The Influence of Cross-Linking/Chain Extension Structures on Mechanical Properties of HTPB-Based Polyurethane Elastomers

Cross-linked and chain-extended polyurethane networks based on hydroxyl-terminated polybutadiene are synthesized by one-step reaction with isophorone diisocyanate. The degree of cross-linking is controlled by varying the triol to diol ratio. Chain extension is optimized by appropriate adjustment in the concentration of the diols. The influence of cross-linking agents and chain-extenders on mechanical properties of cured elastomers is investigated. IR-spectroscopy is employed to observe the curing reaction and inter-segment hydrogen bonding. It is demonstrated that the chain-extended polymer is phase segregated at sub-micron level and registers far better mechanical properties as compared to cross-linked polymer. Incorporation of 1,4 butanediol yields the maximum tensile strength, elongation and modulus. The polymer fabricated with trimethylolpropane is stronger than that of hexanetriol but exhibits lower stretching properties. However, butanediol exhibits superior mechanical properties in comparison to hexanediol. Equilibrium swelling behaviour follows the expected norm; degree of swelling varies inversely with the cross-link density. The chain-extended polymers show reduced degree of swelling due to plausible association between the hard segment domains formed in the micro-phase segregated structures.