Conformational relaxation time in polymer solutions by elongational flow experiments: 1. Determination of extensional relaxation time and its molecular weight dependence

The nature of chain extension of high-molecular weight atactic polystyrene in an elongational flow field was investigated. In particular, we studied the sudden onset of high extension which, according to theory, is governed by the expression $\text{?}\text{?}{}_{\mathrm{c}}\text{τ = 1.}$ The molecular weight dependence of $\text{?}\text{?}{}_{\mathrm{c}}$ (the critical strain rate at which high extension occurs) was examined, and the corresponding relaxation time, τ, which would be relevant to the above expression was considered. The experimental measurements of absolute values and the functional dependence on molecular weight of the relaxation times demonstrates that it is the relaxation time associated with the Zimm non-free-draining model which is appropriate to this coil-stretch transition.