Permeability to liquids of polyethylene and irradiated polyethylene

Permeability constants at four temperatures for twelve organic solvents in three polyethylenes and one electron‐irradiated polyethylene are reported, together with some swelling data, and compared with data on N 2 , O 2 , CO 2 , and CH 3 Br. Irradiation appears to increase permeability and solubility coefficients for large penetrants at low temperatures, decreases the permeability coefficients of small penetrants at all temperatures, has little effect on the solubility coefficients of small penetrants, and tends to decrease both permeability and solubility coefficients of large penetrants at high temperatures. In almost every case irradiation decreased both E p and P 0 {article}{empty}{document}(P = P₀ e- Eₚ /RT );{document} and there exists a temperature, T (inversion) = Δ E p / R (Δ ln P 0 ) at which the permeability constants of a penetrant in polyethylene and irradiated polyethylene are equal. These inversion temperatures tend to increase with increasing polarity of the penetrant. To account for these observations, it seems sufficient to assume that irradiation decreases crystallinity and increases crosslinking, that an increase in polymer crystallinity is accompanied by an increase in the quasicrystalline‐like character of amorphous regions, and that small penetrants may dissolve, and diffuse, through the crystalline regions of polyethylene. It is postulated that activated diffusion in polyethylene induces structural changes in the polymer not unlike local melting. An explanation is given for the observation that flexible molecules tend to have small frequency factors compared to rigid molecules of similar activation energy. And a simple, particle‐in‐a‐box model is proposed for diffusion of small penetrants in a crosslinked polymer. This model predicts that irradiation should leave E p unchanged, and should decrease P 0 , by nearly the same factor for all penetrants at all temperatures, very much as observed for N 2 , O 2 , and CO 2 .