Melt-processed polyethylene nanofilms via layer multiplying coextrusion: Exploring scaling laws relating tensile properties to intrinsic and extrinsic factors

Polymer nanofibers typically show a sharp rise in moduli and strength below some critical diameter (~100nm to 10µm). It has been hypothesized that a combination of increased molecular orientation and decreased surface flaws contribute to the observed scaling laws. However, relations for polymer nanofibers have come primarily from electrospun fibers where the diameters of the fibers are directly linked to chain orientation via extensional flow parameters. Thus, surface effects impacting deformation in the confined state may be difficult to fully decouple. Here, we use an alternative approach to produce nanofilms of high-density polyethylene (HDPE) of varying thickness and crystalline morphology. The melt-based process relies on coextrusion of HDPE with a sacrificial layer, ethylene vinyl alcohol (EVOH). Layers of varying dimension are achieved using a series of layer multiplying (LM) dies which split and stack the melt flows. After solid-state uniaxial orientation of the extruded composite, the EVOH layers are selectively dissolved to yield HDPE nanolayers. Independent control of the molecular orientation (via extent of uniaxial orientation) and layer dimension (via number of LM dies) allows a platform to study the intrinisic and extrinsic factors dictating tensile properties.