Raman Spectroscopy in the Analysis of Cellulose Nanomaterials

Cellulose nanomaterials (CNs) are new types of materials derived from celluloses and offer unique challenges and opportunities for Raman spectroscopic investigations. CNs can be classified into the categories of cellulose nanocrystals (CNCs, also known as cellulose whisker) and cellulose nanofibrils (CNFs, also known as nanofibrillated cellulose or NFCs) which when produced exist as aqueous suspensions at very low concentrations. Analysis of these suspensions by Raman spectroscopy is especially rewarding in that the CNs can be analyzed in the native hydrated state without special considerations. Several kinds of information can be derived from the spectra of the nanomaterials such as, estimation of crystallinity (in suspensions and freeze-dried states), measurement of accessibility of the nanomaterials by water, detection and quantitation of cellulose II polymorph in CNs, and effect of drying on the structure of CNs. Moreover, Raman spectra of the nanomaterials contain bands that are associated with chemical functionalities usually present on the surfaces of prepared/modified materials, for example, sulfate esters present on the surfaces of the sulfuric acid produced CNCs. In particular situations, these groups can be quantified by Raman spectroscopy. To accurately estimate crystallinity of CNs, a new method based on ~ 93 cm-1 Raman band is reported. In contrast to the existing crystallinity estimation methods, the 93 cm-1 method has the capability to distinguish between simply "organized" and "organized and crystalline" celluloses. The measurement of the latter fraction is necessary to accurately measure crystallinity. Finally, a few applications of Raman spectroscopy to CN-composites are considered and it is shown that useful information can be obtained from the Raman investigations.