Good- and bad-solvent effect on the rotational statistics of a long chain molecule

The long-range solvent-mediated intramolecular forces that may either expand or contract a polymer chain, may also change the probabilities of the rotational states (trans and gauche in a polyethylene chain). This effect is calculated in the present paper, using the chain self-consistent free energy optimisation approach, within the Gaussian assumption, and following the rotational-isomeric-state approximation with neighbour interaction; representation of the chain configuration in terms of Fourier normal modes proves to be a most important feature of the algorithm. Except under a very high expansion, the relative change of the rotational probabilities is a function of the reduced temperature tau = (T - Theta)/T, instead of tau root N as it happens with the chain dimensions (N=molecular length). The statistical distribution of the additional, or excess rotational probabilities is markedly non-Markoffian. In the poor-solvent case leading to chain contraction the excess rotational states are very far apart along the chain sequence, whereas in: a good solvent they are clustered within chain strands consisting of no more than (k) over bar congruent to 15 chain bonds (at T = 400 K for polyethylene), separated by unperturbed strands whose length is no less than (k) over bar bonds. The value of (k) over bar is roughly proportional to the number of bonds comprised within the chain persistence length. Although small, the gamma effect on the NMR C-13 chemical shift depending on the change of the relative amount of trans and gauche rotational states, produced by a good solvent, should be measurable under appropriate conditions. (C) 1998 Elsevier Science Ltd. All rights reserved.