Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

Summary

We demonstrate the preparation of siloxane-based and epoxy-based liquid crystal elastomers (LCEs) and LCE nanocomposites. The LCEs are characterized with respect to reversible strain, liquid crystal ordering, and stiffness. As a potential application, we demonstrate their use as shape-responsive substrates in a custom device for active cell culture.

Abstract

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and as soft robots. Here, we demonstrate the preparation of shape-responsive liquid crystal elastomers (LCEs) and LCE nanocomposites along with characterization of their shape-responsiveness, mechanical properties, and microstructure. Two types of LCEs — polysiloxane-based and epoxy-based — are synthesized, aligned, and characterized. Polysiloxane-based LCEs are prepared through two crosslinking steps, the second under an applied load, resulting in monodomain LCEs. Polysiloxane LCE nanocomposites are prepared through the addition of conductive carbon black nanoparticles, both throughout the bulk of the LCE and to the LCE surface. Epoxy-based LCEs are prepared through a reversible esterification reaction. Epoxy-based LCEs are aligned through the application of a uniaxial load at elevated (160 °C) temperatures. Aligned LCEs and LCE nanocomposites are characterized with respect to reversible strain, mechanical stiffness, and liquid crystal ordering using a combination of imaging, two-dimensional X-ray diffraction measurements, differential scanning calorimetry, and dynamic mechanical analysis. LCEs and LCE nanocomposites can be stimulated with heat and/or electrical potential to controllably generate strains in cell culture media, and we demonstrate the application of LCEs as shape-responsive substrates for cell culture using a custom-made apparatus.

Introduction

Materials that can exhibit fast, reversible, and programmable shape changes are desirable for a number of emerging applications^1-9. Shape-responsive stents can assist with wound healing and treatment⁷. Artificial robots can aid in exploration or in carrying out tasks in environments that are harmful or unsafe for humans¹⁰. Shape-responsive elastomers are desirable for use in active cell culture, in which cells are cultured in an active environment.^11-14 Other applications include packaging, sensing, and drug delivery.

Liquid crystal elastomers (LCE) are polymer networks with liquid crystal orderin....

Protocol

1. Synthesis of Aligned Polysiloxane LCEs

1. Combine 166.23 mg of reactive mesogen (4-methoxyphenyl 4-(3-butenyloxy)benzoate), 40 mg of poly(hydromethylsiloxane), and 12.8 mg of crosslinker (1,4-di(10-undecenyloxybenzene)³⁰ with 0.6 ml of anhydrous toluene in a small vial (approximately 13 mm in diameter and 100 mm in length) charged with a stirring bar. Stir the solution at 35 °C for 25 min to dissolve.
2. In a separate vial, prepare a solution of 1 wt % dichloro(1,5-cyclooctadiene)-platinum(II) catalyst in dichloromethane. Add 30 µl of catalyst solution to the reagents from step 1.1 via pipette, stir to mix, and pour the solution....

Results

Monodomain LCEs are shape-responsive due to coupling of network conformation with liquid crystal ordering. Heating LCEs results in a decrease in the liquid crystal order parameter, producing a contraction of the polymeric network along the primary alignment direction. This is easily visualized by placing an LCE on a hotplate, as shown in Figure 1A and 1B. In heating up from RT, the LCE contracts along the length of the sample, and above the isotropic tran.......

Discussion

In order to produce monodomain LCEs, the LCEs need to be uniaxially loaded during crosslinking. This is challenging in practice because the LCE is loaded when it is only partially crosslinked, and therefore is not mechanically robust and can easily break or tear. The procedure described above (steps 1.1 - 1.4) can produce monodomain LCEs consistently. One critical step is the removal of the LCE from the PTFE mold for loading at the appropriate time. If the LCE is removed too quickly, it will easily break or tear. On the.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
4-methoxyphenyl 4-(3-butenyloxy)benzoate	TCI America	M2106	Reactive mesogen
poly(methylhydrosiloxane)	Gelest	HMS-993	Reactive polysiloxane
1,4-di(10-undecenyloxybenzene)	N/A	N/A	see: Ali, S. A., Al-Muallem, H. A., Rahman, S. U. & Saeed, M. T. Bis-isoxazolidines: A new class of corrosion inhibitors of mild steel in acidic media. Corrosion Science. 50 (11), 3070–3077, doi:10.1016/j.corsci.2008.08.011 (2008)
(dichloro(1,5-cyclooctadiene)-platinum(II)	Sigma Aldrich	244937	Pt catalyst
PTFE mold	N/A	N/A	fabricated at Rice machine shop
carbon black nanoparticles	Cabot	VULCAN® XC72R	used in the synthesis of LCE nanocomposites
polystyrene	Sigma Aldrich	331651	linear polystyrene
4,4'-diglycidyloxybiphenyl	N/A	N/A	see:  Giamberjni, M., Amendola, E. & Carfagna, C. Liquid Crystalline Epoxy Thermosets. Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals. 266 (1), 9–22, doi:10.1080/10587259508033628 (1995).
sebacic acid	Sigma Aldrich	283258	C8 linking group for epoxy-LCE synthesis
hexadecanedioic acid	Sigma Aldrich	177504	C16 linking group for epoxy-LCE synthesis
carboxydecyl-terminated polydimethylsiloxane	Gelest	DMS-B12	Siloxane linking group for epoxy-LCE synthesis
1,5,7-triazabicyclo[4.4.0] dec-5-ene	Sigma Aldrich	345571	catalyst for reversible LCEs
carbon rods	Ladd Research	30250	used in cell culture experiments
medical grade silicone adhesive	Silbione	MED ADH 4100 RTV	used to adhere carbon rods to vessel