From the relations between Tg's of the component homopolymers with that of the resulting copolymer, one sees that the limit of the Tg of copolymers is that of the homopolymers. To obtain a low Tg one must avoid polymers with polar groups and/or bulky groups. Polar groups afford opportunities for intermolecular links which hinder rotation necessary for “rubberyness.” Bulky groups cause stiffness, and also serve as obstructions to contractions. On the other hand, the same reasons that polar and bulky groups raise Tg serve to lower overall thermal contraction, since αr is always greater than αg. Fillers can be used to lower contraction, and new ceramic fillers with negative coefficients of expansion might be combined with specific polymers to obtain a material with a desired contraction. The authors feel that the data presented here, together with design data available from commercial sources, should enable the design engineer to find many new uses for elastomers at cryogenic temperatures.

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The study of elastomeric seals at cryogenic temperatures has moved through the stages of initial development and functional test¬ ing!! 1 1 into a more analytical and fundamental program.At present the program consists of four experiments, one of which involves the direct measure of leak rate through a seal.The other experiments are designed to study the low temperature behavior of elastomeric polymers in terms of thermal expansion, resilience, force-tempera¬ ture relationships, and differential thermal analysis .