Monte Carlo simulations are performed to investigate the uniaxial ordering of chain segments induced upon stretching a polymer network. The polymer network is modeled by chains with fixed extremities on a cubic lattice. Segmental interactions are introduced by imposing excluded volume between all segments in the system. A uniaxial contribution to orientation is shown to arise upon stretching even at a relatively low fraction of occupied sites . This uniaxial contribution is studied as a function of . The variation versus is in agreement with that observed experimentally in a previous NMR work and is well described by a mean field model including orientation-dependent interactions between segments. It is shown additionally that a purely entropic model, including the packing entropy of segments, gives a mean field behavior qualitatively similar to that observed herein.
Molecular dynamics simulations of pure benzene and a poly(oxyethy1ene) chain in benzene are performed. The simulation of pure benzene is found to agree excellently with previous simulations despite using a different force field. A comparison is made between the results of simulations of the poly(oxyethy1ene) chain in benzene and in water and of stochastic simulations with respect to mean torsional angles, translgauche fractions, and transition rates. Characteristic deviations are found for the simulation in water and explained by specific atomic interactions, while there is satisfactory agreement with a stochastic simulation based upon the simple Langevin equation using a friction coefficient of 1 ps-I. The characteristic ratio of poly(oxyethy1ene) in benzene is calculated on the basis of the rotational isomeric state model. 0 1992 by John Wiley 8, Sons, Inc.
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.
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