Sol-Gel Transition of a Thermo-Responsive Polymer at the Closest Solid Interface

Thermo-responsive polymers are applied as surface modifications for the temperature switching of hydrophilic and hydrophobic properties through adsorption and grafting on solid substrates. The current understanding of the influence of polymer chains bound to the solid surface on the transition behavior of thermo-responsive polymers is rather restricted. In this study, we aim to elucidate the effect of the bound polymer chains at the interface on the thermo-responsive sol-gel transition behavior of aqueous methylcellulose (MC) solutions by employing a quartz crystal microbalance (QCM) to evaluate the shear modulus near the solid interface. When the sample thickness was thinner on the order of the millimeter scale, the sol-gel transition temperature evaluated by the cloud point decreased because the condensation of MC near the solid interface promoted the sol-gel transition. On the other hand, focusing on the closest solid interface on the nanometer scale by QCM, the sol-gel transition temperature increased when approaching the solid interface. Adsorption and interfacial interactions reduced the chain mobility and restrained the sol-gel transition by preventing MC chain aggregation. We demonstrated the physical properties evaluation at the closest interface between the thermo-responsive polymer and solid substrate by combining a simple analytical model of QCM and controlling the analytical depth of the QCM sensors. In conclusion, the mobility change of the bound polymer chains at the solid interface caused by adsorption and interfacial interactions must be considered when a thermo-responsive polymer is applied as in adsorbed or thin films on solid substrates for the functionalization of biomaterials.