Time-Resolving Analysis of Cryotropic Gelation of Water/Poly(vinyl alcohol) Solutions via Small-Angle Neutron Scattering
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- 作者：Finizia Auriemma ; Claudio De Rosa ; Rosa Ricciardi ; Fabrizio Lo Celso ; Roberto Triolo ; Vitaly Pipich
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：January 24, 2008
- 年：2008
- 卷：112
- 期：3
- 页码：816 - 823
- 全文大小：169K
- 年卷期：v.112,no.3(January 24, 2008)
- ISSN：1520-5207
文摘
The structural transformations occurring in initially homogeneous aqueous solutions of poly(vinyl alcohol)(PVA) through application of freezing (-13 
C) and thawing (20 C) cycles is investigated by time resolvingsmall-angle neutron scattering (SANS). These measurements indicate that formation of gels of complexhierarchical structure arises from occurrence of different elementary processes, involving different lengthand time scales. The fastest process that could be detected by our measurements during the first cryotropictreatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and anunfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallizationof PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of amicrogel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution atsubzero temperatures, and although below 0 C the PVA solutions used for preparation of cryogels should bebelow 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 ~200 min, due toa slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprintedduring 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 crystallizationof PVA. Crystallization of PVA, in turn, is resumed by freezing the gels at subzero temperatures, after watercrystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVAcrystallization during the permanence of these gels at subzero temperatures are the same shown by PVAduring the first freezing step of the solutions.
C) and thawing (20 C) cycles is investigated by time resolvingsmall-angle neutron scattering (SANS). These measurements indicate that formation of gels of complexhierarchical structure arises from occurrence of different elementary processes, involving different lengthand time scales. The fastest process that could be detected by our measurements during the first cryotropictreatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and anunfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallizationof PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of amicrogel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution atsubzero temperatures, and although below 0 C the PVA solutions used for preparation of cryogels should bebelow 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 ~200 min, due toa slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprintedduring 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 crystallizationof PVA. Crystallization of PVA, in turn, is resumed by freezing the gels at subzero temperatures, after watercrystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVAcrystallization during the permanence of these gels at subzero temperatures are the same shown by PVAduring the first freezing step of the solutions.