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 ε̇c 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 ratetion ε̇c 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 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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Polymers and Plastics
- Colloid and Surface Chemistry
- Materials Chemistry