RELAXATION PHENOMENA IN PPG MODEL NETWORKS NEAR THE GLASS RUBBER TRANSITION

The viscoelastic and dielectric properties of polypropyleneglycol (PPG )tris-(4-isocyanatophenyl)thiophosphate (Desmodur(R)) model networks h ave been studied. Attention was focused on the relaxation behaviour ne ar the glass transition Two relaxation mechanisms were observed in the glass transition range, one due to the PPG chain segments and one due to the less mobile cross-link moieties, i.e. the networks show motion al heterogeneity. Mechanically only the first mechanism was observed w hile dielectrically both mechanisms were found. This motional heteroge neity has a profound effect on the dependence of the glass transition temperature on the composition of these networks. The effect of the st age of cure on the temperature of maximum loss and the magnitude of th ese two relaxation mechanisms was studied dielectrically. Finally, the effect of exchanging the cross-linker molecules with chain extender m olecules with similar structure, thus lowering the cross-link density, was studied.