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Structure and morphology of poly(vinyl alcohol) gels prepared by freezing and thawing processes.

机译：通过冷冻和解冻过程制备的聚乙烯醇凝胶的结构和形态。

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The structure and Morphology of poly(vinyl alcohol) (PVA) gels prepared by repeated cycles of 8 hour freezing at --20°C and 4 hour thawing at 25°C were examined. Long-term morphological changes of such gels were determined upon swelling in water at 37°C for 6 months. The preparation conditions were examined by varying such parameters as the number of freezing and thawing cycles, the concentration of aqueous solution, and the PVA molecular weight. The overall structure and stability were examined in terms of water content, fractional PVA dissolution, degree of crystallinity, and crystal size distribution. The analysis was applied to determine the appropriateness of the gels for various biomedical and pharmaceutical applications. An increase in the number of freezing and thawing cycles served to reinforce existing crystals within the structure. Increased initial concentrations of aqueous PVA solutions resulted in hydrogels that contained initially higher crystallinity and added stability upon swelling. An increase in the PVA molecular weight resulted in crystals of higher lamellar thickness and a broadening of the crystal size distribution due to an increase in PVA chain length. The phenomenon of secondary crystallization was found to be more pronounced for more loosely crosslinked samples. An increase in the free volume and mobility within the network allowed for additional crystallization to proceed during swelling. A molecular model was developed to describe the overall dissolution kinetics as a three-step mechanism: detachment-, diffusion-, and disentanglement-controlled dissolution. The lamellar thickness of a PVA crystal was found to significantly change the rate of unfolding and, thus, the overall dissolution kinetics. Modified PVA gels prepared in the presence of NaCl demonstrated enhanced swelling as indicated by an increase in the initial rate of swelling and the overall water content. The addition of linear poly(ethylene glycol) was investigated to enhance the stability of freeze-thawed PVA gels. The diffusional characteristics of freeze-thawed PVA gels were examined for controlled release applications. A model protein was successfully incorporated into thin films and released. Through a feasibility study, the design of novel, freeze-thawed PVA laminates was introduced.
机译：检查了聚乙烯醇（PVA）凝胶的结构和形态，该凝胶是通过在--20°C下冷冻8小时和在25°C解冻4小时的重复循环制得的。在37℃的水中溶胀6个月后，测定这种凝胶的长期形态变化。通过改变诸如冷冻和解冻循环的次数，水溶液的浓度和PVA分子量之类的参数来检查制备条件。根据水含量，PVA溶解分数，结晶度和晶体尺寸分布检查了整体结构和稳定性。该分析用于确定凝胶对各种生物医学和制药应用的适合性。冷冻和解冻循环次数的增加有助于增强结构中现有的晶体。 PVA水溶液初始浓度的增加导致水凝胶最初具有较高的结晶度，并在溶胀时增加了稳定性。 PVA分子量的增加导致晶体具有更高的层状厚度，并且由于PVA链长的增加而导致晶体尺寸分布变宽。对于更疏松的交联样品，发现二次结晶的现象更为明显。网络中自由体积和迁移率的增加允许在溶胀期间进行额外的结晶。建立了分子模型，将整体溶出动力学描述为三步机制：分离，扩散和解缠结控制溶出。发现PVA晶体的层状厚度显着改变了展开速度，因此改变了整体溶解动力学。在NaCl存在下制备的改性PVA凝胶显示出增强的溶胀，如溶胀的初始速率和总水分含量的增加所表明。研究了线性聚（乙二醇）的添加以增强冻融的PVA凝胶的稳定性。检查了冻融的PVA凝胶的扩散特性，以进行控释应用。模型蛋白已成功整合到薄膜中并释放。通过可行性研究，介绍了新型的冻融PVA层压板的设计。

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