Mechanochemical transformations of α-chitin

Margoutidis, Georgios (2019) Mechanochemical transformations of α-chitin. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Comprising 20–30% of the shells of crustaceans, α-chitin has been shown to provide a wide range of valuable products such as therapeutic substances, platform molecules and functional materials. However, the polysaccharide’s crystallinity has been poorly studied, and this has limited its widespread utilization thus far. Filling that research gap, this thesis reports a new mechanochemical method of ball milling which amorphizes α-chitin in a controlled way. Using powder XRD to measure the polysaccharide’s crystallinity index (CrI%), a low balls-to-chitin (BtC) steel system was found to reduce CrI by an average of 6.0 units in regular milling times with good precision (±2.5 CrI units). That data set was correlated for the first time with FT-IR intensity ratios showing an unaffected degree of acetylation (DA), a steady decrease of glycosidic linkage content and α-chitin’s characteristic amide I split. The behaviour of the latter was rationalized as an experimental indicator for the weakening of the polysaccharide’s intermolecular hydrogen bonding network which is hypothesized to arise from the distribution of the average collision force within the nanofibril structure. The combination of increased collision frequency along with the presence of a solid acid catalyst (kaolinite) provided optimum mechanochemical conditions for efficient conversion of α-chitin into water-soluble products. The latter were analysed with a MALDI-TOF-MS method developed in Memorial University revealing oligomers of N-acetyl-D-glucosamine (NAG) with degrees of polymerization (DP) of 1 to 5. The monomer and dimer reached yields of 5.1 and 3.9 wt.%, respectively, within 6 h, which compare well with yields of glucose and cellobiose from literature for cellulose ball milling. The products of this solvent-free oligomerization process were complemented by colorimetric approximations of reducing ends as well as size exclusion chromatography observations. This analysis is expected to stimulate future research for the sustainable production of these likely biologically active chitooligosaccharides. In parallel, the inevitable fraction of higher MW chitin resulting from the ball milling process has been shown to conveniently solubilize in cold NaOH. An optimum concentration of 19 wt.% of the alkali was found to dissolve ~5 wt.% high MW/crystallinity α-chitin via a freeze-thaw process at −28°C and give films of acceptable mechanical properties after a simple casting treatment with HCl. Practically, this method avoids some of the disadvantages of organic-salt-solution solvents like the need for a costly recycling/purification treatment, their life cycle issues, the high temperatures, and the long stirring times. At the same time, it can quickly create homogeneous solutions of predictable viscosities in the 1–10 wt.% range allowing for more efficient and controlled chitin deacetylation.

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