Application-driven studies of photomechanical materials

Abstract

Deep brain stimulation therapy is a well-established medical procedure with well-documented benefits, but this procedure would be enhanced by a flexible therapy device that could bend when photoactivated. To this end, two types of materials, which respond to light by inducing a stress and strain, show promise and are therefore selected to be characterized: photomechanical dye-doped glassy polymer fibers and liquid crystal elastomers. A custom designed and built apparatus, the photorheometer, measures the force created by these materials during photostimulation. Using the developed theory, it is found that the liquid crystal elastomers produce a large strain but small stress while dye-doped glassy polymer fibers produce a large stress but small strain. Photoisomerization in dyes, a mechanism where a molecule changes shape in response to photostimulation, is investigated in dye-doped glassy polymer fibers. Results show glassy polymer fibers doped with isomerizable dyes have a notable polarization dependence, but this dependence cannot be fully explained by isomerization alone, as non-isomerizable dyes also have a polarization dependence. Liquid crystal elastomers from two synthesis methods are characterized. The first method creates side-chain end-on siloxane-based liquid crystal elastomers, and the second method is a two-stage procedure creating main-chain acrylate-based liquid crystal elastomers. Using the photorheometer, materials made with the different synthesis methods are charactered with various liquid crystalline order. The materials made with the two-stage procedure are also characterized as a function of pre-strain, dye concentration, and crosslink density. Results show monodomain (highly ordered liquid crystal elastomer), low dye concentration, and crosslinked liquid crystal elastomers have a larger photomechanical response than polydomain, high dye concentration (0.25%mol - 1%mol), non-crosslinked liquid crystal elastomers.