The Relationship between Crystallization of Water Clusters Dispersed in Sephadex Resins in their Wet Form and the Polymeric Matrix Glass Transition Temperature

Resumo

Abstract—Differential scanning calorimetry (DSC) analysis was used to study the dependences of the main parameters of crystallization and melting of small water clusters dispersed in Sephadex resins on the amount of water (30–55%) in the wet polymer. The temperatures and heats of the processes were shown to depend on the frozen water (FW) concentration in Sephadex, with the dependence being considered as a manifestation of the size effect. Hysteresis between FW melting and crystallization was detected in a certain Sephadex humidity range; i.e., T_m was higher than T_cr and Q_m higher than Q_cr. The two findings reflected the basic properties of low-dimensional systems. Sephadex and amorphous starch, which was examined previously, were compared as two polysaccharide systems with different structural organizations, and a significant difference was observed in FW crystallization in the systems. Nucleation and crystallite growth, which determine water crystallization, were found to occur separately in time in Sephadex with a low humidity under these experimental conditions. Nucleation occurred during cooling, while growth was observed mainly during heating. Both of the processes occurred during cooling and partly overlapped only at a high humidity, far from T_g of Sephadex. In contrast, the two processes always occurred during cooling in amorphous starch regardless of its humidity. The difference in FW crystallization was assumed to arise because water mobility differs between the systems as a result of their difference in molecular mobility of the biopolymer matrix proper near the glass transition region.