Segmental Dynamics of polymers in 1 and 2nm slit-pores
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P9.9.1
Segmental Dynamics of polymers in 1 and 2nm slit-pores V. Kuppa and E. Manias Department of Materials Science and Engineering, The Pennsylvania State University, 325-D Steidle Building, University Park, PA 16802, USA ABSTRACT Molecular dynamics computer simulations are used to study the structure and dynamics of 1nm wide films of poly(ethylene oxide)(PEO) confined between mica type layered silicates of different cation exchange capacities(CEC). The simulation setup mimics the experimental systems formed by intercalation of PEO in silicate clays with differing numbers of inherent charges. Counterbalancing lithium cations are present in the interlayer gallery to offset the negative charge on the silicate platelets created by the isomorphic substitution of trivalent aluminum by divalent magnesium ions. It is seen that the presence of the lithium has a significance influence on the behavior of the system, with PEO segmental motion strongly dependent on the number of Li+ present in the slit pore. In particular, the coexistence of fast and slow modes for C-H bond dynamics is heavily affected due to the strong interaction between the Li+ ions and partially charged PEO oxygen. In conjunction with local density inhomogeneities, the coordination between lithium and oxygen is responsible for the anomalous segmental dynamics of nanoconfined PEO. INTRODUCTION Polymer/layered-silicate nanocomposites have become an attractive set of organic/inorganic materials, not only for their obvious potential as technological composites, but also for providing convenient macroscopic systems to study the fundamentals of nanoscopically confined polymers[1]. One promising way to synthesize polymer nanocomposites is by intercalating polymers in layered inorganic hosts[2, 3, 4, 5]. The structure and properties of the resulting nanocomposite can be conveniently mediated by controlling subtle guest-host interactions. Here, the focus is on mica-type layered silicates in which the polymers are confined in well-defined nanoscopically wide (0.87nm) slits which are formed by self assembly of negatively charged parallel stacks of alumino-silicate layers[6, 7, 9, 8, 10, 11, 12]. These inorganic crystalline layers can be naturally occurring or synthetic and are typically 0.97nm thick and several ยต in lateral dimensions. In order to investigate the fundamental physics behind the experimentally observed phenomena, molecular dynamics(MD) computer simulations have been performed to directly observe the relevant atomistic motions. Here, we perform a molecular modeling study on PEO intercalated in clays of different inherent charges, so as to comparatively explore the effects of intercalation and the role of the counterbalancing lithium ions on the system properties.
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SIMULATION DETAILS Molecular Dynamics simulations of fully atomistic models of PEO oligomers intercalated in mica type layered clays were performed. Full details of the simulation method, and the force fields employed are given elsewhere[9]. Simulations were carried out for four dif
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