Modulating the porosity of cryogels by influencing the nonfrozen liquid phase through the addition of inert solutes.

The freezing of monomeric mixtures is known to concentrate solutes in a nonfrozen phase in the area surrounding the ice crystals. The concentration of such solutes is determined by the freezing temperature. Although salts or solvents do not directly react in the polymerization reaction, they do change the composition and properties of the nonfrozen phase. In this study, we investigated the influence of the addition of various salts and solvents on the structure of macroporous hydrogels formed in a semifrozen state through aqueous free-radical polymerization. The change in composition of the nonfrozen phase was studied using NMR to monitor the freezing of water, and the structural changes of the gels were observed using scanning electron microscopy. It was found that the addition of methanol or acetone caused the formation of reaction-induced phase separation polymerization due to cryoconcentration, which caused a significant increase of methanol or acetone in the nonfrozen phase. This resulted in a material with bimodal pore size distribution with pores of 10-80 μm in diameter caused by cryogelation, and with pores in the polymeric matrix with a diameter of less than 1 μm due to the reaction-induced phase separation. Addition of salts to the monomeric mixture resulted in a structure with only pores of 10-80 μm in diameter due to cryogelation. Increasing the amount of salts added resulted in the formation of thicker pore walls and thus a slight reduction in pore size compared to a sample with no added solute. The possibility of changing the structure and properties of the gels by adding different solutes could open up new applications for these materials, for example, chromatography applications.