Structure and transport properties of polyethylene terephthalate and poly(vinylidene fluoride-co-tetrafluoroethylene) multilayer films

The morphologies of two crystalline polymers, polyethylene terephthalate (PET) and poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDF-TFE)], were probed under nanolayer confinement using forced assembly multilayer film coextrusion. This multilayer system was used as a platform to investigate the effect of nanolayer coextrusion, biaxial stretching, and isothermal melt recrystallization on the confined morphologies of both of these polymers. To determine the effect of each of these variables independently, three sets of PET/P(VDF-TFE) multilayer films were produced, each with comparable film thickness and layer thickness. The morphology and X-ray data of the extruded PET/P(VDF-TFE) multilayer films, which were taken directly from the coextrusion process, indicate that the morphologies of both PET and P(VDF-TFE) were relatively unaffected by nanolayer confinement, even in very thin 40 nm layers. Biaxial stretching of multilayer films, produced from stretching micron thick layers down to nanolayers, facilitated the development of an on-edge P(VDF-TFE) crystal orientation in addition to an oriented PET fibrillar crystal structure. Finally, an approach of isothermal melt recrystallization was conducted on the biaxially stretched samples which revealed the formation of high aspect ratio in-plane P(VDF-TFE) crystals under nanolayer confinement while also further crystallizing the PET fibril crystals. Therefore, in the same multilayer system, three P(VDF-TFE) crystal orientations were achieved by utilizing nanolayer confinement, biaxial stretching, and isothermal melt recrystallization. Oxygen permeability was used as an additional structural probe for these confined PET and P(VDF-TFE) layer morphologies. From the transport data, it was determined that the PET layers possessed similar oxygen transport characteristics to the bulk materials, which was in good agreement with the morphology data of the PET layers in the various PET/P(VDF-TFE) multilayer films. In contrast, the on-edge P(VDF-TFE) orientation induced from biaxially stretching and in-plane P(VDF-TFE) crystal orientation induced from isothermal melt recrystallization of confined P(VDF-TFE) nanolayers yielded substantial reductions in the effective oxygen permeability of the P(VDF-TFE) layers in comparison to the bulk P(VDF-TFE) control. The various confined P(VDF-TFE) crystal orientations and subsequent enhanced barrier properties are enabled by the hard confinement of the PET nanolayers during biaxial stretching at high draw ratios and isothermal melt recrystallization at high temperatures. Finally, the water vapor transport rate (WVTR) was evaluated for these confined systems which mimicked the trends observed for oxygen permeability. The confined nanolayer morphologies, specifically the in-plane P(VDF-TFE) crystals, substantially reduced the WVTR in multilayer films opening new applications for this technology.