| Nanocrystalline cellulose(NCC) is a new functional polymer material. It has the basic structure and function of cellulose as well as the characteristics of nanomaterials, including huge surface area, high reactivity, excellent mechanical properties, etc. Thus playing an important role in polymer composites, bio-medicine, food and paper making. However, NCC contains a large number of surface hydrophilic hydroxyl groups, which makes them easy to aggregate but difficult to be dispersed in hydrophobic solvents/polymers, and thus limits its scope of application. Previous studies showed that acylation of NCC can improve the hydrophobic properties of NCC and expand its application. The traditional chemical acylation methods typically use acids or sulfuric acid as catalyst, require lengthy preparation and response. In addition, the acetylation process was accompanied by the degradation of cellulose. Unlike those chemical processes, enzymatic modification has high selectivity, mild reaction condition, and can be recycled(immobilized enzyme), which has been wide applied in modification of many natural compounds. In this dissertation, the possibility of lipase catalytic synthesis of NCC ester in a mixed solvents was investigated; the influences of various hydrophobic organic solvents, vinyl esters and reaction conditions on lipase catalyzed transesterification in mixed solvents were studied. Besides, the composite membranes composed of enzymatically modified NCC(e-NCC) and polylactic acid were analyzed to confirm the influences of e-NCC in polylactic acid.Results showed that polarity of solvent in the reaction medium play an important role in the enzymatic reaction. To well disperse NCC and maintain the enzymatic activity, a mixture of hydrophobic organic solvents and dimethyl sulfoxide were used as a reaction medium. The best results of the lipase-catalyzed reaction were observed in t-butanol- dimethyl sulfoxide system. In this reaction medium, the optimum t-butanol concentration, lipase dosage(lipase / NCC), molar ratio of acyl donor(vinyl propionate / anhydroglucose unit), reaction temperature and reaction time for the esterification of NCC propionate were found to be 25%(v / v), 0.67(g / g), 15: 1(mol / mol), 50 ℃ and 8 h, respectively; under which the degree of substitution of the product was 0.24.The reaction was also evidently affected by the acyl donor used(the substrate of the reaction). When vinyl laurate was used as the acyl donor, the optimum t-butanol concentration, lipase dosage(lipase / NCC), molar ratio of acyl donor(vinyl laurate / anhydroglucose unit), reaction temperature and reaction time for the esterification of NCC laurate were 25%(v / v), 0.67(g / g), 9: 1(mol / mol), 50 ℃ and 8 h, respectively; under which the degree of substitution of the product was 0.16. Compared with short chain acyl donor, the reaction with long chain acyl donor showed a much lower DS value, which is mainly due to the steric hindrance caused by the long-chain substrates reaching the active cites of the enzyme molecules.The formation of NCC esters were confirmed by the presence of the carbonyl signal in the FT-IR and solid 13 C NMR spectra. Based on changes of carbon proton peak intensity, esterification was confirmed to occur mainly at the C-6 and C-3 sites. XRD results indicated that the original NCC showed a typical structureⅠ. No significant changes occurred to crystal structure and crystallinity during the esterification, the acylation substantially takes place at the surface or amorphous regions. TGA results showed that the thermal stability of e-NCC better than the original one. Water contact-angle measurements revealed the hydrophobic character of the product after esterification of NCC.The composite membranes were prepared by e-NCC and polylactic acid. The performance of the composite membranes were tested to investigate the effects of e-NCC. The results showed that e-NCC can be well dispersed in organic solvent, which has little effect on the transparency of the composite film. Due to the stronger interfacial bonding force between polylactic acid and e-NCC, the mechanical and barrier properties of the composite films were promoted. |
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