Properties and structure of elastomers

A study of the effect of structure on the dilatometric glass transformation temperatures, T g , of poly(vinyl alkyl ethers) of reasonably high molecular weight showed that the T g values of poly(vinyl n ‐alkyl ethers) decrease with increasing length of the n ‐alkyl group from methyl to n ‐octyl. On lengthening the n ‐alkyl group, 14% of the specific volume increase is free volume. Branching of the alkyl group of the polymer increased the T g value. A comparison of poly (vinyl n ‐alkyl ethers) and polymers of normal a‐olefins showed that the ether group and methylene group in side chains are equivalent in influencing the glass transformation temperatures. The side chain alkyl group in poly(vinyl alkyl ethers) was varied systematically and the influence of these variations on the dynamic mechanical properties of their vulcanizates was evaluated at 31, 60, and 98°C, The relative position of the curves, relating the dynamic resilience to the dynamic modulus of these polymers, was approximately in the order of their glass transformation temperatures. Dynamic mechanical property data on poly(vinyl n ‐pentyl ether) and poly(vinyl 2‐ethylhexyl ether), which have the same T g values, fall on a common curve characteristic of the test temperature. Apparently, the T g is a major factor in correlating the dynamic mechanical behavior of these elastomers. The size and shape of the alkyl group appear to be reflected primarily in their effect on the T g . A direct approach was made for testing an existing hypothesis on the requirement of labile crosslinks for attaining high tensile strength in rubber vulcanizates. This direct approach involved the modification of the labile polysulfidic crosslinks already in existence in natural rubber‐sulfur‐diphenylguanidine vulcanizates to relatively less labile disulfidic or nonlabile monosulfidic crosslinks by reaction with triphenylphosphine. It was demonstrated that highly labile polysulfidic crosslinks are not essential for achieving high tensile strength in natural rubber vulcanizates. Data for the samples which had lost significant amounts of polysulfidic crosslinks by reaction with triphenylphosphine fitted the tensile strength versus 300% modulus curve for the control samples.