Freezing behavior of polymer gels, used as models of biological systems or foods, has been studied in view of its importance for their successful cryopreservation or frozen storage. In this connection, water in the gel of Sephadex, a kind of crosslinked dextran, is known to indicate different freezing behavior dependent on the density of crosslink. That is to say, water in the gel of Sephadex G25 containing about 50 wt% water remains partially unfrozen when cooled from the temperature above 273 K accompanied by non-equilibrium freezing; freezing after substantial undercooling, and crystallizes during subsequent warming. With the aim of understanding the mechanism of anomalous freezing behavior during rewarming indicated by the G25 gel, it is necessary to clarify the frozen state of the gel. In this study, we conducted X-ray CT and XRD analyses of Sephadex gels in the frozen state by using synchrotron radiation. Although ice grains were too small to be observed by the X-ray CT measurement (ice grain size: &lt; 1 μm) when the gels in the bead form were rapidly frozen from the temperature above 273 K (cooling rate: ~60 K/min) irrespective of the density of crosslinks, ice grains of several micrometers were identified within the G25 gel beads when they were slowly frozen (cooling rate: 1 K/min). When the frozen G25 gel beads were heated to the temperature where an exotherm due to ice crystallization during rewarming finishes in the DSC scan (264 K) followed by re-cooling and resultant equilibrium freezing, many ice grains of larger size (10~20μm) were observed within the beads in the frozen state. By the XRD measurement, Bragg peaks due to ice I<sub>h</sub> indicated remarkable tailing after non-equilibrium freezing in the case of the G25 gel, which disappeared after equilibrium freezing. These results suggest the presence of glassy water in the frozen state of the G25 gel after non-equilibrium freezing.

Analysis of the frozen state of Sephadex G25 gel by using X-ray CT and XRD

255th National Meeting (2018)

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