(164s) Plasticization in Thermally Rearranged Polymers: Effect of Free Volume Elements and Chain Dynamics from Molecular Dynamics Simulations

Membrane separations are economic alternatives to energy intensive methods like distillation that are predominantly used in petrochemical industries for hydrocarbon separations. Polymers are attractive membrane materials owing to their inexpensive processing and scalability. An important feature of polymer membranes is the inefficient packing of bulky groups on the backbone that creates a distribution of connected void spaces known as free volume elements (FVEs). Thermally rearranged polymers (TRP) are a class of high performing amorphous gas separation membranes that show promise in the area of light olefin/paraffin separation due to tunable FVE distribution achievable through thermal treatment. Despite many advantages, the phenomenon of plasticization – degradation of membranes from irreversible chain reorganization, has impeded the widespread implementation of polymers beyond laboratories. FVEs are structurally transient due to the semi-flexible polymer chains, making it challenging to correlate FVE topology and chain dynamics.

Motivated by the need to understand the molecular underpinning of plasticization, we use molecular dynamics (MD) simulations to relate membrane structure to chain dynamics and flexibility. We perform all-atom MD simulations on two TRP chemistries, which have different backbone rigidities – PIM-6FDA-OH and HAB-6FDA. We vary the crosslinking density to mimic thermal treatment conditions between 350^oC-450^oC. First, we calculate the distribution of cavities in the membrane as a function of chain rigidity and crosslinking density. We capture the 3D structure of the cavities to provide a visual metric for the free volume space. To characterize how FVE structure is correlated with the polymer flexibility, we calculate backbone segmental autocorrelation function for each system. Understanding the link between FVE structure and chain flexibility will allow us to design membrane chemistries that are resistant to plasticization. Future work includes characterizing the transport of olefin/paraffin across TRP membranes using MD simulations and enhanced sampling methods.