Strategies for Cellulose Fiber Modification

This thesis describes strategies for and examples of cellulose fiber modification. The ability of an engineered biocatalyst, a cellulose-binding module fused to the Candida antarctica lipase B, to catalyze ring-opening polymerization of εcaprolactone in close proximity to cellulose fiber surfaces was explored. The water content in the system was found to regulate the polymer molecular weight, whereas the temperature primarily influenced the reaction rate. The hydrophobicity of the cellulose sample increased as a result of the presence of surface-deposited polyester. A two-step enzymatic method was also investigated. Here, Candida antarctica lipase B catalyzed the acylation of xyloglucan oligosaccharides. The modified carbohydrates were then incorporated into longer xyloglucan molecules through the action of a xyloglucan endotransglycosylase. The modified xyloglucan chains were finally deposited on a cellulose substrate. The action of Candida antarctica lipase B was further investigated in the copolymerization of ε-caprolactone and D,L-lactide. Copolymerizations with different ε-caprolactone-to-D,L-lactide ratios were carried out. Initially, the polymerization was slowed by the presence of D,L-lactide. During this stage, D,Llactide was consumed more rapidly than ε-caprolactone and the incorporation occurred dimer-wise with regard to the lactic acid units. Morphological studies on wood fibers were conducted using a sol-gel mineralization method. The replicas produced were studied, without additional sample preparation, by electron microscopy and nitrogen adsorption. Information concerning the structure and accessibility of the porous fiber wall was obtained. Studies of never-dried kraft pulp casts revealed micro-cavities and cellulose fibrils with mean widths of 4.7 (±2) and 3.6 (±1) nm, respectively. Finally, cationic catalysis by simple carboxylic acids was studied. L-Lactic acid was shown to catalyze the ring-opening polymerization of ε-caprolactone in bulk at 120 °C. The reaction was initiated with methyl β-D-glucopyranoside, sucrose or raffinose, which resulted in carbohydrate-functionalized polyesters. The regioselectivity of the acylation was well in agreement with the corresponding lipase-catalyzed reaction. The polymerization was also initiated with a hexahydroxy-functional compound, which resulted in a dendrimer-like star polymer. The L-lactic acid was readily recycled, which made consecutive reactions using the same catalyst possible.