Studies on mechanical and physicochemical properties of polymer gels

The three topics in our study are reviewed in the present paper: (1) molecular understandings of rubber elasticity of end-linked polymer networks and the dynamics of trapped guest chains; (2) elastomers with remarkable extensibility or high damping insensitive to both frequency and temperature via topology control of polymer networks; (3) phase transition in polymer network systems such as sol-gel transition and volume phase transition driven by nematic-isotroptic transition. On the basis of the biaxial elongation data of end-linked polydimethylsiloxane (PDMS) networks, the form of the strain energy density function is evaluated, and the five entanglement models of rubber elasticity are unambiguously assessed. It is shown that the dynamics of linear guest chains trapped in crosslinked networks obey the reptation concept. A dramatic slowing down of the reptation is observed in the host network whose mesh size is smaller than the entanglement spacing. The deswollen networks, prepared by end-linking long precursors in the diluted state and removing the solvent, exhibit a marked extensibility over 3000% as well as a very weak strain dependence of stress. These features stem from a small amount of trapped entanglement and a compact conformation of network chains. The damping of irregular networks containing many pendant chains increases with the amount of pendant chains. The damping is very weak dependent of temperature and frequency as a result of a slow viscoelastic relaxation of irregular network structures. The nematic networks swollen in nematic or isotropic solvents undergo a phase transition between the swollen isotropic and shrunken nematic states.