Preparation of cellulose-graft-polylactic acid via melt copolycondensation for use in polylactic acid based composites: synthesis, characterization and properties

In order to improve the melt strength of poly(lactic acid) (PLA), microcrystalline cellulose-graft-polylactic acid (MCC-g-PLA) copolymer was prepared and introduced into the PLA matrix. The MCC-g-PLA copolymers were synthesized by melt copolycondensation of lactic acid (LA) with microcrystalline cellulose (MCC) which was pretreated to improve accessibility. The MCC-g-PLA copolymers with a molar substitution (MS) of PLA in the range of 1.67-5.97 were synthesized by adjusting reaction temperature, molar ratio of LA monomer to MCC and washing times in the pretreatment process. Compared with MCC, the crystalline structures of MCC-g-PLA copolymers were not perfect due to the existence of PLA side chains. A glass transition temperature (T-g) appeared in the copolymers, which had never been observed in unmodified MCC, and decreased with the increase of MS. When MS was above 4.41, the thermal degradation of PLA side chains emerged. When MCC-g-PLA copolymer was introduced into the PLA matrix, good dispersion of MCC-g-PLA was verified with SEM results. The extension rheology results showed that the melt strength of PLA can be effectively enhanced with the addition of MCC-g-PLA, especially at low elongation rate. Meanwhile, MCC-g-PLA also improved the crystallization ability of PLA in the non-isothermal crystallization process.