Phase transition in strongly stretched polymer gels

We will present the volume phase transition of cylindrical poly N-isopropylacrylmide (NIPA) gels under large distortion along the uniaxial direction. The neutral and slightly ionized NIPA gels were synthesized in submillimeter diameter. The gel was stretched along the uniaxial direction with the longitudinal deformation ratio, alpha(<paralell>) (the ratio of the stretched length to the equilibrium length in the swollen state at 30 degrees C). By keeping the longitudinal length constant, we have simultaneously measured the equilibrium diameter and the force as functions of temperature. The swelling curves of the neutral NIPA gels including the volume phase transition temperature were obtained for several deformations in the range between alpha(<paralell>) = 1 and alpha(<paralell>) = 6. With increasing alpha(<paralell>), the transition temperature increased up to 1 degrees C in the small deformation below alpha(parallel to) similar to 3.5, and it saturated and slightly decreased in the large alpha(parallel to) above alpha(<paralell>) similar to 4. At the transition temperature on heating, the force to keep the length constant increased discontinuously in the smaller region below alpha(<paralell>) similar to 3.5. The magnitude of this steplike change in the force began to decrease in the vicinity of alpha(<paralell>) similar to 3.5, and hereafter exhibited negative change above alpha(<paralell>) similar to 4. In the case of the slightly ionized NIPA gels, these effects came into play at an earlier stage, that is, at smaller alpha(<paralell>), because of the prestretching due to the ionic pressure. The present observations, especially for the strong deformation above alpha(parallel to) similar to 4, were discussed by the equation of states of strongly stretched hydrophobic gels on the basis of the extended Flory-type free energy taken into account the non-Gaussian effect. (C) 1997 American Institute of Physics