Thermal Characterization of Nematic Liquid Crystal Elastomer

Transcription

1 Thermal Characterization of Nematic Liquid Crystal Elastomer Rita A. Gharde 1, Santosh A. Mani 2 1 Department of Physics, University of Mumbai, Mumbai, INDIA 2 K. J. Somaiya College of Engineering, Mumbai, INDIA ABSTRACT We have synthesized and characterized thermal properties of Nematic Liquid Crystal Elastomer (NLCE) using Polarizing Microscopy Studies (PMS), Differential Scanning Calorimetry (DSC) and Thermo gravimetric Analysis (TGA). The synthesis of NLCE was confirmed by Fourier Transform Infrared (FTIR) Spectroscopy. The NLCE used in the present study, has a unique coupling between the anisotropic order of Liquid crystal components and elasticity of polymer network. The investigations revealed that there is a spontaneous change of the NLCE at nematic isotropic transition, which gives a flexing effect, when the material is pinned on the substrate. This shows that this synthesized material can be used for large number of biological applications. Key Words: Nematic Liquid Crystal Elastomer (NLCE), Fourier Transform Infrared (FTIR) Spectroscopy, Differential Scanning Calorimetry (DSC), Thermo gravimetric Analysis (TGA) INTRODUCTION Liquid Crystals are condensed materials with a degree of order intermediate between anisotropy of crystalline solid and isotropy of a liquid phase. There are different liquid crystal phases depending on the degree of positional and/or orientational order like Nematic, Smectic and Cholesteric. Liquid Crystals offer broad range of applications in the field of Liquid Crystal Displays, Liquid Crystal Thermometer, Optical Imaging etc. (P. G. De Gennes; 1975). The advances in the polymer science have opened a new arena for active materials which has attracted significant interest from scientific community. These most promising materials, which combine the properties of liquid crystalline units and rubber elasticity of the cross-linked network, are Liquid Crystal Elastomers (LCEs) (R. Zental and G. Reckert; 1986, E. M. Terentjev; 1999). Liquid Crystal Elastomers (LCEs) have attracted considerable interest from both basic and applied science research points of views due to their anisotropic mechanical and thermal properties. These materials have several unusual properties like spontaneous; reversible shape change on heating, soft elasticity, mechanical instabilities and optical switching, etc. LCEs are weakly cross-linked elastic polymer networks that incorporate rigid, anisotropic units (termed mesogens) bounded to the Copyright 2012, Asian Business Consortium AJASE Page 114

2 polymer chains. The mesogens can exhibit spontaneous orientational ordering of the same type that is observed in ordinary low molar mass liquid crystals. However, since the mesogens are chemically bound to the cross-linked polymer network, any change in their orientation will be coupled to changes in the mechanical as well as thermal properties of the material. As the mesogens undergo a phase transition from the liquid crystalline state to the isotropic state, LCEs exhibit a significant change of their macroscopic shape due to the motion of the polymer network, which is coupled to the mesogens. The transition between the ordered and disordered states is dictated by the thermodynamic equilibrium; hence the shape change is fully reversible. The coupling between the liquid crystal chain and backbone is critical to thermostrictive behaviour of LCE material. (C. H. Legge, J. Davis and G. R. Mitchell; 1991, H. Finkelman, J. Kupfer; 1991, S. V. Fridrikh E. M. Terentjev; 1999).The concept of LCE was put forward by P.G. De Gennes in However, the first LCE was synthesized by Finkelmann et al in LCEs with thermo responsive, photo responsive and electro responsive functions have been developed. (A.R Tajbakhsh, E. M Terentjev; 2001, P. E. Cladis, P. Ehrhard., D.Riley; 2001, M. Bispo, Daniel Guillon, Bertrand Donnio, H. Finkelmann; 2008, P.M Hogan, A. R Tajbakhsh, E. M Terentjev; 2001). Depending on the phase exhibited by liquid crystal, LCEs are divided into nematic LCE, smectic LCE and others (P. E. Cladis, P. Ehrhard. D. Riley; 2008) Nematic LCEs (NLCEs) consist of pendent liquid crystal mesogens attached to a polymer network. They are soft, elastic materials that have wide range of applications. (M. Warner and E. M. Terentjev; 2003, M. Bispo, Daniel Guillon, Bertrand Donnio, H. Finkelmann; 2008). In recent years, their applications as micropumps, microvalves for microfluidic devices and opto-mechanical shutters have been developed. (Dukes, D.; Li, Y.; Lewis, S.; Benicewicz, B.; Schadler, L.; Kumar;2010, Biggins, J.S.; Warner, M.; Bhattacharya;2012, Jayaraman, A. J;2013). In the present study, we synthesized NLCE using Finkelmann procedure and thermal properties of this material were characterized by Polarizing Microscopy Studies (PMS), Differential Scanning Calorimetry (DSC) and Thermo gravimetric Analysis (TGA). Fourier Transform Infrared (FTIR) Spectroscopy is used for structural investigations and confirmation of NLCE synthesis EXPERIMENTAL Synthesis The samples of NLCE was made by preparing partially crosslinked films in a centrifuge, highly swollen in toluene (2-3 ml per 1 g of material), reacting for minutes before evaporating the solvent. A careful study of reaction kinetics ensured that approximately 50% of crosslinks were established in the first stage of this preparation. The orientation is then fixed by the subsequent second-stage reaction, when the remaining crosslinks are fully established. The structure of polymer, monomer and cross linking agent is shown in Figure 1. Copyright 2012, Asian Business Consortium AJASE Page 115

3 Polymer Monomer (Pure NLC) Cross linking Fig. 1.Synthesis of NLCE Characterization Techniques The thermal characterizations of synthesized NLCE were performed by Fourier Transform Infrared (FTIR) Spectroscopy, Differential Scanning Calorimetry (DSC) and Thermo gravimetric Analysis (TGA). The heating rate for DSC and TGA were kept at 100C/min. The heat flow for DSC was recorded upto temperature of 1600C, whereas weight for TGA was recorded upto temperature of 2000C. 3 RESULTS AND DISCUSSION FTIR Analysis FTIR is an effective analytical instrument for detecting functional groups and characterizing covalent bonding information. The spectrum produces a distinctive molecular fingerprint that can be used to scan samples for many different components. The spectra obtained from FTIR spectroscopy is shown in Figure 2. According to FTIR spectra, spectral peak obtained at 3445 cm-1 was due to some moisture in the sample (water), peaks in between 3445 and 2926 cm-1 was due to benzene C-H bond, peaks at 2926 and 2857 cm-1 was due to long chain of -CH2- group, peak at 1731 cm-1 was due to C=O group, peak at 1606 cm-1 was due to C=C group, peak 1509 cm-1 due to benzene (C=C), peak at 1465 cm-1 was C-H deforming,1256 cm-1 was due to Si-C bond, 1196 cm-1 was due to C-O (sample present in the group), 1069 cm-1 was due to C-O of benzene group, 1009 cm-1 was Si-O bonding, peak at 763 cm-1 also confirms benzene in the sample. This spectrum confirms the synthesis of NLCE. Fig.2. FTIR of NLCE Copyright 2012, Asian Business Consortium AJASE Page 116

4 Polarizing Microscopy Studies PMS is the most widely used method to identify different phases. The synthesized material of NLCE under investigation was studied at crossed polarized light for various temperatures of heating and cooling cycles. The textures obtained by PMS are shown in Figure 3. In these textures molecular alignment and defects are seen at different temperatures. It is found that the NLCE material possesses nematic phase upto a temperature of 76 0 C followed by isotropic phase. The nematic liquid crystal (monomer), used in synthesis of NLCE, possesses nematic phase upto 58 0 C followed by isotropic phase. DSC Analysis DSC is a thermo analytical technique in which the difference in the amount of heat required to increase the temperature of sample and reference are measured as a function of temperature. Both sample and reference are maintained at nearly the same temperature throughout. When they undergoes phase transition heat flows to it than to the reference to maintain both at the same temperature. The amount of heat flow into the sample depends on whether the process is exothermic or endothermic. DSC graph of NLCE for heating and cooling cycles is shown in Figure 4. The heat flow (in mw) was recorded upto temperature of C at a heating rate of 10 0 C/min. The value of Cp value was found to be 0.041J/g 0 C at C. Copyright 2012, Asian Business Consortium AJASE Page 117

5 TGA Analysis Thermo gravimetric analysis (TGA) is an analytical technique used to determine a material s thermal stability and its fraction of volatile components by monitoring the weight change that occurs as a specimen is heated. The weight is recorded as a function of increasing temperature. The TGA graph of NLCE is shown in Figure 5. The weight was recorded upto temperature of C at a rate of 10 o C/min. It is found that weight remains same upto temperature of 60 o C and then decreases linearly and becomes minimum at 90 o C. The weight increases linearly for further increase in temperature and becomes constant at 120 o C.The weight remains same for further increase in temperatures. This is due to spontaneous change of the NLCE. CONCLUSIONS The FTIR spectroscopy confirms the syntheses of NLCE. The thermal characterizations performed by DSC and TGA shows there is a spontaneous change of the NLCE due to change of rotation of the liquid crystalline side chains. This shows that this is a unique class of soft material which can be used for wide range of biological applications like artificial muscles. This behavior has a great practical relevance and we are continuing our investigations to understand many other aspects of this soft material. ACKNOWLEDGEMENTS We would like to acknowledge the help and encouragement given to us by Dr. Anuradha Misra, Professor and Head, Department of Physics, University of Mumbai. We are also grateful to Dr. Shubha Pandit, Principal, K. J. Somaiya College of Engineering, Mumbai for her help and motivation REFERENCES [1] P. G. De Gennes, The Physics of Liquid Crystals: Oxford University Press, (1975). [2] R. Zental and G. Reckert, Macromol. Chem. 187, 1915 (1986). [3] E. M. Terentjev, J. Phys. Condens. Matter 11 p (1999) [4] C. H. Legge, J. Davis and G. R. Mitchell, J. Physique, 21, 1253 (1991) [5] H. Finkelman, J. Kupfer, Makromol. Chem. Rapid Commun. 12, (1991) [6] S. V. Fridrikh, E. M. Terentjev, Phys. Rev. E, 60, 1847 (1999). [7] A. R Tajbakhsh, E. M Terentjev, Eur. Phys. J. E. 6, (2001). [8] P. E. Cladis, P. Ehrhard., D. Riley, Proc. Euro Mech,. 408, Kluewer Accademic, pg. 123, (2001). [9] M. Bispo, Daniel Guillon, Bertrand Donnio, H. Finkelmann, Macromolecules. 41, (2008) [10] P.M Hogan, A. R Tajbakhsh, E. M Terentjev, Cond. Mat. (2001) [11] P. E. Cladis, P. Ehrhard., D. Riley, Proc. Euro Mech,.408, Kluewer Accademic, pg. 123, (2001). [12] M. Warner and E. M. Terentjev, Liquid Crystal Elastomers, Clarendon Press Oxford, (2003). [13] M. Bispo, Daniel Guillon, Bertrand Donnio, H. Finkelmann, Macromolecules. 41, (2008) [14] Dukes, D.; Li, Y.; Lewis, S.; Benicewicz, B.; Schadler, L.; Kumar, S.K. Macromolecules 43, (2010). [15] Biggins, J.S.; Warner, M.; Bhattacharya, K. Elasticity of polydomain liquid crystal elastomers. J.Mech. Phys. Solids 60, (2012) [16] Jayaraman, A. J. Polym. Sci., Part B: Polym. Phys. 51, (2013) Copyright 2012, Asian Business Consortium AJASE Page 118

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