EXPERIMENTAL AND MODEL DESCRIPTION OF COMBINED EFFECTS BETWEEN STRESS AND DIFFUSION IN GLASSY POLYMERS

A bending cantilever apparatus has been set up to measure the mass transport properties of gases and vapors in glassy polymers and to study the evolution of the stresses induced in the solid matrix by penetrant sorption. The mechanical and diffusive problems have been solved to obtain a model to evaluate the concentration and stress profile in the polymer during time, which allows to recover valuable information about the transport, volumetric and mechanical properties of the polymer-solvent system. The diffusion of acetonitrile in polycarbonate and of n-pentane in polystyrene at 40°C have been studied through this technique and the results compared with those obtained with more classical methods as gravimetry or FTIR-ATR spectroscopy. The results from the different techniques are in good agreements with one other, even if the kinetics obtained through the bending beam method deviates slightly from the expected Fickian behavior, suggesting a not negligible influence of the stress profile in the polymer on the diffusive flux. The method proved also to be quite sensitive to relaxation process which always accompanies the diffusion in glassy polymers; indeed the cantilever deflection initially increases during the sorption process, it reaches a maximum and then decreases due to the matrix relaxation. Also the swelling of the polymer has been measured through the same apparatus giving results in good agreement with those obtained through other experimental technique.