  Amorphous polymers are considered promising materials for separations due to
their excellent transport properties and low fabrication costs. The separation
performance of a membrane material is characterized by its permeability
(overall throughput of components), and selectivity (efficiency of separation).
Both permeability and selectivity are controlled by the diffusion of different
penetrants through the matrix, which is strongly influenced by the distribution
and morphology of the free volume elements (FVEs). FVEs are void spaces in the
polymer matrix that result from the inefficient packing of bulky and rigid
groups on the polymer backbone. Thus, FVEs dictate the efficiency of membrane
polymers, and it is imperative to understand how processing conditions such as
high pressure influence their structure. In this paper, we apply uniaxial
tensile deformation on three polymers, namely polystyrene (PS),
polymethylpentene (PMP), and HAB-6FDA thermally rearranged polymer (TRP), at
varying temperatures and strain rates. We calculate the stress strain curve,
tensile modulus, and free volume element evolution at these conditions. We find
that PMP and PS with low and moderate glass transition temperature,
respectively, exhibit the most change in mechanical properties as a function of
strain rate and temperature. The properties of TRP, however, do not vary as
much. We also find that FVEs become larger with deformation, and the extent of
this change is in line with the overall change of mechanical properties of the
material.
