Isophase Transitions of Cellulose – A Short Review

Abstract

Cellulose has complicated supramolecular structure consisting of nanofibrils, which are built of ordered crystallites (CR) and low ordered non-crystalline domains (NCD) in various CR/NCD ratios, from 4 for nanocrystalline cellulose to zero for amorphized cellulose. Moreover, crystallites have four main allomorphs, CI, CII, CII and CIV. In this critical review isophase temperature transitions in NCD and phase transition of cellulose crystallites, as well as amorphous cellulose, were described and discussed. It has been shown that due to structural heterogeneity the non-crystalline domains have three isophase temperature transitions, where the α1 and α2 transitions are caused by the segmental mobility in dense mesomorphous and medium packed amorphous clusters, respectively; whereas the β transition is related to the mobility of small segments in loose packed amorphous clusters, which probably are located on the outer surface of nanofibrils. Under the action of water and other plasticizers all three isophase transitions are shifted to lower temperatures. Various crystalline allomorphs and amorphous cellulose can be converted into each other as a result of phase transitions, such as recrystallization, decrystallization, transitions between various crystalline allomorphs, etc. Important phase transition is a sol-gel process, when cellulose is dissolved and then regenerated from the solution. In this review mechanism of the phase transitions and their relation to isophase transitions of cellulose was disclosed.