Evolution of free volume elements in amorphous polymers undergoing uniaxial deformation: a molecular dynamics simulations study†

Amorphous polymers are considered promising materials for separation applications due to their excellent transport properties and low fabrication costs. The separation performance of a polymer membrane is characterized by its permeability and selectivity. Both permeability and selectivity are controlled by the diffusion of 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 pressures and temperatures influence their structure. In this work, we apply uniaxial tensile deformation on three polymers, polystyrene (PS), polymethylpentene (PMP), and HAB-6FDA thermally rearranged polymer (TRP), at varying temperatures and strain rates. We characterize the stress strain behavior, 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, which we attribute to the rigidity of the chains. We also find that FVEs shift to broader distributions with deformation, and the extent of this change is in line with the overall change of mechanical properties of the material.