The dynamics of isolated high molecular weight (MH) polymer chains dissolved in a nonentangled semidilute solution of a low molecular weight (ML) polymer were investigated by monitoring the elongational flow birefringence. Because of its nonentangled nature, a low molecular weight matrix polymer solution is regarded as a pure solvent (a binary pseudo-solvent). A ternary solution consisting of a small amount of a high molecular weight probe polymer and the binary pseudo-solvent is effectively a dilute solution of the probe polymer. It was observed that the birefringence from the orientation and/or stretching of the probe polymer chains starts to increase rather abruptly at a certain critical strain rate, \documentclass[12pt]{minimal}
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\begin{document}$$\dot{\varepsilon }_{{\text{c}}} $$\end{document}, and the spatial birefringence pattern is localized along the elongation axis, characteristics that are reminiscent of the coil-stretch transition of flexible polymer chains in a simple dilute solution. The relaxation time for the chain extension, τel, defined as the reciprocal of the critical strain rate \documentclass[12pt]{minimal}
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\begin{document}$$\dot{\varepsilon }_{{\text{c}}} $$\end{document}, was determined at various temperatures, matrix polymer concentrations cL, and test chain molecular weights MH. It was found that τel varied with molecular weight as τel~MHa, with a ranging from 1.3 to 1.8, which is roughly consistent with the molecular weight dependence of the non-free-draining Zimm relaxation time. A scaled relaxation time τelkT/η, which can be used to estimate the radius of gyration Rg of the probe polymer, decreased with increasing cL, indicating contraction of the high molecular weight polymer due to a screening of the excluded volume effect caused by the matrix polymer in the pseudo-solvent.
