NMR self-diffusion and viscosity of polyurethane formulations for rocket propellants

Diffusion coefficients and viscosity have been measured in three different binary poly(urethane) binder formulations for solid rocket propellants. By means of the FT-PGSE NMR method the diffusion coefficients could be measured simultaneously for both the diisocyanate and prepolymer components with good accuracy. To calculate average diffusion coefficients for polydisperse substances, the stretched exponential function could be used successfully. The diffusion coefficients vary strongly with several orders of magnitude as a function of composition and also between the different systems from 5.04 x 10(-11) m(2) s(-1) for pure isophorone diisocyanate (IPDI) down to 9.34 x 10(-14) m(2) s(-1) for pure glyzidyl azide polymer (GAP). Plotted on a semilogarithmic scale against the prepolymer weight concentration, several of the diffusion coefficients show nearly linear behavior. Viscosities of these mixtures vary from 0.0 15 to 5.85 Pa s. An attempt to correlate the self-diffusion coefficients to the measured viscosity has been moderately successful. For all substances except GAP the self-diffusion coefficients scale close to the -0.6 power of the viscosity, according to the quenched disorder model. For GAP, the scaling factor is close to -0.9. While some systems can be explained quite well by a Rouse-type model, other systems cannot, the most important exception being the HTPB:IPDI system that shows phase separations at hydroxyl terminated polybutadiene (HTPB) concentrations below 50% w/w. The measured self-diffusion data have given valuable information for the interpretation of migration phenomena in rocket propellants.