Synthesis, characterization, and properties of novel polyisobutylene‐based urethane model networks

This paper concerns the synthesis, characterization, and physical properties of novel polyisobutylene (PIB)‐based urethane model networks prepared from diphenylmethane diisocyanate (MDI) and three‐arm star PIBs capped with CH 2 OH end groups (PIB(CH 2 OH) 3 ). The PIB(CH 2 OH) 3 starting materials were produced by the inifer method in the M̄ n = 13,550–27,000 range. The best networks were obtained with NCO : OH = 1. Solvent extractions showed uncatalyzed network formation to be essentially complete and swelling studies indicated the expected architecture, i.e., M̄ c = 2 M̄ n /3 and virtual absence of dangling chains. Stannous octoate was found to increase the rate of network formation in the absence of side reactions (i.e., allophanate formation), while other catalysts also accelerated undesirable reactions. The effect of molecular weight between crosslinks ( M̄ c ) on network physical properties has been studied in the M̄ c = 900–18,500 range. The tensile strength increases with decreasing M̄ c up to a limiting value after which it sharply declines. Elongations at break decrease monotonously with decreasing M̄ c . Low temperature tensile studies show higher tensile data at −20°C, and, surprisingly, a retention of elongations at break. The T g 's decrease with increasing M̄ c 's until a plateau is reached at −73°C. Hysteresis is fairly constant and permanent set is constant and low. The PIB‐based urethane networks exhibit excellent hydrolytic stability, negligible moisture absorption, and outstanding heat‐aging stability, far beyond what is expected for a polyurethane. Conceivably the thermal deblocking of the urethane group may be reversible (NHCOOCH 2 ⇌ NCO + HOCH 2 ) because the isocyanate that arises in the highly hydrophobic PIB matrix recombines with the alcohol, and cannot react with moisture as in conventional urethane networks.