Mechanism of Cryopolymerization: Diffusion-Controlled Polymerization in a Nonfrozen Microphase. An NMR Study

Polymerization of dimethylacrylamide (DMAAm) cross-linked with PEG diacrylate was studied using H-1 NMR both in situ in a semi frozen system and in a supercooled aqueous system. The amount of nonfrozen microphase, in which polymerization proceeds, is defined by the concentration of the starting monomers and the freezing temperature, which depends on the depression in freezing temperature caused by dissolved osmolytes. However, despite there being identical initial concentrations in the nonfrozen microphase, at a chosen temperature of -10 degrees C the cryopolymerization proceeded at different rates depending on the size of the nonfrozen microphase. Further studies of the conditions in the nonfrozen microphase were performed by using pulsed gradient spin echo (PGSE) to study the self-diffusion of solutes. Observations regarding the reaction rate were rationalized in terms of different degrees of long-range diffusion which was seen using PGSE. Cryopolymerization resulted in decreasing osmolyte concentration, and hence in gradual freezing of excess water as defined by the depression in freezing point caused by the remaining monomers. The NMR data provide a way of rationale predicting the effect of the monomer concentrations and freezing temperatures on the amount of nonfrozen microphase and its polymer concentration. These parameters define cryogel properties such as mechanical strength and porosity, which are evaluated using SEM and as flow resistance of cryogels.