Polyurethanes based on hydroxyl terminated polybutadiene: modelling of network parameters and correlation with mechanical properties

Hydroxyl terminated polybutadiene (HTPB) based urethane networks were synth esized by reacting HTPB with toluene diisocyanate (TDI) at various stoichio metric ratios (R) ranging from 1.0 to 1.5. The resulting networks would con tain allophanate linkages in addition to urethane groups. The networks were characterized for mechanical and swell characteristics. The parameter X wh ich is the extent of urethane groups involved in the allophanate formation and other network parameters, such as crosslink density (v(e)) and effectiv e chain length (L-X), were calculated using the alpha-model developed by Ma rsh with suitable modifications. The value of X showed very good correlatio n with R-value, and linear correlations were obtained between mechanical pr operties and the calculated network parameters. Crosslink densities, calcul ated from stress-strain data were found to be nearly 1.2 times the correspo nding values calculated from swell data. Dynamic mechanical studies showed that R value did not influence the glass transition temperature (T-g); howe ver tan delta(max) was shown to be a linear function of the crosslink densi ty. Activation energy for the variation of storage moduli with temperature decreased with R-value, while that for loss moduli remained nearly unaffect ed by the R value. (C) 2000 Elsevier Science Ltd. All rights reserved.