Relationship between the volume phase transition and the surface area of poly ( N -isopropylacrylamide) and poly ( N -isopropylacrylamide- co -sodium methacrylate) hydrogels

The volume phase transition of poly( N -isopropylacrylamide) (PNIPAAm) and poly( N -isopropylacrylamide- co -sodium methacrylate) (P(NIPAAm- co -SMA)) hydrogels was investigated with consideration of the pore characteristics. The volume phase transition temperature of the hydrogel increased by incorporating sodium methacrylate (SMA). Based on the BET equation, the surface area was evaluated by a gas adsorption method. The surface area of PNIPAAm and P(NIPAAm- co -SMA) hydrogels decreased with increasing temperature, resulting from the fact that the depth and the size of pores decreased significantly in the course of the volume phase transition. Hence, it is suggested that the change of surface area has a close relationship with the volume phase transition of PNIPAAm and P(NIPAAm- co -SMA) hydrogels.