VISCOELASTIC RELAXATION OF POLYURETHANE AT DIFFERENT STAGES OF THE GEL FORMATION .2. SOL-GEL TRANSITION DYNAMICS

The viscoelastic relaxation of cross-linked polyurethane systems is in vestigated over a very broad dynamical range using time-temperature su perposition. Stable cross-linked samples were obtained by polycondensa tion of a triol and a diisocyanate at different connectivity extents e ither by varying the amount of excess triol (A) br by addition of diff erent amounts of a monoalcohol (B). The frequency dependence of the sh ear modulus over a range of up to 13 decades could be accurately fitte d assuming a stretched exponential for the high frequency relaxation ( see part 1, preceding paper in this issue) and a relaxation time distr ibution containing two power laws for the lower frequency relaxation. The results up to the gel point are compatible with the percolation mo del if proper consideration is given to the low and high frequency lim its of the model. Above the gel point the relaxation time distribution still has a power law dependence. The exponent decreases initially fr om about 0.73 at the gel point to about 0.5 and then increases to 0.78 for the fully grown gel.