On the polymorphic and morphological changes of cellulose nanocrystals (CNC-I) upon mercerization and conversion to CNC-II

• Gradual polymorphoric transformation from polymorph of Cellulose I to II takes place with the evidences of decreasing crystallite size of Cellulose I and growing crystallite size of cellulose II in the transition conditions.
• Nano-size induced polymorphic transformation, which is strongly correlated with the sample's crystallite size, plays a role in reducing the critical concentration required for polymorph conversion.
• Morphology of individual CNC was imaged by TEM after redispersion of mercerized CNC samples by TEMPO oxidization.
• Morphology of cellulose II exhibited interconnected granules, indicating one cellulose molecular chain may involve in several crystallites forming.
• This work sheds some light on the mechanism of polymorphism transformation of cellulose I to II.

Polymorphic and morphological transformations of cellulosic materials are strongly associated to their properties and applications, especially in the case of emerging nanocelluloses. Related changes that take place upon treatment of cellulose nanocrystals (CNC) in alkaline conditions are studied here by XRD, TEM, AFM, and other techniques. The results indicate polymorphic transformation of CNC proceeds gradually in a certain range of alkali concentrations, i.e. from about 8% to 12.5% NaOH. In such transition alkali concentration, cellulose I and II allomorphs coexists. Such value and range of the transition concentration is strongly interdependent with the crystallite size of CNCs. In addition, it is distinctively lower than that for macroscopic fibers (12–15% NaOH). Transmission electron microscopy and particle sizing reveals that after mercerization CNCs tend to associate. Furthermore, TEMPO-oxidized mercerized CNC reveals the morphology of individual nanocrystal of the cellulose II type, which is composed of some interconnected granular structures. Overall, this work reveals how the polymorphism and morphology of individual CNC change in alkali conditions and sheds light onto the polymorphic transition from cellulose I to II.