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Photopolymerized Liquid Crystal Gels

Module by: Yichen Lu, Aditya Agrawal, Rafael Verduzco. E-mail the authorsEdited By: Yichen Lu, Aditya Agrawal

Summary: Liquid crystals (LCs) are a state of matter that posses the qualities of both conventional liquid and solid phases. They have long range positional or orientational order as in a crystal while showing liquid mobility. Molecules that exhibit liquid crystal phases are known as mesogens1. Liquid crystal elastomers (LCEs), also known as solid liquid crystals, are made up of rubbery liquid crystal polymers. LCEs can reversibly change shape in response to temperature changes, electric fields, and even light2. This report will describe how to prepare a thin ( ~ 25 micrometer) liquid crystal gel by photopolymerizing a reactive mesogens. The long-term goal is to build responsive LC nanostructures using a similar technique on top of a micro- or nano-patterned substrate. Practical information regarding sample preparation and characterization will be provided.

Photopolymerized Liquid Crystal Gels

Yichen Li, Aditya Agrawal, Rafael Verduzco 6100 Main Street, MS-362, Department of Chemical and Biomolecular Engineering Rice University Houston, TX 77005 the following report is the result of a 2010 summer research project.

Introduction

Liquid crystals (LCs) are a state of matter that posses the qualities of both conventional liquid and solid phases. They have long range positional or orientational order as in a crystal while showing liquid mobility. Molecules that exhibit liquid crystal phases are known as mesogens1. Liquid crystal elastomers (LCEs), also known as solid liquid crystals, are made up of rubbery liquid crystal polymers. LCEs can reversibly change shape in response to temperature changes, electric fields, and even light2. This report will describe how to prepare a thin ( ~ 25 μm) liquid crystal gel by photopolymerizing a reactive mesogens. The long-term goal is to build responsive LC nanostructures using a similar technique on top of a micro- or nano-patterned substrate. Practical information regarding sample preparation and characterization will be provided.

Experimental

Materials

4-cyano-4’-pentyl biphenyl (5CB) and 4-cyano-4’-hydroxybiphenyl were purchased from TCI America. bis(cyclopentadienyl)bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium (Irgacure-784) was purchased from BASF. Potassium carbonate and benzene were purchased from EMD. Poly-vinyl alcohol (98-99% hydrolyzed), 1, 6-hexanediol diacrylate (80%), acrolyl chloride (97%), triethylamine (99%) and magnesium sulfate were purchased from sigma Aldrich. All the chemicals were used as received unless otherwise stated. The synthesis of mesogenic monoacrylate monomer (A6OCB) is mentioned in the synthesis section.

Synthesis of reactive mesogen

Figure 1: Reaction scheme for the synthesis of A6OCB in two steps. Step 1 and Step 2 are shown at the top and bottom of the figure, respectively.
Figure 1 (graphics1.jpg)

Steps for the synthesis of A6OCB:

Step 1

  1. Dissolve 4.96g 4-hydroxy-4`-cyanobiphenyl in 25ml DMF.
  2. Add 4.91g 6-bromo-1-hexanol and 6.58g K2CO3 and let the reaction continue for 20hrs at 90 oC. Verification for the completion of the reaction is done by Thin layer chromatography.
  3. Quench the reaction by adding water to dissolve excess K2CO3 and precipitate out the desired product. The product was dried under vacuum and recrystallized in benzene to obtain 4-(1-hexanol oxy)-4`-cyanobiphenyl .

Step 2

  1. In a vacuum dried flask dissolve 4g 4-(1-hexanol oxy)-4`-cyanobiphenyl in 80ml benzene followed by the addition of 3.76ml triethylamine via syringe.
  2. Dropwise add 2.185ml acrolyl chloride to the flask and the let the reaction for 1hr. Verification for the completion of the reaction is done by Thin layer chromatography.
  3. Add 1M HCl solution to quench the reaction. And via liquid-liquid separation collect the product.
  4. Using roto-vaporization distill out the benzene and recrystallize the product in ethanol to increase impurity.

Yield (%) = actual yield/ theoretical yield*100 =1.85/4.73*100=39.1%

Purity of the final product was confirmed by 1H-NMR taken by Bruker 400 MHz NMR Spectrometer (see appendix).

Sample cell preparation

To prepare a sample cell with controlled thickness, we used microscope slides along with glass cover slips. First, we cut the glass slides into 1 inch by 1 inch pieces. Then we placed 3 narrow glass cover slips along the edges of glass slide as boundaries to hold the solution from flowing through. The glass cover slips were not the only choice of the spacers. We tried with Teflon tape, normal tape and glass slides. They also worked well. As long as the spacer itself does not interfere with the reaction and matches your desired thickness of the film, the choice is relatively loose. The optimal set we applied was as follows: one glass slide at the bottom, three pieces of glass cover slips as spacers, and another glass slide on top. Three layers were glued together using epoxy. We left a slit from where the solution was added.

Photopolymerization

The monomer AO6CB was mixed with the solvent 5CB (weight ratio 1:1). AO6CB was solid at room temperature so we heated the mixture to about 60oC to make it completely liquid. The cross linker acrolyl chloride (7 mol % of the monomer) and photointiator Irgacure-784 (0.5 mol % of the monomer) was then added to the mixture. Then we added the liquid mixture to the sample cell via capillary tubes. Photopolymerization was performed by the irradiation using Newport 67005 Arc Lamp Housing and 74125 Oriel Cornerstone 260 1/4 m Monochromator to select an emission wavelength of 526 nm at 333k (see figure-2 for photopolymerization setup). The sample cell gap was 25 µm and the irradiation time was 30 minutes. During irradiation the sample cell temperature was controlled using heating stage.

Figure 2: Setup for the Photopolymerization of A60CB.
Figure 2 (Picture 9.jpg)

Aligned LC Gels

Synthesis of uniformly aligned liquid crystal (LC) molecules is of critical importance for applications. Here we have used rubbing technique to produce planar aligned liquid crystal gels. So for preparing planar aligned gels, glass slides were first coated with 10 wt% aqueous poly-vinyl alcohol (PVA) solution via WS-400BZ-6NPP (lite) spin coater. Coated glass slides were air dried for 3hrs to remove the solvent followed by thermally annealing in vacuum oven at 493K to get rid of remaining residual solvent. PVA coated glass slides were then rubbed with stripe-less tissue paper in the unidirectional way. These glass slides were used for sample cell preparation. Schematic of the whole process is shown in figure 3.

Figure 3: Schematic of the poly-vinyl alcohol (PVA) coating on glass slides for aligned liquid crystals.
Figure 3 (graphics2.jpg)

Results

A variety of methods are available for producing liquid crystal elastomers and gels3, but here we focused on a photopolymerization method due to its versatility for producing aligned or unaligned liquid crystal gels on a variety of substrates4-5. Reactive mesogenic monoacrylate monomer (A6OCB) was synthesized in two simple steps (see section 2.3).

Photopolymerization was carried out by mixing the reactive mesogen (A6OCB), a liquid crystal solvent (1:1 by weight ratio), crosslinker (7 mol% feed) and a photoinitiator (0.5 mol% feed). These were dissolved and loaded into a glass cell with controlled thickness (see section 2.3 for details). Photopolymerization was carried out by irradiating the sample with a Arc lamp. (see section 2.4 for details). Separating the LC gel from the glass cell after photopolymerization was achieved by first swelling the membrane in dichloromethane for several days and then deswelling it by increasing the methanol content.

Swelling/Deswelling experiments

After we completed the photopolymerization, we obtain the yellowish LCE gel coated on a glass surface. We then proceeded with a further experiment: exploring the temperature effect on the gel. We covered the polymer film with a layer of the solvent 5CB and started heating from room temperature. Figure-4 shows transition of the liquid crystal gel from nematic to isotropic in 5CB solvent as temperature ranges between 29 oC to 50 oC.

Figure 4: Swelling and deswelling of the liquid crystal elastomer in 5CB
Figure 4 (Object 9.png)

Anchoring experiments

Before photopolymerization, surfaces of both the glass slides facing each other were spin-coated with poly-vinyl alcohol and rubbed in one direction (see figure-3). After adding the sample, photopolymerization was done 1hr later inorder to properly orient the liquid crystal molecules according to the new boundary conditions. Figure-5 shows the polarizing optical microscope results of the liquid crystal gel.

Figure 5: Transmitted bright field images under crossed polarizers as the rotating stage holding sample makes an (top) 0 degree (middle) 45 degree (bottom) 90 degree angle with one of the polarizers
Figure 5 (graphics3.jpg)

Conclusions

In this study we reported the synthesis of mesogenic monomer (A6OCB) followed by the preparation of liquid crystal elastomer gel by photopolymerization of the monomer AO6CB on the glass surface using lamp. Also we have shown the swelling and deswelling of the liquid crystal elastomer gel in liquid crystal solvent (5CB). Furthermore, orientation of liquid crystal elastomer gel can be tuned by modifying the surface properties of glass slides.

References

1. P. J. Collings and M. Hird, Introduction to Liquid Crystals, Taylor & Francis Inc., Philadelphia, 1997.

2. M. Warner and E. M. Terentjev, Liquid Crystal Elastomers, Oxford University Press, Oxford, 2003.

3. C. Ohm, M. Brehmer and R. Zentel, Adv. Mater., 2010, 9999, NA.

4. K. Urayama, Macromolecules, 2007, 40, 2277-2288.

5. K. Urayama, Y. O. Arai and T. Takigawa, Macromolecules, 2005, 38, 5721-5728.

Appendix

Figure 6: -1H NMR spectrum of A60CB. Spectra were recorded at room temperature in CDCl3 by Bruker 400 MHz NMR Spectrometer
Figure 6 (Picture 9.png)

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