Fabrication of Architectured Multilayers with Mismatched Rheological Behaviors: Layer Stability, Structure, and Confinement Dictate Polyethylene-Based Film Properties

To better understand the processing and final properties of multilayer films obtained by layer forced assembly in coextrusion as well as their structure morphology, the present study focuses on model films with LDPE as the base polymer. More specifically, the layer confinement of LDPE by a glassy amorphous polymer (here, PC or PS) was investigated. First, the viscosity and elasticity ratios of the different neat polymers were measured by rheological tests to simulate the processing conditions in the feedblock and multipliers during coextrusion. These results together with the observation of film transparency at the macroscopic scale and the layer breakup phenomena between layers at the microscopic scale enabled us to build a comprehensive stability map rationalizing the conditions required for a well-controlled multinanolayer architecture. Second, the morphology of the coextruded films was analyzed by SEM and TEM. The onset of the layer breakup in the LDPE/PS system was determined at 2048 layers with a layer thickness of 95 nm, while in the LDPE/PC system, it was at 256 layers with a layer thickness of 980 nm. The layer breakup happened at a fewer number of layers for the LDPE/PC system due to the viscoelastic mismatched properties between the base polymers. Interestingly, we have demonstrated that it is nonetheless possible to prepare some nanolayer structures with 16,380 layers of the PS/LDPE system with some defects but still maintain an overall property improvement despite their high mismatched viscoelastic properties. Finally, the orientation and crystalline structure of the coextruded films were characterized by 2D-WAXS and DSC, and the ultimate properties of the films were determined through tensile testing. The geometrical confinement of the LDPE nanolayer did not affect the thermal crystalline properties of LDPE chains, but it affected the crystalline morphologies as well as the final mechanical response of the obtained multilayer films.