Preparation pressure dependence of structure inhomogeneities and dynamic fluctuations in poly(N-isopropylacrylamide) gels

The structure inhomogeneities and dynamic fluctuations in poly(N-isopropylacrylamide) (PNIPA) gels have been investigated by dynamic light scattering as a function of gel-preparation pressure, P-prep. The ensemble average light scattered intensity ((I)(E)), a measure of structure inhomogeneities, was found to be an increasing function of P-prep and diverged at P-prep = 168 MPa. This indicates that PNIPA gels undergo phase separation by increasing P-prep. The corresponding PNIPA solution was transparent and had no P-prep dependence. It is concluded that the structure of gels depends not only on the pressure at observation P-obs but also on P-prep and the inhomogeneities increase with P-prep. On the other hand, the dynamic fluctuations were independent of P-prep. This shows that cross-linking plays a major role in formation of built-in structure inhomogeneities.