Network formation in polyurethanes based on triisocyanate and diethanolamine derivatives

The effect of the side chain length and the presence of charge in diols on gelation of polyurethane networks from diols based on diethanolamine derivatives and tris(4-isocyanatophenyl) thiophosphate was studied. The samples having different initial ratios of the reactive hydroxy [OH] and isocyanate [NCO] groups (rH=[OH]/[NCO] ranging from 1.1 to 3) were investigated. The number- and weight-average molecular weights in the pregel stage (Mn, Mw) together with the critical ratio of the reactive groups, rHc, at which gelation occurs, were determined. The experimental Mn and Mw values are lower than the theoretical ones calculated for the ring-free case suggesting that cyclization takes place. The fraction of bonds lost in cycles, S, is 0.02–0.04 in the bulk state, increasing with increasing dilution and decreasing rH. A pronounced decrease in rHc with dilution was found; while the length of the side chain of the diol has only a minor effect on the decrease, the introduction of the charge (quaternization of the nitrogen) into the diol leads to a less pronounced decrease (lower extent of cyclization). The S values calculated from Mw and rHc were found to be always higher than S values calculated from Mn. A possible supramolecular structure is proposed for networks prepared from the uncharged diethanolamine-based diols when prepared in the presence of diluent.