Thermally induced crystal transformation from cellulose I-alpha to I-beta

It is known that cellulose I-alpha is metastable and mostly converted into I-beta phase by a heat treatment of 280 degreesC in an inert gas, helium. To elucidate the mechanism of this heat-induced crystal transformation from cellulose I-alpha to I-beta, we measured X-Ray diffraction of the highly crystalline cellulose (I-alpha 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 degreesC. This result indicated that 200 degreesC 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 degreesC as a result of the transformation from I-alpha to I-beta phase. This indicates that I-alpha transformed into I-beta 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-alpha to I-beta.