The Study Of Cellulose Nanofibrils Modified Poly(Lactic Acid) Composites And The Compatibilization Mechanism

In the paper, we focus on the blending effect of cellulose nanofibrils (CNF) and poly(lactic acid) (PLA). In order to improve the interfacial compatibility between hydrophilic cellulose nanofibrils and hydrophobic PLA, compatibilizer, silane coupling agent (APS) modified cellulose nanofibrils (A-CNF), graft copolymerization modification of cellulose nanofibrils (CNF-g-PMMA) were blended with PLA. The properties of different composites were studied and the mechanism of interfacial cohesion was revealed. Poly(lactic acid) porous scaffolds were prepared by liquid-liquid phase separation method and the effects of silane treated CNF and untreated CNF on the properties of the porous scaffold were also investigated.Natural pulp board was treated by acid hydrolysis and high pressure homogeneous process to obtain lignin-cellulose nanofibrils (LCNF) with diameter of 50nm. The crystallinity of LCNF decreased and the thermal stability enhanced. The mechanical properties, crystallization properties, thermal stability, dynamic mechanical properties of LCNF/PLA were significantly increased compared with PLA. The tensile strength, tensile modulus and elongation of LCNF/PLA composites were improved by 28%,44.4% and 44% in the addition of 3 wt% LCNF, which produced a strong interaction between LCNF and PLA.Controling the lignin content to obtain the LCNF with 5 wt%,9 wt%,14 wt% lignin content, the thermal stability and surface energy of LCNF were decreased along with the increase of lignin content. The surface interaction between LCNF and PLA was improved with the increase of lignin content, the highest tensile strength and tensile modulus of composites for PLA/LCNF with 9 wt% lignin content had improved 35% and 51% compared with PLA.The elongation of PLA/LCNF composite with 14 wt% lignin content increased 92%. The thermal stability, crystalline and thermo-mechanical properties improved at first stage then declined with the increase content of lignin. The lignin connected with cellulose through intermolecular hydrogen bond, PLA can interact with lignin by Vander-Waals force and ester groups interaction, arising lignin content can improve the interfacial bonding by increasing the interacted scope.Isothermal crystallization kinetics of LCNF/PLA composite indicated LCNF can obviously improve the crystallization efficiency of PLA, the isothermal crystallization rate improved by 110%at 120℃ and LCNF did not affect the nucleation mechanism and dimensions of crystal growth of PLA. The thermal degradation kinetics of PLA/LCNF composite showed LCNF could improve the thermal degradation activation energy. Pyrolysis process showed nucleation and nuclear growth and phase boundary controlled equation mechanism.A Silane coupling agent (APS KH550) was used to modify the surface of CNF (ACNF). ACNF keeps their integrity and rod-like morphology, the crystallinity of ACNF was decreased. There are less hydroxyl groups in ACNF than in CNF, the oxygen-carbon ratio of CNF and ACNF was 0.71 and 0.55. The highest tensile strength of composites was obtained for PLA with 2 v/v% APS and 3 wt% CNF, the tensile strength, tensile modulus and elongation improved 48.1%,77.8% and 44% compared with PLA. The crystallization and thermo-mechanical properties were enhanced obviously. ACNF can disperse evenly in PLA matrix and interacted with PLA by chemical bond to improve interface bonding.Graft copolymerization of methyl methacrylate (MMA) onto cellulose was conducted with cericammonium nitrate as the redox initiator. PMMA grafted cellulose nanofiber (CNF-g-PMMA) was obtained. Compared to CNF, CNF-g-PMMA was enlarged and rougher. The water contact angle increased with grafting. The crystallinity index dropped, while the thermal stability improved upon grafting. The resultant nanofibers were incorporated into PLA matrix, the optimal adding amount of CNF-g-PMMA was 5wt%, and this resulted in simultaneous enhancements of the tensile modulus, tensile strength and elongation of approximately 19.6%,38.9% and 148%, respectively. The thermal stability improved and the glass transition temperature dropped. It was clearly evidenced that the PMMA grafting of CNF enhances their compatibility with the polymeric matrix and thus improves the final properties of the nanocomposites. In this case, the rigid CNF contributed to the endurance of higher stress, whereas the long grafted CNF chains improved the association between the PLA matric and the nanofiber filler and hence facilitated the transfer of stress to the rigid CNF. The mechanical performances of LCNF/PLA, A-CNF/PLA, and CNF-g-PMMA/PLA were compared. CNF-g-PMMA performed the best plastic ability, while A-CNF/PLA had the best tensile modulus and strength.PLA and its composite porous scaffolds were prepared by liquid-liquid phase separation method. Comparing the effect of ACNF and CNF on the porosity, pore structure, mechanical properties, thermal stability and degradation properties of PLA porous scaffolds. The LCNF/PLA scaffolds had uniform distribution of pore structure, pore size less than 1μm and an 85% increase in compressive strength compared with PLA scaffods, the thermal stability was improved and the degradation process in SBF solution was controllable, which was to meet the needs of different tissue engineering scaffolds.