Self-Similar Relaxation Behavior at the Gel Point of a Blend of a Cross-Linking Poly(-caprolactone) Diol with a Poly(styrene-co-acrylonitrile)

ABSTRACT: Novel polymer properties can be achieved by blending high molecular weight linear chains into a cross-linking system of short linear chains. This study is concerned with the rheological properties that are dominated at first by the highly entangled linear chains. However, with increasing extent of cross-linking, the short chains connect into a network structure and begin to dominate the rheology. The material here consists of cross-linking poly(-caprolactone) diol (PCL) and up to 40 % of linear poly-(styrene-co-acrylonitrile) (SAN) of high molecular weight. The blend was molecularly mixed before cross-linking. Three competing processes determine the structure of the system, (1) chemical cross-linking of the low molecular weight species into a sample spanning network of interpenetrating chains, (2) fluctuations in composition due to phase separation at increasing extents of reaction, and (3) crystallization of the PCL, which we tried to suppress as much as possible. At the gel point, systems with low SAN content show the typical scaling behavior of the critical gel with a self-similar relaxation spectrum, H(ì)) Go/¡(n) (ì/ìo)-n, ì> ìo, at low probing frequencies, ö < 1/ìo. However, for the systems with high concentrations of the inert component, the self-similar region did not develop, possibly due to the phase separation induced by the cross-linking. The relaxation exponent, n, decreased with increasing concentration of the highly entangled linear component. The results suggest that dynamic mechanical methods are applicable for determination of the gel point for homogeneous semi-IPN systems