Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale.

A polymer network can imbibe copious amounts of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as stretchable conductors, ionic cables, and neuroprostheses, the thermal conductivities of hydrogels should be understood clearly. In the present work, we build molecular dynamics (MD) models of random cross-linked polyacrylamide hydrogels with different water volume fractions through a reaction method. On the basis of these models, thermal conductivities of hydrogels at the nanoscale are investigated by a none-equilibrium MD method. This work reveals that when the water fraction of hydrogels is under 85%, the thermal conductivity increases with the water fraction, and can be even higher than the thermal conductivities of both pure polymer networks and pure water because of the influence of the interface between polymer networks and water. However, when the water fraction in hydrogels is bigger than 85%, its thermal conductivity will decrease and get close to the water's conductivity. Accordingly, to explain this abnormal phenomenon, a 2-order-3-phase theoretical model is proposed by considering hydrogel as a 3-phase composite. It can be found that the proposed theory can predict results which agree quite well with our simulated results.