Water Solubility Characteristics of Poly(vinyl alcohol) and Gels Prepared by Freezing/Thawing Processes

Abstract

Poly(vinyl alcohol) (PVA) is a water soluble polymer whose water solubility depends on its degree of hydrolysis, molecular weight, and tendency to hydrogen bond in aqueous solutions. PVA exhibits both upper and lower critical solubility temperatures and can be readily solubilized in water. For long-term dimensional stability, a new method involving freezing and thawing of aqueous PVA solutions was used to prepare insoluble PVA gels held together by physical crosslinks formed predominantly by crystallites. Solutions containing 15% PVA were frozen at −20 {\textdegree}C for, 1, 8, and 18 hours and thawed at room temperature for 30 minutes to 6 hours. These cycles were repeated for up to 10 times. The ensuing gels were analyzed by equilibrium swelling studies. Each cycle led to further crystallization of PVA leading to stable gels. Differential scanning calorimetry was used to analyze the gel morphology. Degrees of crystallinity varied from 4 to 16% on a swollen basis. Upon exposure to swelling temperatures of up to 37 {\textdegree}C, the crystallites of these gels remained remarkably stable for a period of several weeks. However at 60 {\textdegree}C relatively fast crystal melting occurred. The dissolution process was followed using complexation with boric acid.