The Effect of Templating on the Mechanical Properties of Smectic Liquid Crystal Elastomers

Abstract

In 1969, de Gennes predicted that macromolecules templated in different liquid crystal phases would have distinct mechanical properties. In this work, we explore his prediction, specifically examining smectic liquid crystal elastomers (LCEs) that have been polymerised either in a nematic or a smectic phase. The LCEs are chemically similar; they can be defined as primarily side-chain systems in which the mesogenic moiety is connected to an acrylate backbone by an alkoxy spacer unit. All of the LCEs are lightly crosslinked using a bifunctional acrylate mesogenic unit at a ratio of 7.7 mol%. When polymerised in the nematic phase, the smectic LCEs show relatively short smectic correlation lengths, extending over less than 5 smectic layers. The samples have nematic-like mechanical properties. This includes: low anisotropy of their Young’s moduli (E_(||)⁄(E_⊥~) one order of magnitude), and similar resistance to compression parallel to their initial director as for comparable nematic LCEs. Alternatively, when polymerised in a smectic phase, the LCEs exhibit much larger long-range smectic order (correlation lengths > 10 layers) and incompressible smectic layers. Surprisingly, in samples polymerised in the smectic phase have rather low anisotropy of their Young’s moduli, which we attributed to a more isotropic backbone conformation than anticipated by De Gennes. Regardless of the phase in which the polymerisation is conducted, all of the smectic LCEs show the emergence of biaxial smectic A order upon mechanical deformation perpendicular to their director.