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.
Tapered disk thermal fatigue tests have been conducted on one cobalt- and one nickel-base superalloy for the purpose of characterizing crack growth behavior. It is shown that linear elastic fracture mechanics can be used to represent the crack growth rate data, and that the propagation mode prefers favorably-oriented carbides.
Infrared absorption spectra were used to study the structural changes resulting from oxidation in a number of unvulcanized, purified polymers under a variety of experimental conditions. The results showed that as oxidation progressed, the structural changes which were detected were qualitatively the same for all of the polymers under all of these experimental conditions. Correlation of volumetric measurements with spectral data indicated that considerably more oxygen was absorbed by the polymer samples before structural changes could be detected than the minimum amount required for the initial detection of the hydroxyl and carbonyl structures. From the interpretation of information from the absorption spectrum, it is believed that the structural evidence is in substantial agreement with current theories on autoxidation. Although the initial hydroperoxide formation is not observed in the spectrum, it is quite probable that this reaction takes place with almost immediate decomposition which results in a net hydroperoxide concentration so low as to make infrared methods insensitive for its detection. Correlation of physical properties with structural studies indicate that little change in these properties take place until the hydroxyl and carbonyl structures become evident in the infrared spectrum.
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