Theoretical investigation on thermal aging mechanism and the aging effect on mechanical properties of HTPB–IPDI polyurethane

The CO bond connected directly with CH2 moiety is the weakest bond. The half-life time of the cleavage of CO bond exhibits a temperature dependent exponential decay function, which shows clearly that the thermal-aging of HTPB–IPDI is accelerated with increased temperature. The MD simulations show that the aged HTPB–IPDI densities decrease when the carbon dioxide formed during thermal aging disperses inside the cell or diffuses out of the system. The decreased tensile modulus and shear modulus is attributed to the dissociation of hard segments and breaking of hydrogen bonds in HTPB–IPDI cross-linking network. [Display omitted] •The CO bond connected directly with CH2 moiety is the weakest bond in HTPB–IPDI molecules.•We propose a thermal aging degradation mechanism for HTPB–IPDI polyurethane.•The thermal-aging of HTPB–IPDI is accelerated with increased temperature.•The aged HTPB–IPDI tensile modulus and shear modulus decrease. The thermal aging mechanism of hydroxy-terminated polybutadiene/isophorone diisocyanate (HTPB–IPDI) polyurethane was studied theoretically by density functional theory (DFT) method. The results of bond dissociation energy (BDE) calculated at B3LYP/6-311++G(2d,p) level show that the CO bonds connected with the CH2 group are the weakest bonds. The weakest CH bond was calculated by BDE method is the CH bond in the alpha position connected with the ester group, and the secondary weak CH bonds are in the cyclohexane tertiary carbon moiety whatever in model 1 or model 2. The HTPB–IPDI decomposition reactions toward the unsaturated cyclohexene, methyl carbamic acid, methylamine, and CO2 products were also calculated to be thermodynamically feasible. The mechanical properties changes of aged HTPB–IPDI polyurethanes were analyzed by molecular dynamic simulation. The aged HTPB–IPDI with CO2 molecules inside the cell had a decreased Poisson’s ratio and bulk modulus, while that of the aged system with CO2 diffused out of the system tended to increase. The decreased Young’s modulus and shear modulus are attributed to the hard segment dissociation and the hydrogen bond breaking in the HTPB–IPDI cross-linking network.