Translational and Rotational Motion of Small Penetrants in AF1600 Nanocomposites

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Translational and Rotational Motion of Small Penetrants in AF1600 Nanocomposites Darryl Aucoin, Junyan Zhong, Gouxing Lin, Wen-Yang Wen, and Alan A. Jones# Chemistry, Clark University, 950 Main St., Worcester, MA, 01610

ABSTRACT Translational and rotational motions of dichloromethane were observed in a composite of poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene), also referred to as AF1600, with fumed silica using NMR. Pulsed field gradient diffusion measurements show that adding fumed silica to AF1600 results in a great enhancement of the diffusion coefficient of dichloromethane. The diffusion enhancement behavior is similar to that of previously reported pentane, cyclohexane, and toluene in AF1600 nanocomposites. Spin-lattice relaxation time measurements of this system indicate two domains: one containing larger free volume elements (FVEs) that support faster dynamics and one containing smaller free volume elements that support slower dynamics. Adding fumed silica disrupts the packing of polymer chains, which results in an increase of free volume and improved connections between domains supporting rapid transportation. The lattice model simulation is performed to assist in understanding the mechanism of how the addition of fumed silica improves penetrant diffusion in nanocomposite polymer systems.

INTRODUCTION Poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene), also referred to as AF1600, is an ultrapermeable, reverse selective, supperglassy polymer, which has important industrial application for gas separation [1]. There are two types of free volume elements (FVE) in this polymer. Positron lifetime annihilation spectroscopy (PALS) indicates that the radius of the large FVEs in AF1600 is approximately 0.52 nm [2], which is about twice the size of FVEs in conventional glassy polymers. The small FVEs have a size of approximately 0.3 to 0.4 nm [3]. It has been shown that adding fumed silica nanoparticles to supperglassy polymers can significantly enhance permeability and induce reverse selectivity [1]. Fumed silica disrupts the packing of the polymer chain, which increases the amount of accessible free volume elements, and increases the average size of these large FVEs [1]. This increase of FVE size will allow faster penetrant rotational and translational motion. The diffusion behavior inside such a nanoparticle composite polymer is complicated. AF1600 has a glass transition temperature of 160 oC [4], which means at room temperature it will be a glass and the polymer chains will move very slowly. Thus the free volume plays an important role in penetrant diffusion. The influence of increasing free volume on diffusion by nanoparticle addition, and the dependence of nanoparticle concentration upon diffusion are still difficult to predict. # Posthumous

To improve our understanding of the diffusion enhancement mechanism by nanoparticle addition, we applied pulse field gradient NMR to directly monitor the diffusion of small penetrants through the