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摘要
In the industry of polymer coextrusion processing, efforts are being made toward a better control of multilayer flow stability that is governed by the interface/interphase between neighboring layers, the understanding of which is considerably inadequate. This study aims to explore the relaxation dynamics of polymer chains located in the interphase between two compatible polymer melts based on transient rheology in step shear experiments, an area that is often overlooked without justification. First, transient rheology was investigated based on pure polymers of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) melts, focusing on a feature of abrupt stress decline observed in the transient period after a large step shear. This feature is considered to be the phenomenological onset of rheological nonlinearity (e.g., stress damping) commonly observed in the long-time window of a relaxation for an entangled polymer. From the molecular viewpoint, the nonlinearity onset is a result of polymer chain disentanglement under a large flow as interpreted based on a refined version of the Doi Edwards tube model and Wang's force imbalance theory, etc. A decreased entanglement number of polymer chains, either by decreasing molar mass in the melts or by reducing polymer concentration in solutions, was demonstrated to accelerate the onset of the rheological nonlinearity. In particular, a noticeable stress break-off in the transient period can be observed upon large deformations in the solutions due to their very weak entanglements. Second, large step shears were given to PMMA/PVDF bilayer-structured melts and a new model has been developed to determine the relaxation behavior of the interphase triggered between the layered polymers. Similar to the solutions, an abrupt stress decline was observed in the transient period for the interphase upon a large step deformation, indicating an analogous entanglement weakness of the interphase to the solution. Hence, a close correlation can be established between the interphase and the pure melts and solutions based on their transient rheology and the concept that the polymer chain entanglements can be weakened either by solvent dilution or by blending with dissimilar chains.
- 出版日期2018-6-13
