Liquid Crystal Elastomer Films and their Potential in Sensing

1. Liquid Crystal Elastomer Films and their Potential in Sensing Sarah Hicks EARS-IGERT project Lake Lacawac Workshop 2010

2. Introduction • The project on creating liquid crystal polymer elastomers was started up after receiving the EARS-IGERT (Environment Aquatic Resource Sensing -Integrative Graduation Education and Research Training) program grant from NSF. • We hope to create a flexible film that can be patterned in a macro and micro-scale and can respond to various stimuli. There is a possibility that these films can be used in environmental sensing.

3. Liquid Crystal Elastomers • LCEs are flexible materials consisting of a liquid crystal material and polymer backbones connected by cross linkers1. • Just like liquid crystal fluids, LCEs are anisotropic1. In other words, physical or optical behavior along the LC director will differ from that perpendicular to the director n. • The director orientation can be achieved by either mechanical stretching 2,3 or by cross-linking4. backbone cross linker n

4. Previous Research: micro-actuators • Keller et al. were able to create a micro structured array of liquid crystal elastomer pillars that responded to changes in temperature5,6. • Used a soft lithography technique to create the LCE array7. • Reported a contraction of 30-40% for side chain LCE pillars5 ; 300-500% contraction for main-chain LCE pillars6. Side-chain LCE pillars contracting under temperature change1 Pillar diameter ~ 20 microns1

5. Materials A combination of: Bi-functional LC monomers Mono-functional LC monomers and add Photoinitiator hν and then But monomers are solids at RT! Solution: Add a component to induce miscibility. It can be a solvent and/or a liquid crystal fluid.

6. Soft lithography process for LC elastomer Clean substrate Pour liquid PDMS over substrate. Cure mold. Spin-coat photoresist (SU-8). Bake. Peel off PDMS mold Expose to UV light through photomask Stamp PDMS over elastomer. Cure with UV light5. Bake. Develop. Remove. Peel off mold

7. Process on making LCE film • Make a mixture of 85% silicone and 15% crosslinker and pour over silicone wafer etched with microstructures. Bake in oven at 65 °C for two hours to harden. Peel off PDMS. • Make LCE mixture consisting of liquid crystal and monomers. Dispense on glass substrate. Place PDMS mold on top. Expose to UV light for 30 minutes to initiate polymerization. Take off mold and rinse and dry LCE film.

8. PDMS mold

9. LCE film

10. Nematic LCE with pillars • Two different arrays of pillars were observed • Banana shaped • Circular shaped • Pillars were heated up from RT to 100 °C at a rate of 5 °C/min. Then samples were cooled at no specified rate. • Images were taken from a camera mounted on top of a polarizing microscope. Polarizers were crossed. • Pillar thickness was measured using software program ImageJ.

11. Banana shaped pillars

12. Circular pillars

13. Other types of LCE films • Have made other kinds of LCE films that can have microstructure arrays patterned into them. Thermochromic Reflective cholesteric

14. Conclusion • Progress has been made in creating these kinds of films. • Next we would like to see how these films will respond to temperature changes and under applied fields.

15. References 1. W. Gleim and H. Finklemann, Side Chain Liquid Crystalline Polymers, edited by C. B. McArdle, Glasgow: Blackie (1989). 2. T. Eckert, H. Finkelmann. Macromol. rapid Commum., 17 767 (1996). 3. K. Semmler, H. Finkelmann, Macromol. Chem. Phys., 196 3197 (1995). 4. R. Kohler, R. Stannarius, C. Tolksdorf, R. Zentel. Appl. Phys. A 80 381 (2005). 5. Buguin, A.; Li, M.H.; Silberzan, P.; Ladoux, B.; Keller, P. J. Am. Chem. Soc.2006, 128, 1088-1089. 6. Keller, P.; Yang, H., Buguin, A. 5th International Liquid Crystal Elastomer Conference, Kent State University, 2009. 7. Xia, Y., Whitesides, G.M. Angew. Chem. Int. Ed.1998, 37, 550-575.