Understanding the effect of chain entanglement on the glass transition of a hydrophilic polymer

In this article, we report an interesting phenomenon of the glass transition temperature (T_g) deviation of a hydrophilic polymer. Polyacrylamide (PAL) samples with different extents of chain entanglement were prepared by spray drying and solution casting. We found that the glass transition temperature increases as the extent of chain entanglement decreases upon the sub-T_g annealing. The water content in the PAL matrix is found with no direct correlation to T_g. However, the observation of a faster diffusion process of water in the disentangled PAL matrix offers an evidence of a faster relaxation process of disentangled PAL molecules. The T_g increase of the disentangled PAL samples is believed to be associated with the increased molecular interaction during the chain relaxation process upon the sub-T_g annealing. A macroscopic evidence is the fact that the density of the hot-laminated samples increases as the extent of chain entanglement decreases. A thermodynamic analysis suggests that enthalpy more than entropy favors an elevated T_g of a disentangled hydrophilic polymer upon the sub-T_g annealing. We believe that this research provides new understanding of T_g of the hydrophilic polymers, which are being extensively used in bio-related studies.