Nonlinear Bending Mechanics of Hygroscopic Liquid Crystal Polymer Networks

Abstract

A chemically responsive liquid crystal polymer network is experimentally characterized and compared to a nonlinear constitutive model and integrated into a finite element shell model. The constitutive model and large deformation shell model are used to understand water vapor induced bending. This class of materials is hygroscopic and can exhibit large bending as water vapor is absorbed into one side of the liquid crystal network (LCN) film. This gives rise to deflection away from the water vapor source which provides unique sensing and actuation characteristics for chemical and biomedical applications. The constitutive behavior is modeled by coupling chemical absorption with nonlinear continuum mechanics to predict how water vapor absorption affects bending deformation. In order to correlate the model with experiments, a micro-Newton measuring device was designed and tested to quantify bending forces generated by the LCN. Forces that range between 1 and 8 μN were measured as a function of the distance between the water vapor source and the LCN. The experiments and model comparisons provide important insight into linear and nonlinear chemically induced bending for a number of applications such as microfluidic chemical and biological sensors.