(57a) 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.