Increasing the Rigid Amorphous Fraction In Semi-Crystalline Homopolymers by Solid-State Shear Pulverization Leads to Major Reductions In Oxygen Permeability

Abstract

In semi-crystalline polymers, a rigid amorphous fraction (RAF) exists at the interface of crystal and amorphous phases as a result of the immobilization of the polymer chains due to the crystal. Previous studies have shown that increasing RAF fraction in the homopolymer can have a significant impact on physical properties.  Here, we show for the first time that the RAF content in semi-crystalline homopolymers can be significantly increased by a process called solid-state shear pulverization (SSSP), which employs a modified twin-screw melt extruder that is cooled rather than heated, and thus results in solid-state processing. Following SSSP, overall crystallinity remains constant, while RAF content increases, leading to major reductions in the oxygen permeability coefficient. For example, the RAF content of nylon 11 compression molded films increases from ~ 16% for the pellet (no exposure to SSSP) to ~ 37% for the pulverized sample, which leads to a ~ 40% reduction in the oxygen permeability coefficient after SSSP processing. Similar results have been demonstrated with several other semi-crystalline homopolymers. The origin for these large decreases in oxygen permeability is due to the major reduction in crystallite size of the SSSP processed material. The formation of smaller crystallites allows for a larger interface between the crystal and amorphous phases, ultimately resulting in greater immobilization of the amorphous polymer chains. Polarized optical microscopy images demonstrate that crystallite sizes decrease ~ 10 fold following pulverization. These results demonstrate that SSSP processing of homopolymer prior to melt-processing can be very beneficial for creating high oxygen barrier films, which is important for packaging applications.