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

本研究旨在定量地揭示聚合物溶液粘弹性相分离的整体特征。特别是，我们专注于聚合物的分子量如何影响聚合物溶液的相分离行为。我们发现瞬态凝胶的形成在粘弹性相分离过程中起着至关重要的作用，我们研究了粘弹性相分离过程在宏观尺度上的进行，着重讨论了瞬态凝胶和永久凝胶的异同。我们发现，在相分离时，富聚合物相的体积收缩，如化学凝胶的体积收缩。然而，瞬态凝胶在体积收缩后保持变形，并松弛到由界面张力决定的形状。换句话说，瞬态凝胶在最终阶段表现为流体。这与我们用显微镜观察到的完全一致。我们还发现，就在瞬间凝胶停止收缩的时候，富含聚合物的相的透明度也发生了剧烈的变化。这表明内部相分离结构在那时急剧变化。这种行为应该是由富聚合物相从弹性体转变为流体引起的，这使得富聚合物相难以支撑其自身重量。我们还发现，当富含聚合物的相开始松动其弹性性能的特征时间增加成比例的淬火深度。这个时间变为零的温度与低于瞬时凝胶出现的温度Tt吻合得很好，这支持了我们的picture.This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。这一事实不仅对理解粘弹性相分离，而且对理解聚合物溶液的静态和动态相行为都很重要。

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.