Diffusion‐controlled stress relaxation in polymers. III. Stress relaxation in a swelling polymer

An approximate theory of the relaxation of stress in amorphous linear polymers accompanying sorption of a low molecular weight penetrant is worked out on the assumption that the relaxation time of each Maxwellian relaxation mechanism involved is changed in the presence of penetrant by a factor dependent on penetrant concentration. A method is derived from the theory which permits approximate evaluation of the integral diffusion coefficient D̄ of the penetrant in the polymer from stress‐relaxation data on swelling systems. The theory is checked on experimental data for the systems polymethyl acrylate‐water and polymethyl acrylate‐methanol at 40°C., and it is found that, for both systems studied, the values of D̄ computed from mechanical data agree reasonably with those evaluated directly from usual sorption experiments. The theory described presents only a first attempt to the quantitative interpretation of the phenomenon discussed. Further improvement and modification of it is apparently desirable, in view of some drastic approximations incorporated in its mathematical development.