Effect of time of annealing on gas permeation through coextruded linear low-density polyethylene (LLDPE) films

The effect of annealing on the permeation of oxygen, nitrogen, and carbon dioxide through coextruded linear low-density polyethylene (LLDPE) films is studied. The results indicate that the permeability coefficient P of nitrogen does not show a definite dependence on the time of annealing, t(a), whereas for the other gases this parameter increases with t(a). The analysis of the variation of the diffusion coefficient of O-2 and N-2 with t(a) indicates that D undergoes a sharp decrease from t(a) = 0 to t(a) = 2 h, but for larger times of annealing the diffusion parameter only undergoes a slight diminution; on the contrary, the diffusion coefficient of CO2 gradually decreases with increasing t(a). The fact that annealing increases the solubility of the gases in the polymer films suggests that thermal treatments may favor the formation of microcavities or molecular packing defects in the crystalline amorphous interface that can accommodate individual site molecules without disturbing the natural dissolution process in the rubbery region of the polymer matrix. Finally, free volume theories are not sensitive enough to interpret the effect of annealing on the permeation characteristics of coextruded LLDPE films.