Knowledge-based Approach to Gas Sorption in Glassy Polymers by Combining Experimental and Molecular Simulation Technique
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1130-W06-10
Knowledge-based Approach to Gas Sorption in Glassy Polymers by Combining Experimental and Molecular Simulation Techniques Matthias Heuchel1*, Ole Hölck2+, Martin Böhning2, Martin R. Siegert1 and Dieter Hofmann1 1
Center for Biomaterial Development, Institute of Polymer Research, GKSS Research Center, Kantstraße 55, 14513 Teltow, Germany 2 Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany + current address: Micro Materials Centre Berlin, Fraunhofer Institute for Reliability and Microintegration IZM, Volmerstrasse 9B, 12489 Berlin, Germany
ABSTRACT We present a method which allows to calculate gas sorption in complex polymers where, as slow processes, gas induced plasticization and volume dilation are important factors. Since the relaxational swelling of the polymer matrix that is observed at elevated gas concentrations takes hours or days, the swelling process is orders of magnitudes too slow to simulate the respective molecular dynamics in reasonable time and effort. To address this apparent incompatibility of experiment and simulation, we use single representative reference states from experiment and construct atomistic packing models according to these specifications. Gas sorption of CO2 and CH4 was successfully calculated on polysulfone, a 6FDA-polyimide, and a polymer of intrinsic microporosity, PIM-1, at 308 K and pressures up to 50 bar.
INTRODUCTION Describing sorption of highly soluble gases (CO2) in glassy polymers often requires consideration of swelling / plasticisation phenomena. Experimentally investigated polymer/gas systems show often a two-stage sorption (and dilation) process consisting of an initial rapid stage controlled by Fickian diffusion (often completed after 1 h) and, especially at higher pressures, a much slower stage dominated by relaxational processes of the polymer matrix (often not finished after 24 h). We have already reported such experiments for polysulfone/CO2 [1]. So often, at experimental observation timescales, the systems may have not reached a thermodynamic equilibrium state which is generally a point of reference for the molecular simulation methods to predict polymer/gas properties. Such simulation methods show well agreeing results in somewhat idealized cases (see [2] as a recent example for polystyrene), but they frequently fail when applied to less moderate conditions, e.g., more complex polymers, high penetrant concentrations, large penetrants, or long time scales. Due to the current limitation of theoretical / modeling approaches, we propose here to include such “experimental information” which “represents” the experimentally observed non-equilibrium state of the system into a simulation strategy.
In the following we present investigations on three different glassy polymers: (1) polysulfone as a conventional polymer, (2) a polyimide as a high performance membrane material and (3) a polymer of intrinsic microporosity (PIM) as a high free-volume material. Based on the experimental values for density and
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