Simple scaling rules on swollen and shrunken polymer gels

The equilibrium swelling ratios of temperature sensitive polymer gels have been studied as a function of the initial monomer concentration at preparation in both the swollen and the shrunken states. The linear swelling ratio, d/d(0), was independent of the initial monomer concentration of N-isopropylacrylamide, C-0, where d and d(0) are the diameters of gels at observation and at preparation, respectively. On the other hand, d/d(0) was proportional to C-0 in the shrunken state. The swelling curves of gels with different C-0's were successfully reproduced with a modified Flory-Rehner equation, where the effective cross-link density was modified to be proportional to C-0 by taking account of the contribution of ''excess cross-linking'' by entanglement. Scaling rules for the polymer fractions were derived for both the swollen and the shrunken states; i.e., phi(sw) similar to C-0(1) and phi(sh) similar to C-0(0), where phi(sw) and phi(sh) are the equilibrium volume fractions of the network at the swollen and shrunken phases, respectively.