Solubility and dynamic mechanical behaviour of polyurethane systems at the critical molar ratio of the reactive groups for gelation and at the gel point

The dynamic mechanical behaviour of fully cured soluble and insoluble polyurethanes from two poly(oxypropylene)triols and a diisocyanate with initial ratios r(H) = [OH]/[NCO] from 1.44 to 1.8 were investigated. The dynamic behaviour during the curing reactions of two samples with r(H) = 1 was also measured. From extraction experiments the critical ratios for gelation r(H)(c) of fully cured samples were determined. As expected, both critical gel (CG) samples in which the gel point was reached by changing the conversion exhibit a power-law rheological behaviour. On the other hand, both fully cured CG samples prepared at r(H)(c) show a small frequency dependence of the loss tangent so that critical power-law behaviour is not exactly obeyed. New conditions representing critical rheological behaviour are suggested. Copyright (C) 1996 Elsevier Science Ltd.