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

本研究旨在从定量水平上揭示聚合物溶液粘弹性相分离的总体特征。特别是，我们关注聚合物的相对分子质量如何影响聚合物溶液的相分离行为。我们发现瞬时凝胶的形成在粘弹性相分离中起着至关重要的作用，我们在宏观长度尺度上研究了粘弹性相分离的过程，重点研究了瞬时凝胶与永久凝胶的异同。我们发现，在相分离时，富聚合物相的体积缩小，就像化学凝胶的体积收缩一样。然而，瞬时凝胶在体积收缩后继续变形，并松弛到由界面张力决定的形状。换句话说，瞬时凝胶在最后阶段表现为流体。这与我们用显微镜观察到的完全一致。我们还发现，就在瞬时凝胶停止收缩的时候，富含聚合物的相的透明度也发生了巨大的变化。这表明当时内部的分相结构发生了巨大的变化。这种行为应该是富聚合物相从弹性体转变为流体所引起的，这使得富聚合物相难以支撑其自身的重量。我们还发现，富聚合物相开始松弛其弹性性质的特征时间与淬火深度成正比增加。此时变为零的温度与瞬间凝胶出现的温度TT吻合得很好，这支持了我们的图景。这项研究清楚地表明了聚合物溶液中的相分离和聚合物凝胶中的体积相变之间的相似性。这一事实不仅对于理解粘弹性相分离，而且对于理解聚合物溶液的静态和动态相行为都是重要的。

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.