Thernial Expansivity and Relaxational Behavior of Amorphous Polymers at Low Temperatures

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Abstract
A linear dilatometer is described. It differs from one previously employed by us by making possible the use of thin specimens. The linear thermal expansivity is obtained in the range 7 ≤ T ≤ 350 K with an accuracy of ± 1 × 10−3(K)−1. Three widely different families were studied, namely polystyrene and two poly(methyl styrenes), four tough so-called engineering plastics and six esters of poly(methacrylic acid), including the series of five to eight-membered saturated ring polymers. In all but one of these systems, at least one sub-Tg relaxation region is observed. Excellent agreement between these and relaxations detected by dynamic techniques is obtained, with appropriately enhanced resolution by the slower dilatometry. In order to make meaningful comparisons we adopt a semi-empirical method for computing the temperature-frequency shift. This, it is shown, can be based on an Arrhenius equation with a universal value of the pre-exponential factor. The effective frequency of the dilatometer turns out to be about 10−3 Hz. Relaxation processes associated with sidechain motions are dilatometrically more intense than others, as seen by a comparison of the engineering plastics with the cyclo-methacrylates. Finally we apply the low temperature equation of state, derived and discussed by Nanda and us in connection with the present experimental data, to our earlier measurements. These involved primarily homologous series of alkyl methacrylates and vinyl ethers. Again a low temperature mastercurve, identical with that for our present polymers, results. Exceptions in the pattern for some methyl containing polymers are discussed.