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首页> 外文期刊>The journal of physical chemistry, B. Condensed matter, materials, surfaces, interfaces & biophysical >Time-resolving analysis of cryotropic gelation of water/poly(vinyl alcohol) solutions via small-angle neutron scattering
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Time-resolving analysis of cryotropic gelation of water/poly(vinyl alcohol) solutions via small-angle neutron scattering

机译：水/聚乙烯醇溶液低温凝胶化的小角中子散射时间分辨分析

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摘要

The structural transformations occurring in initially homogeneous aqueous solutions of poly(vinyl alcohol) (PVA) through application of freezing (-13 degrees C) and thawing (20 degrees C) cycles is investigated by time resolving small-angle neutron scattering (SANS). These measurements indicate that formation of gels of complex hierarchical structure arises from occurrence of different elementary processes, involving different length and time scales. The fastest process that could be detected by our measurements during the first cryotropic treatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and an unfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallization of PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of a microgel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution at subzero temperatures, and although below 0 degrees C the PVA solutions used for preparation of cryogels should be below the spinodal curve, occurrence of liquid-liquid phase separation could not be detected in our experiments. Upon thawing, ice crystals melt, and transparent gels are obtained that become opaque in similar to 200 min, due to a slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprinted during the fast freezing by the crystallization of water. During the permanence of these gels at room temperature (for hours), the presence of a high content of water (higher than 85% by mass) prevents further crystallization of PVA. Crystallization of PVA, in turn. is resumed by freezing the gels at subzero temperatures, after water crystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVA crystallization during the permanence of these gels at subzero temperatures are the same shown by PVA during the first freezing step of the solutions.
机译：通过时间分辨小角度中子散射（SANS），研究了通过应用冷冻（-13摄氏度）和融化（20摄氏度）循环在聚乙烯醇（PVA）最初均匀的水溶液中发生的结构转变。这些测量表明，复杂的分级结构的凝胶的形成是由不同的基本过程的发生引起的，涉及不同的长度和时间尺度。在第一次低温处理过程中，通过我们的测量可以检测到最快的过程是溶剂的结晶。然而，溶剂结晶不完全，并且还形成了比初始溶液更浓缩在PVA中的未冷冻液体微相。 PVA的结晶发生在未冻结的液相微相内部，并且由于形成了微凝胶级分而减慢了结晶速度。水的结晶发生在溶液在零度以下的温度下处理的前10分钟，尽管低于0摄氏度，用于制备冰凝胶的PVA溶液应低于旋节线曲线，但不能发生液-液相分离在我们的实验中检测到。解冻后，冰晶融化，透明凝胶在约200分钟内变得不透明，这是由于通过水的结晶在快速冷冻过程中压印的微异质性（稀疏和致密区域）的大小逐渐缓慢增加。在这些凝胶在室温下永久保存数小时期间，存在高含量的水（高于85质量％）会阻止PVA进一步结晶。依次是PVA的结晶。在水结晶并随后形成未冷冻的微相后，通过将凝胶冷冻至零度以下温度来恢复凝胶。这些凝胶在零度以下温度下永久存在的过程中，PVA结晶的动力学参数与溶液的第一个冷冻步骤中的PVA相同。

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