Kinetics of volume phase transition in poly(N-isopropylacrylamide-co-acrylic acid) gels

The shrinking/swelling behavior across the volume phase transition temperature, T-c, has been investigated for cylindrical gels made of poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) and poly(N-isopropylacrylamide) (NIPA). NIPA/AAc gel shrank at T-c = 43 degrees C by quasistatic heating. On the other hand, the gel underwent a shrinking transition at a lower temperature, i.e., T-c approximate to 40 degrees C, when a temperature jump was applied to the gel from 20 degrees C. More interestingly, the shrinking process consisted of three stages: (i) a uniform shrinking stage where the gel diameter decreases exponentially, (ii) a plateau stage where the gel shrinks further from both ends of the cylinder while the middle (swollen) part remains in the swollen state, and (iii) a collapsing stage where the middle part of the gel shrinks linearly with time. This sequential process was found to be characteristic of weakly charged gels with a large aspect ratio. To the contrary, the swelling process was fitted with a single exponential function given by the Tanaka-Fillmore theory on gel swelling. This interesting shrinking behavior was discussed by comparing the results of swelling kinetics. In the case of NIPA homopolymer gel, the shrinking process was strongly decelerated due to phase separation, while the swelling process was well reproduced by the Tanaka-Fillmore theory.