Structural relaxation in poly(ethyleneoxide) and poly(ethyleneoxide)–sodium iodide systems: a molecular dynamics study

Abstract

Results of molecular dynamics simulations of poly(ethyleneoxide) and poly(ethyleneoxide)–sodium iodide melts are presented. In particular we focus on the segmental motion of the polymer backbone. It is argued and verified that a unified atom model accounts adequately for the long time dynamics of the polymer segments in the pure PEO system. The mean square displacement of the segments displays a power law behaviour with respect to time 〈Δ r 2( t))〉∼( Dt) α . The temperature dependence of the coefficient D can be described by a Vogel–Tamman–Fulcher (VTF) law at low temperatures. At higher temperatures Arrhenius behaviour is recovered. The self-intermediate scattering function F s( k, t) has been computed for the pure PEO-system and two PEO x –NaI systems with x=8.67 and x=27.8 respectively. The long time decay of F s( k, t) can be described accurately by a stretched exponent exp(−( t/ τ k ) β k ), with a temperature and wavelength ( k-) dependent relaxation time τ k . The exponent β k is independent of temperature and depends only weakly on k. The long wavelength behaviour of F s( k, t) can be explained in terms of the Gaussian approximation: F s( k, t)=exp(− k 2〈Δ r 2( t))〉). This implies β k = α and τ k ∼ k −(2/ α) in the limit of vanishing wave-number k. Mode coupling models provide a suitable framework for the interpretation of F s( k, t). Addition of salt slows down segmental relaxation and stiffens the polymer backbone. Experimentally however the increase in relaxation time is much larger than in our simulations. We attribute the discrepancy to the neglect of manybody polarization effects in our force model.