| Polylactic acid（PLA）is produced from a wide range of raw materials and is sustainably produced.It has its good processing and mechanical properties compared to other biodegradable polymers.In addition,its mechanical properties can be comparable or even better than those of petroleum-based polymers such as polyethylene polymers（PE）,polypropylene（PP）and polystyrene（PS）.However,the inherent high brittleness and slow crystallization rate of PLA limits the development and application of PLA.toughening and modification strategies for PLA focus on plasticization,copolymerization and blending.Among these methods,polymer blending is a simple and economical way to prepare the material.In this paper,small molecule substances are selected as toughening agents to improve the brittleness of PLA by using simple melt blending and hot pressing to prepare small molecule toughening agent/PLA materials.The effects of small molecule plasticizers and processing methods on the rheological behaviors and mechanical properties of the composites are investigated by adjusting the processing process.Details of the study are as follows.（1）Industrial small molecule plasticizer N-ethyl-p-toluenesulfonamide（N-PTSA）was selected for simple melt blending to prepare a series of PLA/N-PTSA with different mass ratios,and the mechanical properties of PLA matrix were optimized by heat treatment of the material.The rheological behaviors,mechanical properties,thermal stability,crystal structure and micromorphology of PLA/N-PTSA were systematically investigated and DSC,XRD and SEM tests revealed that the N-PTSA and PLA matrix were fully interactable systems.Rheological tests revealed that the addition of N-PTSA increased the critical strain value（γc）for the transition from the linear to the non-linear region of PLA/N-PTSA,while decreasing the molecular chain relaxation time（1/ωc）of the PLA matrix.γc and 1/ωc both obeyed a similar power-law relationship.The steadystate rheological behavior suggests that the mechanism of viscosity reduction is related to the reduction of hydrogen bonding in the PLA matrix caused by the inhibition of PLA hydrogen bond formation by N-PTSA.The results suggest that the hydrogen bonding interaction between PLA and N-PTSA plays a dominant role,rather than crystallinity.During heat treatment,an increase in crystallinity was found to improve the notched impact resistance of PLA/N-PTSA,but to the detriment of tensile properties.12 wt% N-PTSA as the optimum content improved the machinability of PLA,with the elongation at break of the sample being 480.4 %,a 63.2-fold improvement over pure PLA（7.6 %）,and the brittleness of PLA being significantly improved.At the same time,the tensile strength and Young’s modulus were 36.4 MPa and 718.6 MPa respectively,which remained at 70 % and 80 % of that of pure PLA（50.8 MPa and 888.2 MPa）,significantly improving the toughness of PLA.（2）In this part,cellulose nanocrystals（CNCs）with a "hydrophilic-hydrophobic" structure were prepared by ion exchange using a core of cellulose nanocrystals（CNCfs）.PLA materials with different ratios of CNCs,CNCfs and CNCfs/CNCs were prepared by melt mixing and hot pressing.DSC and XRD revealed that the addition of CNCs and CNCfs improved the crystalline properties of PLA.The steady-state rheological behaviors revealed that the interaction force between the nanoparticles（CNCs）and the polymer（PLA）molecular chains was stronger than that of the polymer（PLA）molecular chains themselves,resulting in a decrease in the steady-state viscosity of PLA/CNCs.The incorporation of CNCfs resulted in a decrease in the molecular chain entanglement points and an increase in the molecular chain mobility of PLA/CNCfs.The improved flow properties confirm that CNCfs has a plasticizing effect and that the processing properties of the material are greatly improved.The results show that the tensile strength and tensile modulus of PLA/CNCs increase with increasing CNCs content when CNCs are added to the PLA matrix.However,there is little effect on the elongation at break of PLA/CNCs,which still exhibits the typical brittle structure.When the content of CNCs was increased to 2 wt%,the tensile strength,tensile modulus and elongation at break of the material increased to 65.61 MPa,1414 MPa and 6.6 %respectively.The addition of CNCs had a significant strengthening effect compared to pure PLA（56.25 MPa,1265 MPa and 5.38 %）.When CNCfs were added to PLA,the elongation at break of PLA/CNCfs gradually increased,while the tensile strength and tensile modulus gradually decreased（but remained higher than that of pure PLA）.The synergistic effect of CNCfs（fixed at 1 wt%）and CNCs increases the elongation at break of PLA/CNCfs/CNCs by 15.3 % compared to 5.38 % for pure PLA.The toughness of the material is significantly improved,with its brittle structure transformed into a ductile one.At the same time,the slightly reduced tensile modulus has a good effect on promoting the flowability of PLA during processing.It was found that the synergistic effect of CNCfs and CNCs contributed well to the balance of strength,modulus and ductility of PLA/CNCfs/CNCs,with both strengthening and toughening effects. |
