Stretch induced thermal conduction anisotropy of hydrogel

Abstract

Soft materials involving both temperature and deformation, as represented by multi-functional hydrogels for temperature sensing and thermal actuation, have been widely mentioned in recent researches. In many circumstances, the hydrogels are working under combined actions of the thermal stimuli and the mechanical forces. The complex conditions require accurate experimental techniques in characterizing thermal and mechanical properties, and call for theoretical models to quantify the deformation dependent thermal properties. In this work, we proposed a novel experimental method to determine thermal conductivity and surface heat transfer coefficient in the hydrogel, using thermochromic capsule powders (TCPs) as an indicator for temperature change. A microscopic statistical model was developed for the hydrogel, to relate the anisotropy of the thermal conductivity to the deformation. The new model takes the hydrogel as a 3-phase composite and relates the stretch-induced orientation change of polymer chains to the change of thermal conductivity. Using the new experimental technique, we measured the thermal properties of PAAm in both hydrogel and dried network states, under different extension ratios. The model enables us to determine two microscopic scale thermal properties from the experimental data of the dried network, and predict the thermal conductivity of hydrogel under different stretches. The prediction accurately captures the stretch induced thermal conduction anisotropy of hydrogel and agrees well with the experimental results.