Conformations and structures of poly(oxyethylene) melts from molecular dynamics simulations and small-angle neutron scattering experiments

Abstract

An ensemble of H(CH2OCH2)12H chains has been studied by molecular dynamics simulations as both melt chains and unperturbed phantom chains as a model System to investigate condensed phase effects on chain conformations of poly(oxyethylene) (POE). In addition, conformations of high molecular weight POE chains in the melt have been determined by small-angle neutron scattering (SANS) experiments over a temperature range of 347-459 K. Our simulations show that POE chains in the melt are more extended than the phantom chains which represent the unperturbed chains in Θ solution. Moreover, the melt chains exhibit a negative temperature coefficient of chain dimensions in contrast to a positive value for the phantom chains. The difference in chain dimensions and the difference in the temperature dependence of chain dimensions between melt and phantom chains are corroborated by the results of our SANS measurements when they are compared with experimental results for POE chains in Θ solution. We attribute these significant deviations in conformational properties of POE chains in the melt from those of unperturbed ideal chains to condensed phase effects, similar to those found in 1,2-dimethoxyethane (DME), a dimer molecule of POE, from both experiments and simulations. That is, simulations show that the population of the C-C-O gt conformation is greater in melt chains than in the phantom chains, while the C-C-O g±g∓ populations are much smaller in the melt, the latter effect largely accounting for the more extended dimensions of the melt chains. As in DME, the conformation-dependent intermolecular polar attractions (O⋯H interactions, for example) account for these condensed phase effects, which become more pronounced at lower temperatures. Such intermolecular polar attractions in POE melts also result in increased interatomic packing order, but do not appear to enhance the intermolecular orientational order when compared to simulation results for polymethylene melts.