Effects of the gel size on the volume phase transition of poly(N-isopropylacrylamide) gels: A calorimetric study

Effects of the gel size on the thermal behavior and the shrinking profile of a chemically cross-linked poly(N-isopropylacrylamide) (NIPA) gel in water were investigated by differential scanning calorimetry (DSC) and the volume measurement at the same heating rate as used for the DSC. The DSC thermogram of NIPA gel depended on the shrinking process at the transition. The DSC thermogram of the NIPA gel of the smallest size (0.3-mm diameter at the preparation) gave double endothermic peaks, and the gel underwent a discontinuous volume shrinkage (i.e. a volume transition) at about 34.5 degrees C in the DSC run with a heating rate of 0.1 degrees C/min. This suggests that the two-step cooperative dehydration of NIPA chains takes places in the volume shrinkage. It was proposed that the peak at the low temperature (peak 1) originated from the dehydration of NIPA chains without the volume transition while the peak at the high temperature (peak 2) was associated with the discontinuous volume shrinkage. A single broad endothermic peak was observed for the NIPA gel of the largest size (3-mm diameter at the preparation) that shrank only slightly during the DSC run. For the largest gel, the volume transition cannot occur within the time scale of the DSC run due to the slow volume shrinkage while dehydration of the polymer chains takes place locally. The peak of the largest gel and peak 1 were suggested to be due to the dehydration of NIPA chains coupled with the spinodal decomposition without the volume transition.