Nanocellulose Dimensions and Crystallinity

In the work of Meier [43] the term elementary fibril was reported to have a diameter of 3.5 nm, and Heyn [44] stated that elementary fibrils are universal structural units of natmal cellulose, as the same biological structure had been encoimtered in cotton, ramie, jute and wood fibers. Blackwell and Kolpak [45] also reported the occmrence of elementary fibrils with diameters of approximately 3.5 nm in cotton and bacterial cellulose. All of these studies provide supportive evidence about the basic fibrillar unit in cellulose microfibrils. 

Most methods have typically been applied to the investigation of dried nanocellulose dimensions, although a study was conducted by Paakko et al. [47] where the size and size-distribution of enzymatically pretreated nanocellulose fibrils in a suspension were studied using cryotransmission electron microscopy, atomic force microscopy, and cross-polarization/magic-angle 

Zhou et al. [53] studied the effect of nanocellulose isolation techniques on the quality of nanocellulose and its performance in reinforced nanocomposites. They employed three different techniques including acid hydrolysis (AH), TEMPO-mediated oxidation (TMO) and ultrasonica-tion (US) to isolate nanocellulose from microcrystalline cellulose (MCC) and to evaluate the quality of nanocellulose and the reinforcing ability of these nanocelluloses in PVA matrices. The characterization of nanocellulose indicated that nanocellulose with higher aspect ratio, surface charge (-47 mV) and yields (37%) was obtained by TMO treatment, while acid hydrolysis treatment resulted in higher crystallinity index (88.1 %) and better size dispersion. 

Nanocelluloses isolated from AH technique have individual crystallites and disperse uniformly showing needle-shaped structures (namely nanocrystals), with diameters of 30-40 nm and lengths of 200-400 nm, while the TMO-derived nanocelluloses are interconnected webs showing nanofibrils with diameters of 40-80 nm and lengths ranging from 200 nm to several micrometers, having a wide range of aspect ratio. 

Both the TMO-derived and AH-derived nanocelluloses could homogeneously disperse in the PVA matrixes. The TMO/PVA films were better than AH/PVA films for tensile modulus and strength but were lower for elongation. The thermal behavior of the PVA nanocomposite films was more highly improved with addition of TMO-derived nanofibrils. It has been found that because of the mild reaction condition, the environmentally friendly attribute, the good quality of resulted nanofibrils and the superior properties of the final reinforced nanocomposites, the TMO technique has significant potential in the field of composite reinforcement. 

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