Significance of Surface Resistance in Absorption by Polymers

Solvent absorption has been followed in injection-molded samples for 13 solvents in cyclic olefinic copolymer (COC), 4 solvents in two different grades of polycarbonate (PC), and 2 solvents in the terpolymer acrylonitrile/butadiene/styrene (ABS). It was discovered that a surface resistance to absorption was significant in 19 of these 23 cases. Approximate surface mass transfer coefficients and approximate diffusion coefficients were determined where possible. There is no surface resistance to absorption where the absorbing molecules are smaller and linear. When the molecules of the test chemicals are too large or bulky, no absorption occurs. The molecules can simply not get through the surface layer, and its resistance is therefore effectively infinitely large. Between these extremes are situations where surface resistance clearly affects the absorption process. Surface resistance becomes significant when molecules can be transported away from the surface into the bulk of the polymer faster than they can be adsorbed/absorbed just at/in the surface.