Applications of Infra-Red Methods in the Structural Examination of Synthetic Rubber

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Abstract
The physical properties of a synthetic polymer are associated with variations in molecular structure which are dependent on the conditions of the polymerizing reaction and the monomers used. In some cases, structural differences of significance for physical properties cannot be detected by infra‐red methods because the relative number of chemical linkages affected is too small. An important structural detail which can be followed by infra‐red analysis is the relative amount of 1,2 and 1,4 polymerization occurring in polymerization reactions of butadiene. Absorption curves are reproduced to show the wide range in the relative amounts of these two structures. A description is given of an attempt to obtain a quantitative measure of this ratio by means of a calibration curve derived from known mixtures of pure octene 1 and octene 2. For polyisoprene, variations in the proportions of 1,4 and 1,2 or 3,4 structure also occur depending upon the type of polymerization. Comparison with Hevea rubber and Balata fails to disclose definite evidence of trans‐isomerism in synthetic polyisoprene. Structural differences due to oxidation of polymers may be readily apparent in infra‐red spectra, hydroxyl and carbonyl groups being especially prominent. The effectiveness of anti‐oxidant in preventing structural changes caused by oxidation is shown in a series of absorption curves for samples of GR‐S with and without anti‐oxidant. When the samples were heat‐treated in air, pronounced structural changes occurred for the sample without anti‐oxidant, but no perceptible changes were evident for the sample with anti‐oxidant. On the other hand, the anti‐oxidant used (phenyl‐β‐naphthylamine) was ineffective for stabilizing the structure toward ultraviolet light.

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