Liquid Crystal Liquid Crystal Elastomer Films and Elastomer Films and

their Potential in their Potential in SensingSensingSarah HicksSarah Hicks

EARS-IGERT projectEARS-IGERT project

Lake Lacawac Workshop Lake Lacawac Workshop 20102010

IntroductionIntroduction

The project on creating liquid crystal The project on creating liquid crystal polymer elastomers was started up after polymer elastomers was started up after receiving the EARS-IGERT (Environment receiving the EARS-IGERT (Environment Aquatic Resource Sensing -Integrative Aquatic Resource Sensing -Integrative Graduation Education and Research Graduation Education and Research Training) program grant from NSF.Training) program grant from NSF.

We hope to create a flexible film that can be We hope to create a flexible film that can be patterned in a macro and micro-scale and patterned in a macro and micro-scale and can respond to various stimuli. There is a can respond to various stimuli. There is a possibility that these films can be used in possibility that these films can be used in environmental sensing.environmental sensing.

Liquid Crystal Liquid Crystal ElastomersElastomers

LCEs are flexible materials consisting of a liquid LCEs are flexible materials consisting of a liquid crystal material and polymer backbones connected crystal material and polymer backbones connected by cross linkersby cross linkers11..

Just like liquid crystal fluids, LCEs are anisotropicJust like liquid crystal fluids, LCEs are anisotropic11. . In other words, physical or optical behavior along In other words, physical or optical behavior along the LC director will differ from that perpendicular the LC director will differ from that perpendicular to the director to the director nn..

The director orientation can be achieved by either The director orientation can be achieved by either mechanical stretching mechanical stretching 2,32,3 or by cross-linking or by cross-linking44..

n

backbone

cross linker

Previous Research: micro-Previous Research: micro-actuatorsactuators

Keller et al. were able to create a micro structured array of Keller et al. were able to create a micro structured array of liquid crystal elastomer pillars that responded to changes liquid crystal elastomer pillars that responded to changes in temperaturein temperature5,65,6..

Used a soft lithography technique to create the LCE Used a soft lithography technique to create the LCE arrayarray77..

Reported a contraction of 30-40% for side chain LCE Reported a contraction of 30-40% for side chain LCE pillarspillars55 ; 300-500% contraction for main-chain LCE pillars ; 300-500% contraction for main-chain LCE pillars66..

Pillar diameter ~ 20 microns1

Side-chain LCE pillars contracting under temperature change1

MaterialsMaterialsA combination of:

But monomers are solids at RT!

Solution: Add a component to induce miscibility. It can be a solvent and/or a liquid crystal fluid.

Mono-functional LC monomers

Bi-functional LC

monomers

and

add

Photoinitiator and then hν

Soft lithography process for LC Soft lithography process for LC elastomerelastomer

Clean substrate

Spin-coat photoresist (SU-8). Bake.

Expose to UV light through photomask

Bake. Develop.

Remove.

Pour liquid PDMS over substrate. Cure mold.

Peel off PDMS mold

Stamp PDMS over elastomer. Cure with UV light5.

Peel off mold

Process on making LCE Process on making LCE filmfilm

Make a mixture of 85% silicone and 15% Make a mixture of 85% silicone and 15% crosslinker and pour over silicone wafer crosslinker and pour over silicone wafer etched with microstructures. Bake in etched with microstructures. Bake in oven at 65 °C for two hours to harden. oven at 65 °C for two hours to harden. Peel off PDMS.Peel off PDMS.

Make LCE mixture consisting of liquid Make LCE mixture consisting of liquid crystal and monomers. Dispense on glass crystal and monomers. Dispense on glass substrate. Place PDMS mold on top. substrate. Place PDMS mold on top. Expose to UV light for 30 minutes to Expose to UV light for 30 minutes to initiate polymerization. Take off mold initiate polymerization. Take off mold and rinse and dry LCE film.and rinse and dry LCE film.

PDMS moldPDMS mold

LCE filmLCE film

Nematic LCE with pillarsNematic LCE with pillars

Two different arrays of pillars were observedTwo different arrays of pillars were observed Banana shapedBanana shaped Circular shapedCircular shaped

Pillars were heated up from RT to 100 °C at a Pillars were heated up from RT to 100 °C at a rate of 5 °C/min. Then samples were cooled at rate of 5 °C/min. Then samples were cooled at no specified rate.no specified rate.

Images were taken from a camera mounted on Images were taken from a camera mounted on top of a polarizing microscope. Polarizers were top of a polarizing microscope. Polarizers were crossed.crossed.

Pillar thickness was measured using software Pillar thickness was measured using software program ImageJ.program ImageJ.

Banana shaped pillarsBanana shaped pillars

Banana pillar thickness over temperature

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

20 30 40 50 60 70 80 90 100

T (Celsius)T

hic

kn

es

s (

mic

ron

s)

heating

cooling

Circular pillarsCircular pillars

Circular pillar thickness over temperature

0

2

4

6

8

10

12

14

16

18

20

20 30 40 50 60 70 80 90 100

T (Celsius)T

hic

kn

es

s (

mic

ron

s)

heating

cooling

Other types of LCE films Other types of LCE films

Have made other kinds of LCE films Have made other kinds of LCE films that can have microstructure arrays that can have microstructure arrays patterned into them.patterned into them.

Thermochromic Reflective cholesteric

ConclusionConclusion

Progress has been made in creating Progress has been made in creating these kinds of films.these kinds of films.

Next we would like to see how these Next we would like to see how these films will respond to temperature films will respond to temperature changes and under applied fields.changes and under applied fields.

ReferencesReferences

1. W. Gleim and H. Finklemann, 1. W. Gleim and H. Finklemann, Side Chain Liquid Side Chain Liquid Crystalline PolymersCrystalline Polymers, edited by C. B. McArdle, Glasgow: , edited by C. B. McArdle, Glasgow: Blackie (1989).Blackie (1989).

2. T. Eckert, H. Finkelmann. 2. T. Eckert, H. Finkelmann. Macromol. rapid Commum.Macromol. rapid Commum., , 1717 767 (1996). 767 (1996).

3. K. Semmler, H. Finkelmann, 3. K. Semmler, H. Finkelmann, Macromol. Chem. Phys.Macromol. Chem. Phys., , 196196 3197 (1995). 3197 (1995).

4. R. Kohler, R. Stannarius, C. Tolksdorf, R. Zentel. 4. R. Kohler, R. Stannarius, C. Tolksdorf, R. Zentel. Appl. Appl. Phys. A Phys. A 8080 381 (2005). 381 (2005).

5. Buguin, A.; Li, M.H.; Silberzan, P.; Ladoux, B.; Keller, 5. Buguin, A.; Li, M.H.; Silberzan, P.; Ladoux, B.; Keller, P. P. J. Am. Chem. Soc.J. Am. Chem. Soc. 20062006, 128, 1088-1089., 128, 1088-1089.

6. Keller, P.; Yang, H., Buguin, A. 6. Keller, P.; Yang, H., Buguin, A. 55thth International Liquid International Liquid Crystal Elastomer ConferenceCrystal Elastomer Conference, Kent State University, , Kent State University, 20092009..

7. Xia, Y., Whitesides, G.M. 7. Xia, Y., Whitesides, G.M. Angew. Chem. Int. Ed.Angew. Chem. Int. Ed. 19981998, , 37, 550-575.37, 550-575.