical shift is a function of the nucleus and its environment. It is measured relative to a reference compound. For 1 H NMR, the reference is usually tetramethyl silane, Si (CH 3 ) WORKING PRINCIPLE OF POLARIZING OPTICAL MICROSCOPE Nikon polarizing microscope (Eclipse E600W POL) is a versatile instrument used for detection of optical textures and tilt angle of the molecules in liquid crystal compounds. This microscope works on the principle of polarization of the light. The monochromatic un-polarized light from lamp-house of the instrument is made to pass through a filter and crossed polarizer and analyzer. Since the liquid crystal is anisotropic media which rotates the plane of polarization of the incident light, when it is placed between two crossed polarizers,

7 31 The polarizing microscope consist of an objective whose resolutions can be chosen from 5x, 10x, 20x and 50x respectively, while the eyepiece resolution is fixed at 10x. Thus desired magnification can be achieved by changing suitable objective piece. The detailed microscope diagram is illustrated in Figure 2.2 where in component and operational parts of Nikon microscope are detailed. The microscope is equipped with a digital camera which constitute of a camera control unit (DS-U1), camera head (DS-5M). The former is connected to a computer via a USB interface while the latter is mounted on the polarizing microscope. Camera control unit (DS-U1) can perform the following operations: Figure 2.2 Component and operational parts of Nikon Polarizing microscope

8 32 a) Displays images captured by the microscope on the computer monitor. Images can be saved in many formats such as BMP, TIFF, JPEG etc. b) It stores the images to a specified location, retrieves and analyzes them at a convenient time of the operator. c) Further, it displays and controls the operating status of the microscope. d) The capture size of the image depending on the pixels is classified into four categories namely i) fine (2560 x 1920), ii) fine (1280 x 9600), iii) normal (1280 x 960) and iv) normal (640 x480) respectively. The exposure mode of the camera depends on the use. Program mode is designed for the general use, speed mode is used when the exposure time is fixed at a speed between 1/1000 sec and 1 sec. Manual mode allows manual setting of exposure time in the range from 1/1000 sec to 60 sec. Focus mode is designed for situations involving dark subjects, where slow image updating can cause focusing problems. 2.5 TEMPERATURE CONTROLLER AND ITS ACCESSORIES The Instec STC200 series of precision temperature controllers are designed to provide the best possible control of heating and cooling instruments, by integrating the STC200 with specifically selected control components such as HCS40 2 heating and cooling stage to accomplish the temperature resolution of ± C. The Instec hot stage use 100 Platinum RTD`s as temperature sensor elements. The STC200 comes equipped with an RS 232 serial communication port for host computer control. A specific windows based host computer software program for the STC200 called

9 33 WinTemp is provided. This software program is capable of enabling and disabling the temperature and monitors the temperature rate and percentage of power transferred and many other commands Figure 2.3 shows STC200 standalone temperature controller can be used in standalone mode or remote mode with the Wintemp programme to adjust sample temperatures for observations and measurements. Figure 2.3 HCS402 heating and cooling stage Instec Heating and Cooling Stage HCS402 heating and cooling stage is equipped with dual thin film heater, one is located underneath the sample chamber and the other is located at the part of the cover, above the sample chamber as depicted in Figure 2.3. This configuration provides a thermally uniform sample oven HCS402 which is provided with gas and liquid purge ports. When cooling below ambient temperatures, an argon gas is used to prevent water vapor from condensing on the windows as the inner chamber cools.

10 34 Polarizing Optical Microscope associated with sensitive temperature controller is employed to record the liquid crystalline textures with 5 mega pixels and pixel resolution along with various phase transition temperatures of mesophases that exist in a sample. 2.6 DESCRIPTION OF LIQUID CRYSTAL CELL Commercially available liquid crystal cells of various thicknesses such as 4 m, 10 m and 15 m are imported from Instec Company, USA. Instec empty cells are manufactured in clean rooms. The adhesive can withstand temperatures up to 200 C. All cells have a patterned ITO area, 5mm X 5mm, with ITO resistance of 100 ohms per sq. alignment layers are of 1 to 3 pre-tilted angles. Physical dimensions and spacing of cells vary depending on the models. Cell gap tolerance is ±0.2 µm. 1 to 3 pre-tilted angles Calibration of Liquid Crystal Cells To establish the electrical contact with the cell, silver leads are drawn from the glass electrodes of the cell and sliver paste is used as adhesive. The empty liquid crystal (LC) cell is calibrated against temperature in the range of 25 o C to 150 o C and with the frequency in the range of 100Hz to 1MHz respectively. Empty LC cells which show negligible change with thermal and frequency variation are chosen for the parameter measurement The capacitance of the electrodes and the silver wires are found when the cell is filled with a compound (benzene) whose dielectric constant is known. Thus the leads capacitance of the cell can be estimated. Hence the calibrated cell which is filled with the sample is effectively been used for different analysis such as textural observation, tilt angle measurement and dielectric studies respectively.

11 WORKING PRINCIPLE OF DSC The Shimadzu DSC-60 is a heat flux type differential scanning calorimeter. The block diagram of DSC-60 is shown as Figure 2.4. As power is supplied to the heater, heater block temperature Tb is detected and controlled so that it rises or falls according to the program. Heats Q s and Q r flow per unit time from the block through heat resistance R and to the sample section respectively, so that respect temperature T s and T r rise or fall after the programmed temperature T b. At this time, the sample section temperature from the reference material section temperature T= T s -T r is detected. A substance ordinarily selected as a reference material is thermally inert in the temperature range order study. When the sample is also thermally inert, its temperature is stabilized by a value with appropriate difference from the reference material temperature due to the heat capacity difference there between, after the initial transient period of temperature rise or drop. The curve obtained by plotting T signals with respect to time or the sample section temperature T s is called the baseline. When compound melts, its temperature remains constant where as the reference material temperature increases continuously. As a result T will deviate from the value obtained before the onset of melting. When melting completes, since T s is largely different from T b compared with T r, a great amount of heat flows from the heater block to the sample section, so that the equilibrium state is resumed quickly or in other words, T returns to the baseline.

12 36 Figure 2.4 Block diagram of DSC Accordingly, the DSC curve plotting T signals with respect to time or the sample section temperature is a succession of the baseline for the equilibrium state, the peak obtained during melting, and the baseline. Theoretically the peak area (the area between the DSC curve and the extrapolated baseline during melting and the following transient state) is approved to be proportional to the heat energy (heat of fusion x sample quantity) required for melting. Under conditions of constant pressure, the heat energy supplied to a sample generally conforms to the increase in enthalpy of the sample. Therefore, in DSC in which measurement is made under one atmosphere of pressure, primary phase transitions such as melting and crystallization or the like phenomenon accompanied by a discontinuous change of enthalpy (with respect to temperature) are rendered as a peak whose area corresponds to the changes in enthalpy. The DSC-60 uses a Chromel-alumel thermocouple for detection of T b and T s and Chromel- Constantan differential thermocouple for detection of T.

13 37 In this present work DSC has been effectively emphasized to identify transition temperatures of various liquid crystalline phases, its corresponding enthalpy values and height of enthalpy peaks at various scan rates which are essential to measure an order parameter of a molecule in a mesophase respectively. 2.8 FUNCTIONALITIES OF AGILENT IMPEDANCE ANALYSER 4192A Figure 2.5 shows an Agilent 4192A impedance analyzer performs both network analysis and impedance analysis on devices such as telecommunication filters audio, video electronic circuits, and basic electronic components respectively. This instrument can be employed to test both floating and grounded devices. The impedance analyzer interfaced with computer with the aid of an automated setup comprising of GPIB module (NI ) through HP-IB and LabVIEW platform. This instrument is successfully employed to study the dielectric permittivity variation and relaxation of molecule at various frequencies in the range between 5Hz to 13 MHz. Figure 2.5 Block diagram of Impedance analyzer

14 X RAY- DIFFRACTION STUDY The X-Ray diffraction studies are conducted with CuK Radiation with wave length = 1.54 A o using rotating anode generator (Rigaku Ultrax 18) and an image plate (Marresearch). A sample which is to be analyzed is taken in a thin walled glass capillary tube of 5mm diameter (Hampton Research, Model HR6-112). A magnetic field of 2.3 k gauss is applied in a direction perpendicular to incident X -Ray beam to pole the molecule in a sample. The sample is heated to isotropic temperature and subjected to controlled cooling in the presence of magnetic field. The X- ray diffraction pattern of oriented sample and the intensity of scattered X-ray are measured at different temperature and recorded. Detail discussion pertaining to XRD measurements are given in Chapter 8.