(122c) Physical Aging of Glassy Polymers In Confined Environments

Amorphous polymers below their glass transition temperature (Tg) are almost always in a non-equilibrium state.  They spontaneously undergo a densification process known as physical aging, which changes many of their physical properties, including gas permeability, modulus, refractive index, and enthalpy.  Over the past decade, repeated observations have shown that thin, free-standing polymer films (<1 μm in thickness) can undergo physical aging much more rapidly than thicker “bulk” samples.  This is of great importance in many areas, including gas separation membranes, which often utilize a glassy skin layer of ~50-100 nm in thickness to effect the separation.  Other studies of physical aging in confinement have shown both accelerated and retarded aging in confined polymers depending on the nature of the polymer-substrate interactions.  The fundamental reasons why aging can be dependent on thickness are not completely understood.

A novel co-extrusion process, using an extruder with two feed streams and several layer-multiplying dies, was used to produce films of polysulfone (PSF) and a rubbery co-layering material.  The co-layering material does not undergo physical aging (since it is above its Tg) and merely provides confinement for the PSF layers.  Two sets of films were made: Dow Engage 8100 (EO, an ethylene-1-octene copolymer) was used as the rubbery co-layering material in the first set, and Dow Infuse 9007 (OBC, an olefin block copolymer) was used in the second set.  The films have PSF layer thicknesses ranging from ~180-500 nm, but have overall thicknesses in the range of 50-100 μm.  The O2, N2, and He permeabilities of these multilayered films were measured over time to determine the effect of layered confinement on physical aging.  Prior to study, the films were annealed for 10-15 minutes at 10-15 °C above the Tg of PSF (PSF Tg~186 °C) and then quenched to room temperature to define a starting point for the aging studies. 

The results of experiments with these films show that the rate of decline in permeability for all layered films is similar to that of bulk films, regardless of PSF layer thickness, co-layering material, or penetrant gas.  For free-standing PSF films with overall thicknesses similar to the PSF layer thicknesses in our films, physical aging would have been expected to proceed much more rapidly.  We believe that the lack of free surfaces and possible chain entanglement at the PSF-rubber interfaces in layered films restricts the rapid, near-surface relaxation that can occur in thin polymer films.