Further computer simulation studies of the orientational behaviour of poly(ethylene terephthalate) chains

Abstract

The energy parameter governing the glycol gauche conformation of the Williams–Flory rotational-isometric-state (RIS) model of poly(ethylene terephthalate) (PET) is varied to give values of (〈r²〉/M)_∞ and % gauche-glycol conformers that agree with experiment for bulk PET. Such agreement is not given by the original Williams–Flory model. The RIS Metropolis Monte-Carlo (RMMC) model developed by Molecular Simulations Incorporated (MSI) can also be modified, through values of the effective relative permittivity, to match the same experimental quantities. The orientational behaviours of the glycol segments, terephthaloyl segments and the repeat units with respect to the chain end-to-end vector are investigated by calculating the Legendre polynomials 〈P₂(ξ_gly)〉, 〈P₂(ξ_ter)〉 and 〈P₂(ξ_rep)〉, respectively, as functions of end-to-end chain extension, r/r_max, for selected RIS and RMMC parameters. The calculations show that the more flexible glycol segments orientate more readily than the terephthaloyl segments and the repeat units show orientations between these two extremes. The various models of segmental orientation presented in this paper will enable more realistic interpretation of the experimental stress-optical behaviour of PET in bulk. In this context, the Langevin model based on freely-jointed chains is seen to be completely inadequate for describing changes in segmental orientation with chain extension.