Rotational Isomeric State Analysis of Poly(oxyethylene). Conformational Energies and the Random-Coil Configuration

Abstract

Configuration-dependent properties of the poly(oxyethylene) (POE) chain were calculated within the framework of the rotational isomeric state (RIS) approximation. Conformational energies of the chain have been determined by the 1H and 13C NMR studies on 1,2-dimethoxyethane (DME) and POE as described in our recent paper as well as in the present work. The unperturbed chain dimension C(=‹r2›0/nl2), the dipole moment ‹μ2›/nm2, and the molar Kerr constant ‹mK›/x were calculated as a function of RIS parameters such as bond angles, rotational angles, and conformational energies. The value of C(=5.2) determined in organic solutions by Beech and Booth was found to be favorably reproduced by these calculations. The SANS result (C=6.9) recently reported by Kugler et al. on the melt exceeds the limit of a probable range estimated from uncertainties involved in each of the RIS parameters. Within the limit mentioned above, a set of RIS parameters was chosen so as to give an optimum agreement with observed values of the configurational properties of POE. Conformational energies thus estimated are as follows: Eσ=−0.5 kcal mol−1 for the C–C bond and Eρ=0.8 kcal mol−1 for the C–O bond, both representing the energy of the gauche form expressed relative to the trans state. Use of the conformational energies Eσ=−1.2 and Eρ=1.1 kcal mol−1 determined in D2O leads to a value of C=3.2, suggesting that the end-to-end dimension of the chain for the state unperturbed by longrange interactions may be considerably small in pure water.