The effect of the initial molar ratio of the reactive groups, rNc o (= [NCO]0/[OH]o = 1.0 or 0.8), on the gelation of polyurethane networks based on tris(4-isocyanatophenyl)thiophosphate and poly(oxypropylene) diol, was studied. The number-average molecular weight, h4n, in the pre-gel state, the critical conversion at the gel point, P~co, and the critical ratio, r~co, at which gelation occurs, were measured. The decrease in 1/r~co with increasing dilution in xylene suggests that cyclization takes place. The experimental h4, values are always lower than the theoretical ones, with the latter being calculated for the ring-free case. The fraction of bonds that are lost in the cyclization process, s, calculated from the difference between the experimental and theoretical values of/~n, increases with conversion and has a value near the gel point of 0.04--0.06. It is demonstrated both theoretically and experimentally that the values of s calculated from the shift of the gel point conversion are not equivalent to those obtained from Mn, but depend on rNCO.
Посилання на статтю:
Effect of the ratio of reactive groups on gelation and cyclization during polyurethane network formation / J. Budinski-Simendic and Z. S. Petrovic and M. Ilavsky t and K. Dusek // Polymer. – 1993. – Vol 34. – P. 5157-5162.