It is known that cellulose Iα is metastable and mostly converted into Iβ phase by a heat treatment of 280°C in an inert gas, helium. To elucidate the mechanism of this heat-induced crystal transformation from cellulose Iα to Iβ, we measured X-Ray diffraction of the highly crystalline cellulose (Iα rich type) samples as a function of temperatures. In the heating process, d-spacings of equatorial reflections increased in line with the temperature, and furthermore the rate of change of the d-spacings increased considerably above 200°C. This result indicated that 200°C was the critical temperature for breaking intermolecular hydrogen bonds. Above that temperature, cellulose molecular chains became more flexible, inducing a thermal expansion of the crystal lattice, and it formed a transformation intermediate, a "high-temperature" structure. In the cooling process, new types of hydrogen bonds may form at 200°C as a result of the transformation from Iα to Iβ phase. This indicates that Iα via the above expanded intermediate. The heat-induced expansion of the crystal lattice may be a trigger for the rearrangement of the hydrogen bonds, which may enhance the transformation from Iα to Iβ.
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