Diffusion of ethyl acetate vapor in strained low density polyethylene

At a fixed vapor pressure p of the penetrant and constant temperature of the experiment, the sorption S = c / p or concentration c of the ethylacetate vapor in the uniaxially strained low density polyethylene (LDPE) increases most rapidly at low strains. If, however, on the basis of strain relaxation one separates the total strain ϵ into an elastic ϵ e , and a plastic ϵ pl , deformation, one obtains an almost linear increase of the concentration c or sorption S of the sorbate with elastic strain ϵ e . The separation of ϵ = ϵ e + ϵ pl depends very much on the time t h the sample is kept elongated and the vapor pressure p of the sorbate. The elastic component decreases and the complementary plastic fraction increases with t h and p . An almost stationary state is reached after t h of about 1/2 h . The calculation of the diffusion coefficient D s 1 from the first sorption immediately after the stretching is affected by this slow adjustment in the interval 0 ≤ t h ≤ ½ h and shows a pseudo maximum at a strain of ϵ∼ = 10 percent. The first desorption experiment and all the later sorptions and desorptions yield the same D D = D S < D S 1 that is the correctly calculated diffusion coefficient D . The coefficient D decreases with the strain ϵ or ϵ e in contrast with the expected increase of D a of the amorphous component. Such an increase of D a is expected as a consequence of the fractional free volume (FFV) increase caused by the elongation. According to the FFV concept, a decrease of the measured apparent diffusion coefficient D = ψ D a requires that with increasing ϵ, the tortuosity factor ψ decreases faster than the increase in D a .