CONFORMATIONS AND STRUCTURES OF POLY(OXYETHYLENE) MELTS FROM MOLECULAR-DYNAMICS SIMULATIONS AND SMALL-ANGLE NEUTRON-SCATTERING EXPERIMENTS

An ensemble of H(CH2OCH2)(12)H chains has been studied by molecular dy namics simulations as both melt chains and unperturbed phantom chains as a model system to investigate condensed phase effects on chain conf ormations of poly(oxyethylene) (POE). In addition, conformations of hi gh molecular weight POE chains in the melt have been determined by sma ll-angle neutron scattering (SANS) experiments over a temperature rang e of 347-459 K. Our simulations show that POE chains in the melt are m ore extended than the phantom chains which represent the unperturbed c hains in Theta 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 t he difference in the temperature dependence of chain dimensions betwee n melt and phantom chains are corroborated by the results of our SANS measurements when they are compared with experimental results for POE chains in Theta solution. We attribute these significant deviations in conformational properties of POE chains in the melt from those of unp erturbed ideal chains to condensed phase effects, similar to those fou nd in 1,2-dimethoxyethane (DME), a dimer molecule of POE, from both ex periments and simulations. That is, simulations show that the populati on 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 smal ler in the melt, the latter effect largely accounting for the more ext ended dimensions of the melt chains. As in DME, the conformation-depen dent intermolecular polar attractions (O ... H interactions, for examp le) account for these condensed phase effects, which become more prono unced at lower temperatures. Such intermolecular polar attractions in POE melts also result in increased interatomic packing order, but do n ot appear to enhance the intermolecular orientational order when compa red to simulation results for polymethylene melts.