Abstract
The link between wood and corresponding cellulose nanofiber (CNF) behavior is complex owing the multiple chemical pretreatments required for successful preparation. In this study we apply a few pretreatments on aspen wood and compare the final CNF behavior in order to rationalize quantitative studies of CNFs derived from aspen wood with variable properties. This is relevant for efforts to improve the properties of woody biomass through tree breeding. Three different types of pretreatments were applied prior to disintegration (microfluidizer) after a mild pulping step; derivatizing TEMPO-oxidation, carboxymethylation and non-derivatizing soaking in deep-eutectic solvents. TEMPO-oxidation was also performed directly on the plain wood powder without pulping. Obtained CNFs (44–55% yield) had hemicellulose content between 8 and 26 wt% and were characterized primarily by fine (height ≈ 2 nm) and coarser (2 nm < height < 100 nm) grade CNFs from the derivatizing and non-derivatizing treatments, respectively. Nanopapers from non-derivatized CNFs had higher thermal stability (280 °C) compared to carboxymethylated (260 °C) and TEMPO-oxidized (220 °C). Stiffness of nanopapers made from non-derivatized treatments was higher whilst having less tensile strength and elongation-at-break than those made from derivatized CNFs. The direct TEMPO-oxidized CNFs and nanopapers were furthermore morphologically and mechanically indistinguishable from those that also underwent a pulping step. The results show that utilizing both derivatizing and non-derivatizing pretreatments can facilitate studies of the relationship between wood properties and final CNF behavior. This can be valuable when studying engineered trees for the purpose of decreasing resource consumption when isolation cellulose nanomaterials.Graphic abstract
Highlights
Improved processes for isolating cellulose nanofibers (CNFs) from recalcitrant wood celluloses have received increased attention recently
Other DES-based treatments (Li et al 2017) gave resulting films high strain-to-break which indicates variations based on which specific solvent systems were used, which may expand the potential for DEStreatment as a conservative, coarse grade CNF-isolating method, and demands systematic studies regarding mechanism, optimization and formulations in order to understand how subsequent crack initiators (Nakagaito and Yano 2004) are removed during fibrillation in relation to ‘‘reference’’ pulp
This type of behavior was not observed with the TOCNFs in the study, which agrees with early investigators where cellulose derivatives are formed in a different way for oxidative treatments compared to conventional ones (Isogai and Kato 1998)
Summary
Improved processes for isolating cellulose nanofibers (CNFs) from recalcitrant wood celluloses have received increased attention recently. Aside from significant energy consumption reduction, enhanced degree of fibrillation is often associated, where nanofibers resembling the characteristic individual cellulose micro- or elementary fibrils are attainable These resemble the pristine fiber structures as they are biosynthesized in the plant and show a range of interesting properties in both suspension and as solids, whilst being obtained from renewable resources. The final product can be processed in either too harsh or too mild conditions relative native fibril morphologies The former can be identified as variable cellulose depolymerisation where corresponding CNFs has been observed to decrease in length (Shinoda et al 2012). Through more appropriate processing it is expected that the relation between tree breeding and isolation of cellulose nanomaterials becomes more pronounced
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