| Summary: | 碩士 === 國立中正大學 === 化學工程所 === 93 === In large step-strain flows, short-time higher Rouse modes are crucial in describing the full stretch relaxation of an entangled polymer liquid. In this work, we wish to shed light on the chain retraction behavior of monodisperse entangled polymer solutions on time scales sufficiently shorter than the Rouse time. Additionally, the impact of polymer entanglement on the nonlinear stress relaxation is discussed on the basis of the current theory/data comparisons. At the same time, we conduct step-strain shear experiments on bidisperse polystyrene solutions so as to investigate the retraction dynamics of long chains in the presence of short chains, which results in an environment distinct from monodisperse solutions in view of the long-chain dynamics. The central findings are as follows: In the monodisperse system, there is, in general, an evident deviation between the experimental nonlinear relaxation modulus and the tube model prediction based on the Rouse model prediction for chain retraction. This result suggests that polymer entanglement or its nonequilibrium properties has an important impact on short-time chain retraction of entangled polymers. Accordingly, we propose a specific modification to describe chain retraction behavior more realistically. In the bidisperse system, a preliminary investigation of the predictions of the extended Mead-Larson-Doi model for bidisperse solutions is performed in order to gain a better understanding of the coupled stretch relaxation between short- and long-chain components. The theory/data comparisons indicate that the coupling in the stretch relaxation is generally over-predicted by the extended Mead-Larson-Doi model, especially for the short-chain-rich system. Besides, the linear mixing rule, which assumes the stretch relaxation is uncoupled in binary blends, is also found to be inappropriate for the bidisperse solutions investigated.
|
|---|
