Thermally induced crystal transformation from cellulose I_α to I_β

It is known that cellulose I_α is metastable and mostly converted into I_β 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 I_α to I_β, we measured X-Ray diffraction of the highly crystalline cellulose (I_α 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 I_α to I_β phase. This indicates that I_α 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 I_α to I_β.