Effects of chain flexibility on polymer solution dynamics

Abstract

An experimental study of the dynamics of polymer chains in concentrated solutions is described. The effects of molecular structure, molecular weight (and its distribution), concentration, temperature and solvent on the chain dynamics, and attempts to quantify the effect of chain flexibility on the dynamic polymer behaviour are covered.

The experimental techniques employed were dynamic electric birefringence (Kerr effect) and photon correlation spectroscopy (PCS). The polymers studied, chosen to cover as wide a range of molecular flexibility as possible, were poly-γ-benzyl-L-glutamate, poly(n-butyl isocyanate), poly-(2-methylpent-1-ene sulfone), polypropylene glycol/oxide and ethyl cellulose.

Two types of relaxation process were identified in the Kerr experiments. The first, associated with whole molecule rotation, displayed qualitatively the same behaviour for all the systems. The relaxation times τ were well described by an expression of the form τ∞C^ξcM^ξm, where the exponents ξ_c and ξ_m changed dramatically at some characteristic concentration, C_r. The second type, corresponding to segmental motions, showed much weaker molecular weight and concentration dependences. In the PCS experiments, marked changes in the shape of the correlation functions were observed at the same critical concentration.

The experimental exponents ξ_c and ξ_m are compared with various theoretical models for concentrated polymer chain dynamics, including recent extensions of reptation models to semi-flexible chains. Even for the most rigid of systems the experimental exponents are in general lower than those predicted by the Doi–Edwards rigid-rod model. Although the results can in some cases be explained semi-quantitatively by other models, a number of significant discrepancies remain. A number of quantitative criteria for assessing the flexibility of polymer chains are proposed and applied to the results of this work.