Conversion of cellulose Iα to Iβ via a high temperature intermediate (I-HT) and other cellulose phase transformations

Abstract

The observation that the hydrothermal conversion of cellulose Iα to cellulose Iβ is irreversible has been assumed to be due to the relative free energy of these polymorph phases. We propose an alternative explanation: when cooling the high temperature phase, the barrier to forming Iβ is much smaller than the barrier to forming Iα, so kinetics favor the formation of Iβ. This explanation is consistent with all available experimental data, and is consistent with the general observation of polymer solid–solid phase transformations via metastable intermediate states. While cellulose Iβ may be lower in free energy than Iα, this has not been shown experimentally. Phase transformations of other cellulose polymorphs may be subject to similar kinetic effects when converted via metastable intermediate states.