Redox initiated aqueous graft copolymerization of nanocellulose and xylan

Aqueous graft copolymerization of nanofibrillated cellulose (NFC) and different xylans with several acrylates and methacrylates, was studied using cerium(IV) initiated free radical method. The most important reaction parameters affecting the yield were the monomer type and concentration. The selectivity of monomers with higher homopolymerization tendency was improved by a higher initiator concentration. Grafting patterns obtained with different monomers differed significantly in graft length and density. Thermoplastic behavior was observed in NFC copolymers with more than 50% of synthetic polymer. The composition of the xylan substrate also had a high impact on the yield and selectivity. The branching degree of 4-O-methylglucuronic acid was the most important reactivity-increasing factor, probably both by solubilizing xylan and attracting the positively charged cerium ion. High lignin content was found to hinder the reaction, most likely due to decreased solubility and radical scavenging by the phenolic hydroxyl groups in lignin. The graft yield was especially poor with xylan having very low molecular weight, apparently because short polysaccharide chains prevented the successful separation of the copolymer by precipitation. Polymer grafting reduced the affinity of arabinoxylan towards cellulose surface. The affinity of glucuronoxylan was slightly increased, most likely due to the grafted polymer screening the negative charge of the xylan, and hence decreasing the electrostatic repulsion and compensating for weaker hydrogen bonding. Nanocomposites were prepared by solution casting from poly(methyl methacrylate) (PMMA) and NFC grafted with the same polymer. Formation of a percolating NFC network took place between 1 and 5 wt% NFC loading. Elastic moduli of the composites increased and strain at break decreased with increasing NFC content. Tensile strength also decreased at all NFC concentrations between 0.5 and 5 wt%, which was the maximum processable NFC loading. Using grafted NFC instead of unmodified NFC did not improve the mechanical properties, suggesting negligible entanglement formation between grafted polymer and matrix. Cationic NFC was synthesized by grafting NFC with a monomer containing a quaternary ammonium group. The product exhibited limited antimicrobial properties against Gram positive and negative bacteria and a type of yeast. Comparison to another NFC derivative, cationized by etherification, showed that the etherified product was more antimicrobial, most likely due to its higher charge density. Preliminary cytotoxicity screening showed that neither NFC derivative released soluble chemicals harmful to human cells.