| This thesis concerns the synthesis and characterization of urethane networks constructed from three-arm star polyisobutylenes (PIB) terminated with primary alcohol endgroups and 4,4'-diphenyl- methane diisocyanate (MDI). The PIB (CH(,2)-OH)(,3) starting materials were prepared with (')M(,n) values ranging from 1350 to 27,000 g/mol, using the inifer method for the polymerizations. Analysis of the polymers after a selective oxidative degradation of the central aromatic ring revealed them to be truly radial in nature, and UV spectroscopy showed that initiation occurs from all three cumyl chloride sites of the trinifer. An NCO:OH ratio of 1:1 resulted in networks with the lowest sol fractions. Solvent extractions showed uncatalyzed network formation to be essentially complete and the results of swelling studies indicated that the expected architecture had been formed, i.e., (')M(,c) = 2(')M(,n)/3, with the virtual absence of dangling chains. Stannous octoate was found to increase the rate of network reaction while not accelerating undesirable side reactions (such as allophanate formation). Other catalysts examined did catalyze these undesirable reactions. The effect of molecular weight between crosslinks ((')M(,c)) on the physical properties in the range of (')M(,c) = 900-18,500 was examined. The tensile strength increases with decreasing (')M(,c) up to a limiting value at which point it sharply decreases. Elongations at break increase monotonically with increasing (')M(,c). Low temperature tensile strengths increase at -20(DEGREES)C and, surprisingly, the samples retain their ultimate extension. The T(,g)s decrease with increasing (')M(,c) until a plateau is reached at -73(DEGREES)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.;Use of these materials as model networks for testing theories of rubberlike elasticity supported the argument that interchain entanglements do not contribute significantly to the equilibrium modulus. |
