Diffusion of polar and non-polar molecules in a polymer membrane

Abstract

Diffusion of small molecules of different polarities in polymeric membranes is the most interesting in terms of studying thermodynamic and kinetic parameters of the separation process of liquid multicomponent organic mixtures. In the paper, the principle emphasis is made on diffusion mechanisms of polar and non–polar molecules in flexible, non–polar organic–silicon polymers. For estimating the diffusion rate and determining diffusion mechanisms, the experiments on adsorption and desorption of a number of samples at constant temperature and pressure have been conducted. Dynamics of the curves shows possible kinetic processes, occurring in polymer at its dissolution into non–polar solvents differing in a molecular size. The similar studies have been carried out using polar solvents with different dipole molecular moments. It has shown that the limited dissolution rate for polar solvents compared to non–polar ones is low. It has been found that the principal mechanism of diffusion in the polymer under its dissolving in polar solvents is the Fickian diffusion. Herewith, at the beginning of a sorption process the diffusion runs very fast, and then the process slows down significantly and a macromolecular matrix manages to complete relaxation, not allowing the solvent molecules to deform the lattice further. In the process of dissolving the polymer in solvents with non–polar molecules the main mechanism of diffusion is not the Fickian diffusion. Moreover, a more low–molecular solvent causes a high degree of swelling that is associated with a higher mobility of molecules and the possibility of their rapid penetration into the polymer depth at the beginning of dissolving. A long time “tail” while desorbing the polymer, dissolved in non–polar solvents shows that there is a second diffusion type. There are obvious micropores in the polymer, through which small non–polar molecules can be trapped and can form clusters with polymer alloys. When desorbing the temperature of carrying out the process is not sufficient for activating such centers.