Free volume and polymeric structure analyses of aromatic polyamide membranes: A molecular simulation and experimental study

Molecular simulation techniques were adopted to investigate the membrane free volume morphologies and alcohol sorption behaviors of aromatic polyamide (PA) membranes for pervaporation applications. A molecular dynamics (MD) technique was adopted to analyze the fractional free volume (FFV), the fractional accessible volume (FAV), and the size and shape distributions of the free volume. Experimental FFV results from Bondiʹs group contribution method and positron annihilation lifetime spectroscopy were also adopted to compare with the simulation data. The radial distribution function was also used to analyze the polymer chain stiffness and mobility. A Monte Carlo (MC) method was used to analyze the membrane absorption behavior. The FFV, FAV, and free volume morphology analyses revealed that bulky groups in the PA membranes contributed to the formation of a larger and more continuous free volume. The sorption analysis indicates that high carbon number alcohols have a higher affinity with the membranes, which might lead to a higher attractive force, reducing the permeability. The MD and MC results demonstrate good agreement with the experimental data, validating the feasibility of molecular simulation techniques in PV membranes at the molecular scale.