Molecular sorption mechanism of solvent diffusion in polymers

Some experimental results from the literature are difficult to explain by t he existing diffusion theories using a concentration gradient or a swelling rate. A new diffusion mechanism is proposed. in which the driving force fo r diffusion is the sorption of penetrant molecules onto polymer chains. Res istance of the matrix to the Row of penetrant opposes this force. Both forc es are a direct consequence of molecular interactions. The interaction betw een the solvent molecules can be expressed by viscosity eta and by surface tension T-s. The interaction of solvent with polymer matrix can be describe d by wettability (or contact angle alpha). The molecular structure of the p olymer and its density fluctuations (average capillary radius r) are also i mportant. Swelling of the matrix will change the molecular geometry and thu s solubility. The rate of diffusion can be expressed as nu = (T-s/4 eta )rc os alpha 1/(x(0) + B) and the diffusion distance as x(0) = B((kr + 1)(1/2) - 1). where B and k are diffusion constants. The concentration dependence o n distance and time is well described by an empirical equation C/C-0 = exp (-(x/x(0))(ax0)), where constant a describes the sharpness of the diffusion front. This model is applicable to swelling or non-swelling polymers, gels and por ous media and can so far explain all the observed features of the diffusion process. (C) 2001 Elsevier Science Ltd. All rights reserved.