Comparative study of sorption and permeation techniques for the determination of heptane and toluene transport in polyethylene membranes

The establishment of specific concentration profiles inside the LDPE membrane during permeation and sorption experiments evoke time-dependent structure changes of the LDPE matrix and such chain reorganisations consequently influence the transport parameters and they are also related to a certain form of high concentration-evoked molecular aggregation (clustering). X-Ray diffraction measurements excluded that the observed phenomena could be due to significant irreversible changes in the crystallinity of the material. Toluene and heptane favourably influence one another's transport and consequently both sorption and permeation experiments reveal a distinct coupling effect. Permeation experiments show a gradual increase of the integral diffusion with increasing vapour activity whereas VPC and gravimetric sorption analysis find a maximum around activity a = 0.7. This difference is due to the deviation from Fickian behaviour at high activity and to fundamental differences in the measurement principle.