INVESTIGATION OF A POLY(OXYETHYLENE) CHAIN BY A MOLECULAR-DYNAMICS SIMULATION IN AN AQUEOUS-SOLUTION AND BY LANGEVIN DYNAMICS SIMULATIONS

Molecular dynamics simulations of poly(oxyethylene) (POE) in water are used as a reference to examine the effect of different friction coefficients in Langevin dynamics with respect to properties like the translational diffusion coefficient, the position and velocity correlation functions of the POE-chain and its centre of mass. It turns out that these dynamic properties found for the simulation of POE in water are properly described by choosing an appropriate friction coefficient in the stochastic approach. Also the transition rates between the trans and gauche conformation of the torsional angles around the C-O bonds of POE can be described in the stochastic approach, but with a different friction constant. Distributions of torsional angles and transition rates involving the C-C bonds of POE under the influence of the explicit polar solvent are compared with those of the stochastic simulation, and a physical explanation is given for the specific interaction of POE with water causing a conformational change of POE. The consequences for the value of the characteristic ratio are analysed and used to explain contradictory measurements of the characteristic ratio. Furthermore, it is concluded that for the description of the static properties of a polymer without specific solvent interactions, stochastic or even simulations in vacuo are appropriate.