Diffusion‐controlled stress relaxation in polymers. II. Stress relaxation in swollen polymers

Abstract

Stress relaxation in the systems PVAc (polyvinyl acetate)‐water, PVAc‐methanol, PMA (polymethyl acrylate)‐water, and PMA‐methanol was measured at several temperatures above the glass‐transition points of the pure polymers in ranges of sufficiently low concentrations of diluents. It was found that the time‐concentration superposition principle as proposed by Ferry is applied successfully to these data to obtain master relaxation curves of the pure polymers at fixed temperatures. The master relaxation curves so obtained at different temperatures were further superposed with each other by horizontal shift along the log time axis, in agreement with the time‐temperature superposition principle. The concentration shift factor ac as a function of diluent concentration C at fixed temperature was well interpreted in terms of the modified Doolittle's free volume equation for the viscosity of supercooled systems on the assumption that the fractional free volume of the system increases linearly with C. The proportionality coefficient, β, of this linear relation provides a measure of the plasticizing effect of a given diluent on a given polymer. It was found that β is practically independent of temperature at sufficiently small C and is larger for water than for methanol in both polymers studied. On the basis of Doolittle's viscosity equation modified to account for the concentration effect, it is shown that the glass‐transition temperature Tg of a given polymer‐diluent pair decreases with C in a linear fashion in the range of small C. The value of β can also be calculated from the slope of this linear relation. Values of β determined in terms of this procedure from literature data on a variety of polymer‐diluent pairs are given in a tabular form. For a given polymer the β values generally become smaller as the size of diluent molecule is increased, but no unique relation can be obtained between the two quantities. Equilibrium sorption and integral sorption processes were studied on the four pairs of polymer and diluent mentioned above, not only to obtain data needed for analyzing the present stress‐relaxation data but also to find information which will serve a study described in Part III of this series which is concerned with diffusion‐controlled stress‐relaxation in amorphous linear polymers. Some correlation (though not definite) appears to exist between the diffusion coefficient D (at zero concentration) and the plasticizing parameter β for a given polymer. This may be expected because both D and β are quantities depending on the frequency at which polymer segments jump cooperatively in the presence of a low molecular weight diluent.