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.