This work is the first and unique comparative study on a so wide range of vapour activities and a so vast number of different techniques, giving important insight into the transport phenomena themselves and on how results are affected by the specific technique used. Results are demonstrated and discussed in detail on the basis of a pair of penetrants with different physico-chemical properties and molecular shape (toluene vs. heptane). Information about differences between concentration dependent permeation and sorption, coupling effect, molecular clustering and polymer–solvent interactions is reported as well.
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.