Chemo-enzymatic preparation and characterization of cellulose nanofibers-graft-poly(lactic acid)s

Production of nanocomposites of poly(lactic acid) (PLA) grafted on surface-modified cellulose nanofibers (TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized cellulose nanofibers), or TOCN-graft-poly(lactic acid) (PLA), is reported via an improved process. The surface-modified cellulose nanofibers forming the backbone of the composite matrix, were prepared from sugarcane bagasse bleached pulp (BBP). The temperature-time profile during the reaction between L-lactide and BBP-TOCN was controlled (raised from 25 °C to 140 °C over 20 min; held at 140 °C for 15 min; then reduced to 100 °C over 20 min; and held at this temperature for 24 h). At 8 h, the PLA-oligomers prepared separately by lipase catalyzed ring-opening polymerization of L-lactide, were added to the above mentioned reaction mixture. This production protocol enhanced the formation of ester bonds between the L-lactide and carboxylate groups on the surfaces of the modified cellulose nanofibers, to improve grafting. With the new preparation protocol, the carbonyl groups of PLA were covalently linked to the surface functional groups of BBP-TOCN with a 4.6-fold higher grafting ratio compared with the BBP-TOCN-g-PLA produced under otherwise identical reaction conditions but without adding the separately prepared PLA. The BBP-TOCN-g-PLA nanocomposites had a high crystallinity (>76%) and a small crystallite size (3.8 nm).