Spatially Nonuniform Heating and the Nonlinear Transient Response of Elastomeric Photomechanical Actuators

Recently various nanomaterials, such as carbon nanotubes and graphene, have been added to rubbery elastomers, such as poly dimethyl siloxane (PDMS), to enable generation of stress and displacement in response to remote illumination. While the response is primarily due to heat-induced generation of stress; i.e., the thermoelastic effect in rubbers, illuminated samples have shown unexpected deviations between the transient waveforms of sample temperature and induced stress. In this report we have created a new and simple lumped element model to explain the stress behavior of these photomechanical nanocomposites. The model consists of two parameters that describe the spatially averaged steady state temperature rise due to optical absorption of the structure (typically a long strip of pre-strained elastomer) and the spatially averaged convective cooling rate of the strip, together with a time-varying function that effectively represents the temperature distribution and thermal convection along the length of the strip. The model is used to compare two actuators that each have a thin embedded layer of carbon nanotubes, in which the one film consists of randomly aligned nanotubes and the other has a much more ordered alignment. The model not only fits both transient responses, but the differences between the parameters suggests that the ordered film conducts heat across the strip more rapidly than the disordered film, leading to it more rapidly reaching the steady state level of maximum stress. This model should be helpful in future experimental studies that work to observe, delineate and identify possible nanoscale and molecular contributions to photomechanical stress.