Controlling Microstructure and Polymer Deformation with Polarized Light in Liquid Crystal Polymer Networks

Abstract

Photoresponsive polymers offer unique opportunities in aerospace morphing structure applications as these afford shape changes spatially and temporally with light. To understand the constitutive behavior of these materials, we quantify the interactions between light excitation and molecular conformation changes in photoresponsive polymer networks using non-equilibrium thermodynamics, nonlinear photomechanics, and combined finite difference/finite element simulations. The analysis is conducted by introducing a set of electronic order parameters to represent light driven molecular conformation changes which are coupled to mechanics of a continuum scale polymer network and time-dependent electromagnetics. The approach provides a general framework for studying material behavior of solids in the presence of complex photoisomerization and photomechanical deformation. In the example described here, we apply the model to understand photoisomerization of azobenzene as it evolves and deforms a polymer network. The model focuses on examples of local surface deformation from laser beam excitation previously described by mass transport. These modes of deformation, from linear and circularly polarized laser beam excitation, are described by liquid crystal microstructure evolution and affine deformation of the host polymer network without the need of mass transport or higher order field gradient theories. The model is implemented numerically using a three-dimensional finite difference time domain method and coupled to finite element modeling to illustrate its ability to quantify light absorption and stress during nonlinear photoisomerization and polymer deformation.