POLYETHYLENE EXPOSED TO LIQUID PROPANE - SORPTION AND PERMEATION KINETICS AND MECHANICAL-PROPERTIES

Data obtained by sorption and permeation measurements of liquid propan e in medium-density polyethylene were analyzed with an analytical diff usion model, assuming a constant diffusion coefficient, and a numerica l model, based on the free volume theory of Fujita. The stress-strain properties of propane-swollen medium-density polyethylene were recorde d in an Instron tensile tester. The sigmoid sorption curves (modeled h ere for the first time) were adequately described using nonstationary boundary conditions. The numerical model was capable of representing s orption, desorption, and permeation data, whereas the analytical techn ique gave satisfactory results only for sorption/desorption data in th e initial, transient stage. The difference between diffusion coefficie nts determined from sorption and desorption measurements and the time dependence of the modulus-solute concentration relationship can be exp lained by assuming a relaxation of constrained amorphous chain segment s. The penetrant-induced structure rearrangement is, according to diff erential scanning calorimetry measurements, confined to the amorphous phase.