Investigation of the Origin and The Domain Growth Mechanism of Viscoelastic Phase Separation

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This study aims at revealing the overall features of viscoelastic phase separation of polymer solutions on a quantitative level. In particular, we focus on how the molecular weight of polymer affects the phase-separation behavior of a polymer solution. We found that the formation of a transient gel plays a crucial role in viscoelastic phase separation.We study how viscoelastic phase separation proceeds in a macroscopic length scale, focusing on the similarity and difference between a transient gel and a permanent gel. We found that upon phase separation the volume of the polymer-rich phase shrinks as in the volume-shrinking of chemical gel. However, a transient gel keeps deforming after the volume shrinking and relaxes to a shape determined by the interface tension. In other words, a transient gel behaves as a fluid in the final stage. This is fully consistent with what we observed with microscopy. We also found that just at the time when a transient gel stops shrinking, the transparency of the polymer-rich phase also changes drastically. This indicates that the internal phase-separated structure changes drastically at that time. This behavior should be induced by the transformation of the polymer-rich phase from an elastic body to a fluid, which makes it difficult for the polymer-rich phase to support its own weight. We also found that the characteristic time when the polymer-rich phase starts to loose its elastic properties increases in proportional to the quench depth. The temperature which this time becomes zero coincides well with the temperature below that a transient gel appears, Tt, which support our picture.This study clearly indicates the similarity between phase separation in polymer solutions and volume phase transition in polymer gels. This fact is important in understanding not only viscoelastic phase separation, but also the static and dynamic phase behavior of polymer solutions.