Freezing of aqueous polyvinylpyrrolidone solutions

Abstract

The freezing of aqueous polyvinylpyrrolidone solutions has been studied by means of differential thermal analysis, dilatometry and calorimetry. A differential thermal analysis apparatus for use at low temperatures has been constructed. A fast method for the determination of thermal diffusivities has been elaborated. Freezing and thawing of aqueous polyvinylpyrrolidone solutions has been carried out at definite heating and cooling rates. The melting point curve as a function of the polymer concentration has been determined. Specific heats of polymer solutions near room temperature have been ascertained for the purpose of obtaining thermal conductivities from thermal diffusivities for these polymer solutions. The freezing of the polymer solutions at equal cooling rates gave satisfactory agreement for data obtained from differential thermal analysis and dilatometry, respectively, whereas on heating of frozen solutions the agreement was not as satisfactory. The results could be represented by straight lines in a logarithmic coordinate system. The slopes of these straight lines were a function of the heating and cooling rates, respectively. It was ascertained that the amounts in grams of unfrozen water left for each gram of polymer increased with decreasing initial polymer concentration for any one freezing temperature. From the melting point data, activities for varous percentages of W/W of polymer in water, could be calculated, using the ratio of the vapor pressure of ice in equilibrium with the respective polymer solution to the vapor pressure of the corresponding supercooled water. A theory for polymer solutions byMaron gave reasonalble results for the interaction parameters for these solutions, which were calculated with the help of their activites. A plot of the grams of unfrozen water/g PVP against the activities of the water in the solutions, gave a curve resembling a Type III adsorption isotherm. Apparently the concentration is the predominant factor and the temperature has little influence. The plot of g of unfrozen water/g PVP against a H2O/(1− a H2O) (where a H2O=water activity) gave a straight line in accordance with the equation for the Type III isotherm, where the constant isc ≅ 1. The unfrozen water can be considered as bound by hydrogen bonds to the polymer molecules. From the isotherms, the vapor pressure for a polymer solution of known composition at 25°C can be predicted and, vice versa, the composition of the solution can be ascertained, if the vapor pressure at 25°C is known. The vapor pressure of water at 25°C must, of course, also be known. The increasing amounts of unfrozen water/g PVP with increasing water activity can be accounted for by the different freezing path of a fairly dilute solution compared with that for a more concentrated solution. The dilute solution passes through stages of higher water activities than the more concentrated one. Hence, more water is adsorbed or bound in the more dilute solution. Apparently this adsorption is not easily reversible and, hence, more water per g PVP remains unfrozen in the initially more dilute solutions in contrast to the initially more concentrated one. Polymer molecules apparently have ice-like hydrate shears.