Penetrant Diffusion in Triblock Polymer by Pulse-Field-Gradient NMR and Lattice Model Simulation
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Penetrant Diffusion in Triblock Polymer by Pulse-Field-Gradient NMR and Lattice Model Simulation Xueqian Kong, Tabitha Hargrove, Sara Ouellette, Marcus V. Giotto, Darryl Aucoin, Gouxing Lin, and Alan A. Jones# Chemistry, Clark University, 950 Main St., Worcester, MA, 01610
ABSTRACT The three versions of poly-styrene-ethylene/butylene-styrene (SEBS) triblock copolymers under investigation have different micro-scale morphologies: polystyrene (PS) spheres in polyethylene/butylene (EB) matrix, PS cylinders in EB matrix and EB cylinders in PS matrix. The diffusion phenomena of 2,2,4-trimethyl pentane (TMP) in SEBS probed with pulse field gradient NMR show distinct features related to the morphological characteristics. The micron scale size of grain boundaries may be obtained. In addition, the lattice model simulation was performed for the TMP diffusion in the membrane with spherical morphology. INTRODUCTION The poly-styrene-ethylene/butylene-styrene (SEBS) triblock copolymer has important industrial applications due to its excellent chemical resistibility, mechanical properties and low cost. The partially sulfonated SEBS has also been found to be a potentially less expensive alternative to Nafion as a permselective membrane for chemical protection and fuel cell applications [1, 2]. The block copolymers may be phase separated into spherical, cylindrical and lamellar morphologies at nano-scale and the domain size can be quantitatively predicted with self-consistent mean field theory [3, 4]. In cast film of this type of block copolymer, the film commonly consists of many small grains, in which the orientation of the EB phase changes randomly from one grain to another grain. In addition, various dislocations at the grain boundaries for the cylindrical and lamellar morphology have been reported [5]. The morphology and grain boundary structure of SEBS may have significant influence on the penetrant diffusion, which is an important factor for applications of block copolymers as membranes [6]. For the SEBS block copolymer, TMP turns out to be an excellent candidate to probe the structural influence on penetrant diffusion. The solubility and the expected order of diffusion constant for TMP in pure EB phase are 625 wt% and 10-7 cm2/s, compared to 2.85 wt% and 10-14 cm2/s in pure PS phase [6]. The combination of the solubility and diffusion differences assures that the conductive phase will be EB and PS will act as a barrier. The diffusion phenomena of TMP could be affected by several factors: the first is the segmental motion of EB midblock; the second is the nano-sized block copolymer phase separation morphology; the third is the grain boundary effect. In order to understand the relationship between penetrant diffusion and # Posthumous
structural properties, three types of SEBS with different morphologies have been investigated. The expected morphologies of them are PS spheres in EB matrix, PS cylinders in EB matrix and EB cylinders in PS matrix respectively. The segmental motion of the EB midblock was extrapolat
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