VISCOELASTIC RELAXATION OF POLYURETHANE AT DIFFERENT STAGES OF THE GEL FORMATION .1. GLASS-TRANSITION DYNAMICS

We report results on the local segmental relaxation (alpha-relaxation) of a cross-linked polyurethane system formed by polycondensation of a triol and a diisocyanate at different connectivity extents. Stable sa mples with different connectivity extents were obtained either by vary ing the amount of excess triol (A) or by addition of different amounts of a mono alcohol (B). The glass transition temperature (T-g) increas es linearly with increasing connectivity extent for system A, while fo r system B it is almost constant. T-g is thus mainly related to the am ount of urethane groups. The temperature dependence of the alpha-relax ation is independent of the connectivity extent and is the same for bo th systems. The shape of the alpha-relaxation can be described by a st retched exponential which decreases with an increasing connectivity ex tent, but in a different way for both systems.