Precisely controlled two-step synthesis of cellulose-graft-poly(l-lactide) copolymers: Effects of graft chain length on thermal behavior

Abstract

This study presented a method to polymerize to cellulose-graft-poly (l-lactide) (cellulose-g-PLLA) copolymers as chemical modification process to overcome disadvantage of melt processing of pristine cellulose due to the strong intermolecular hydrogen bonds. In order to maximize the chain length at the end of cellulose, we designed a precisely controlled polymerization based on a two-step synthesis; 1) short-chain copolymerization of cellulose in an ionic liquid, 2) further graft polymerization in N,N-dimethylformamide and the bulk phase. Accordingly, we could synthesize a copolymer in high yield, with the number of substitutions per glucose unit (DSPLLA) being close to the theoretical maximum value of 3 and the number of lactyl repeating units introduced per glucose unit (MSPLLA) of 212, at a high yield. While the glass transition temperature (Tg) of the copolymer was greatly decreased owing to dissociation of the hydrogen bonds of cellulose and plasticizing effects at a low PLLA content (MSPLLA = 11.2), the Tg gradually increased to attain a value close to that of pure PLLA as the content increased.The cellulose-g-PLLA with low molecular weight was amorphous phase, but the crystallinity started to show from the sample with MSPLLA of 42.0 and WPLLA = 95.0%. As the molecular weight contents of PLLA were increased, the cellulose-g-PLLA of the melting point and the heat of fusion was increased. The cellulose-g-PLLA of the strong inter- and intramolecular hydrogen bonds between the hydroxyl groups was found to affect PLLA crystallinity.