Mechanical Properties of Substances of High Molecular Weight. X. The Relaxation Distribution Function in Polyisobutylene and Its Solutions

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Abstract
Previous twin transducer measurements of the dynamic rigidity and viscosity of a polyisobutylene sample of viscosity-average molecular weight 1.2 million have been extended over a wider temperature range, and previous wave propagation data on solutions of this polymer in xylene have been extended by transducer measurements at lower concentrations. Values of the relaxation distribution function are derived from all these data and are compared with values obtained from stress relaxation measurements on solutions of the same polymer in Decalin, as well as stress relaxation data of Andrews and Tobolsky on solid polyisobutylene and dynamic measurements on butyl rubber from several sources. When reduced to a common reference state at 25°C by the assumption that all relaxation processes depend identically on temperature, the data for solid polyisobutylene provide a picture of the distribution function over eleven cycles of logarithmic time. It appears to have a plateau from 0.1 to 104 sec with a sharp rise at shorter times and a sharp drop at longer times. From 10−5 to 0.1 sec the functions for polyisobutylene and butyl rubber are similar in shape, the latter being somewhat higher. When data for polyisobutylene solutions from 5 to 25 percent concentration are reduced to the reference state of the solid polymer by the assumption that all relaxation processes depend identically on concentration, a single distribution function is obtained which from 10 to 106 sec is similar in shape to that derived from direct measurements on the solid polymer but lies somewhat below it.