| Abstract: | Cast films of a metallocene Linear Low Density Polyethylene (mLLDPE) have been cold‐drawn in the machine direction in two sequential steps to form ultra‐oriented films. The initial films were cast under low shear conditions to form essentially isotropic films, as shown by very low orientation birefringence. The first draw yields moderately oriented films, which display block‐shear type chevron morphology. Under controlled conditions, void formation occurs during the second draw and the ultradrawn films whiten (become opaque), and display a fine crystalline morphology. Surprisingly, the films do not become more permeable; rather, they become high barrier films. In their ultra‐oriented state, the water vapor transmission of the films is equivalent to that of poly(vinylidene chloride) (PVDC). The transport behavior of the films to various gases was studied using transient permeation methods. The decrease in permeability with orientation is attributed to an increase in the degree of crystallinity and increase in tortuosity due to the blocky crystalline morphology. A decrease in the permeability of the amorphous phase due to an increase in the amorphous phase density is also suggested by the data. |
