Solvent accelerated polymer diffusion in thin films

Abstract

We have investigated the absorption of cyclohexane vapor into polystyrene films and the resulting polymer interdiffusion that is enabled by this process. The swelling of films due to solvent absorption was relatively insensitive to the film thickness or polymer molecular weight but increased with increasing temperature. The interdiffusion enabled by absorbed solvent was examined in multilayered films comprising alternating layers of deuteriopolystyrene (dPS) and hydrogenous polystyrene (hPS). Deuterium concentration gradients were measured in dry films following exposure to cyclohexane vapor at controlled temperatures and times to establish the relationship between the coefficient of solvent accelerated interdiffusion, D*, and (i) M-w of hPS, (ii) M-w of both hPS and dPS, (iii) temperature, and (iv) film thickness. Scaling relationships were established for D* as a function of molecular weight. D* &SIM; M-w(-1) for increasing hPS molecular weight for M-w(hPS) < M-w(dPS) and was independent of M-w(hPS) for M-w(hPS) > M-w(dPS). This indicates that the lower molecular weight polymer, in keeping with predictions of "fast-mode" diffusion theory, dominates the rate of interdiffusion. When both dPS and hPS molecular weights were increased simultaneously, the interdiffusion coefficient scaled with M-w(-1.8). A marked increase in D* with increasing solvent temperature was observed, consistent with the temperature dependence of film swelling. A strong and unexpected correlation between D* and total film thickness was found. This effect was attributed to prolonged solvent retention in thicker films following exposure to solvent vapor.