Structure and morphology of poly(vinyl alcohol) gels prepared by freezing and thawing processes
Abstract
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.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering
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