2022 Annual Meeting

The Effect of Mechanical Deformation on the Distribution and Morphology of Free Volume Elements in Glassy Polymer Membranes

Polymer membranes serve important roles in separation operations, including water desalination and hydrocarbon separations. A primary factor that defines the performance of a polymer membrane in these operations is its diffusion dynamics, including permeability, selectivity, and diffusivity. These qualities are facilitated by the morphology of free volume elements (FVEs) within the membrane. FVEs are void spaces within the polymer matrix that are a result of inefficient packing of bulky groups along the polymer backbone. When routinely subjected to high operating pressures, these membranes and their FVEs deform, which impacts their mechanical stability and performance. Thus, the relationship between membrane deformation and the evolving distribution of FVEs can provide valuable insight into a membrane’s potential to operate under high pressures.

To investigate this, we employ molecular dynamics simulations and quantify the evolution of FVEs as a function of external strain rate. We use an entirely open-source workflow to build, deform, and analyze glassy polymers from all-atom simulations. Specifically, we consider three chemistries that represent a broad range of structures, allowing us to understand the interplay between membrane chemistry and mechanical properties. These are polymethylpentene (PMP), polystyrene (PS), and HAB-6FDA thermally rearranged polymer (TRP). Each system is subjected to non-equilibrium uniaxial tensile deformation under three different strain rates. We first calculate the stress-strain curves of each system until fracture. Next, we capture the evolution of FVEs quantitatively through the void distribution and qualitatively through snapshots of the voids at regular intervals throughout the deformation period. Relating the mechanical deformation of polymer membranes to the evolving free volume elements can guide the rational design of enhanced, high-performance, mechanically robust polymer membranes for future separation applications.