Cryostructuring of polymeric systems. 48. Influence of organic chaotropes and kosmotropes on the cryotropic gel-formation of aqueous poly(vinyl alcohol) solutions

Physical (non-covalent) macroporous cryogels of poly(vinyl alcohol) (PVACGs) are formed as a result of the consecutive freezing of concentrated PVA solutions, their incubation in a frozen state and then thawing. If some additional low-molecular-weight solutes are present in the feed system to be gelled cryogenically, such additives can affect more or less the physicochemical properties and the microstructure (porous morphology) of the resultant PVACGs. In this study the influence of the additives of organic non-ionic and ionic chaotropes (urea and guanidine hydrochloride) or kosmotropes (trehalose and hydroxyproline) on the PVA cryotropic gel-formation, as well as on the properties and structural features of the respective PVACGs was explored. It was shown that both the strength and the heat endurance of cryogels formed in the presence of chaotropes progressively decreased with increasing the additive concentration. In turn, in the case of cryogels prepared in the presence of kosmotropes the dependences of the physicomechanical characteristics on the additive concentration were of a bell-like (concave) type, while the heat endurance of the samples grew monotonically. The microstructure of the PVAGCs containing various amounts of the chaotropes and the kosmotropes was studied using the optical microscopy of thin sections followed by the mathematical analysis of the images. The average size of the pore crosssection and the "conditional porosity" of the respective specimens were characterized quantitatively.