Effects of Molecular Structure on Physical Properties of Butyl Rubber

Abstract

Abstract The investigation was undertaken in an attempt to establish the fundamental connections between the physical properties of a typical vulcanized rubberlike polymer and its chemical structure. The structural variables to be considered are the molecular weight of the “primary molecules” entering the vulcanizate, their molecular-weight distribution, and the concentration (or frequency) of cross-linkages introduced during vulcanization. The molecular weights of Butyl rubbers were determined by previously established procedures ; the effects of molecular-weight heterogeneity were suppressed by careful fractionation from very dilute solution. An indirect method, based on the theory of gelation and on the observation of critical molecular weight for incipient gelation (partial insolubility) in “vulcanisates” formed when the cross-linking capacity is fixed, was employed to determine the frequency of occurrence of cross-linked units—a quantity not hitherto evaluated in a vulcanized rubber. In representative pure-gum vulcanizates of Butyl the molecular weight per cross-linked unit ranges from about 35,000 to 20,000, depending (inversely) on the diolefin content of the raw rubber. Micro compounding and testing procedures have been devised for evaluating the necessarily small samples ob- tained in fractionation. Complete evaluation of tensile strength, stress-strain characteristics, swelling in solvents, and creep rate can be obtained with as little as 3 grams of rubber. Results are no less reproducible than those obtained with conventional procedures requiring 50 grams or more. A number of rela- tionships between vulcanizate structure and physical properties have been established. The feasibility of a rational approach to the interpretation of properties of rubber vulcanizates in terms of molecular structure has been demonstrated.