Dynamic Stiffening of Liquid Crystal Elastomers

Dynamic Stiffening of Liquid Crystal Elastomers

Aditya Agrawal¹, Prabir K. Patra⁴, Pulickel M. Ajayan², Walter G. Chapman¹, and Rafael Verduzco¹

¹Rice University, Department of Chemical and Biomolecular Engineering, Houston, TX

²Rice University, Department of Mechanical Engineering and Materials Science, Houston, TX

³Rice University, Department of Biochemistry and Cell Biology, Houston, TX

⁴University of Bridgeport, Departments of Biomedical Engineering and Mechanical Engineering, Bridgeport, CT

Liquid crystal elastomers (LCEs) are rubbery, crosslinked polysiloxane networks with liquid crystal side-groups. Due to the presence of liquid crystalline order, LCEs respond to heat, light, and magnetic and electric fields. Monodomain LCEs, which have uniform ordering of the nematic director, exhibit reversible shape changes to external stimuli. Here, we explore the rheological properties of LCEs in response to low-amplitude, repetitive deformations. While conventional elastomers respond reversibly, nematic LCEs exhibit a dramatic increase in stiffness – up to 90 % - when subjected to a low-amplitude, repetitive compression. By studying a systematic series of materials, we demonstrate that the stiffness increase is directly influenced by the liquid crystal content of the elastomers, the presence of a nematic liquid crystal phase and the use of a dynamic as opposed to static deformation. Through a combination of rheological measurements, polarizing optical microscopy and 2-D X-ray diffraction, we demonstrate that self-stiffening arises due to rotations of the nematic director in response to dynamic compression, and show that the behavior is consistent with the theory for nematic rubber elasticity. We explore the response of nematic LCEs to uniaxial extension and to varying frequencies and amplitudes of deformation. We also investigate the use of dynamic deoformations to align higher order LCEs, including chiral nematic and smectic networks. The use of a low-amplitude, dynamic deformation represents a potentially straightforward route to achieving uniform alignment in LCEs.Previous work with liquid crystal elastomers has focused primarily on ‘soft elastic' deformations at large strains, but our findings indicate rich behavior at previously overlooked low-strain, dynamic deformations.

 

Keywords: Liquid crystal elastomers, stiffening, dynamic mechanical analysis.