Comparative Behavior of Poly(ethylene glycol) Hydrogels and Poly(ethylene glycol) Aqueous Biphasic Systems

Abstract

The properties of covalently cross-linked poly(ethylene glycol) (PEG) hydrogels have been studied. The degrees of expansion of the hydrogels in aqueous solutions containing different types of salt and the expansion with respect to temperature have been examined. The apparent polarity of the hydrogels in these differing environments has been measured using Reichardt's betaine dye. The uptake of hydrophobic solutes with respect to temperature and salt concentration has also been examined. We attempt to relate these observations to a consideration of the PEG molecular conformation and hydrogen-bonding ability in aqueous solution. Hydrogel expansion, apparent polarity, and solute uptake all appear to be continuous functions of temperature and salt concentration without any discontinuity typical of critical phenomena. It is suggested that phase formation in aqueous biphasic systems and miscibility gaps in PEG solutions in general result from molecular collapse and the loss of H-bonding ability, followed by the critical coacervation of preexisting molecular domains within the aqueous solution. These insights might aid in devising new theoretical and practical approaches to problems in many areas, such as separations science, pharmacology, and materials fabrication.