Freezing characteristics of acrylamide-based aqueous solution used for the preparation of supermacroporous cryogels via cryo-copolymerization

Solvent freezing crystallization is a crucial step to the satisfactory formation of homogenous and interconnected supermacropores within cryogels during the preparation of supermacroporous cryogel beds via cryo-copolymerization. In this work, the freezing characteristics of an aqueous solution system containing acrylamide (AAm), N′,N′N′,N′-methylene-bis-acrylamide (MBAAm) and allyl glycidyl ether (AGE) for the production of polyacrylamide-based cryogels were investigated experimentally under various cooling conditions. Freezing curves of solution of AAm+AGE+MBAAmAAm+AGE+MBAAm under various concentrations were measured to determine the corresponding values of freezing point temperature TmcTmc and the actual initial solvent crystallization temperature TcTc. The effects of freezing rate and monomer concentration on TcTc and TmcTmc were addressed. The results showed that the freezing rate had a limited effect on TmcTmc when the freezing rate was very low under the present work. At the same time, TcTc changed randomly in a certain range with monomer concentration and freezing rate, while TmcTmc decreased with the increase of monomer concentration under freezing condition at a constant freezing rate. Based on these results, the in-column freezing behaviors of 7% (w/w) aqueous solution of AAm+AGE+MBAAmAAm+AGE+MBAAm reactive system initiated by ammonium persulfate (APS) and N,N  ,N′N′,N′N′-tetramethylethylenediamine (TEMED) under different freezing-temperature variation conditions were measured to reveal the actual solvent crystallization processes occurring within the column during the cryo-copolymerization. The properties of these cryogels were also measured and discussed.