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 investigated over a very broad dynamical range using time-temperature superposition. Stable cross-linked samples were obtained by polycondensation of a triol and a diisocyanate at different connectivity extents either by varying the amount of excess triol (A) br by addition of different amounts of a monoalcohol (B). The frequency dependence of the shear modulus over a range of up to 13 decades could be accurately fitted assuming a stretched exponential for the high frequency relaxation (see part 1, preceding paper in this issue) and a relaxation time distribution containing two power laws for the lower frequency relaxation. The results up to the gel point are compatible with the percolation model if proper consideration is given to the low and high frequency limits of the model. Above the gel point the relaxation time distribution still has a power law dependence. The exponent decreases initially from about 0.73 at the gel point to about 0.5 and then increases to 0.78 for the fully grown gel.