Study On The Phase Evolution Mechanism And Properties Of Poly(Lactic Acid)-Based Blends Under Different Processing Flow Fields

The development of bio-based biodegradable polymers can not only solve the environmental pollution problem caused by traditional petroleum-based plastics,but also reduce the carbon footprint and carbon emissions,helping the country achieve the carbon peaking and carbon neutrality goals.Polylactic acid(PLA),a renewable and biodegradable polyester,has great application potential in biomedical polymers,agricultural mulching films,and packaging fields due to its excellent mechanical properties,biodegradability,and biocompatibility.However,the poor toughness,low impact and tear resistance of PLA greatly limit its practical application.As one of the most cost-effective methods,blending modification is widely used to improve the mechanical properties of PLA.The mechanical properties of polymer blends largely depend on their micro-phase morphology,which is greatly affected by the processing flow field.Compared with shear flow field,extensional flow field has more advantages in controlling submicron-phase morphology of polymer blends.However,at present,there is no systematic study on the phase morphology evolution of bio-based incompatible blends under the action of tensile flow field.Therefore,the different PLA based blends with different viscosity ratio were prepared by the twin screw extruder(shear flow field)and the eccentric rotor extruder(extensional flow field),respectively.The phase morphology evolution mechanism of different PLA based blends with different viscosity ratio prepared by the eccentric rotor extruder was studied,and compared with that of PLA based blends prepared by the twin screw extruder.Finally,the correlation model between processing flow field,microphase morphology and mechanical properties is established,which provides technical support and theoretical guidance for the phase morphology characteristics and performance evolution of bio-based incompatible blends.The detailed research content and conclusions are as follows:On the basis of Taylor’s droplet dispersion and fragmentation mechanism under steady flow and Grace’s capillary number experience,a droplet morphology evolution model was established,and the deformation and fragmentation mechanisms of dispersed droplets under shear flow field and extensional flow field were analyzed.Based on theoretical analysis,the Polyflow software was used to conduct numerical simulation to study the flow field characteristics of melt conveying section of twin screw extruder and eccentric rotor extruder.The results showed that the mixing index(λ)of the eccentric rotor extruder dominated by the extensional flow field changed periodically throughout the simulation period,and there were regions with λ greater than 0.5,indicating that the extensional flow field can better realize the mixing and dispersion of the polymer blends compared with shear flow field.For the PLA/polycaprolactone(PLA/PCL)blends with viscosity ratio of 0.78,typical “seaisland” phase morphology was observed for the blends prepared by the twin screw extruder,while fibrous dispersed phase morphology was presented for the blends prepared by the eccentric rotor extruder.The crystallinity study showed that the crystallinity of PLA phase in PLA/PCL blends prepared by the eccentric rotor extruder was significantly higher than that of PLA/PCL blends prepared by the twin-screw extruder,which can be attributed to the heterogeneous nucleation of fibrous PCL phase and the tensile induced crystallization behavior of PLA phase.Due to the fibrous phase morphology of PCL phase and the high crystallinity of PLA phase,PLA/PCL blends prepared by the eccentric rotor extruder can maintain the same strength as neat PLA while improving the toughness.For the PLA/ thermoplastic starch(PLA/TPS)blends with viscosity ratio of 2.42,typical two-phase structure was observed for the blends prepared by the twin screw extruder.With the increase of TPS content,the particle size of TPS of the blends prepared by the twin screw extruder varies greatly,the particle size distribution is wide,and the adhesion of two-phase interface is poor.For the PLA/TPS blends prepared by the eccentric rotor extruder,no obvious TPS particle morphology is observed in all the samples,and the two-phase interface is well bonded.The mechanical properties showed that the yield strength and elongation at break of PLA/TPS blends prepared by the eccentric rotor extruder were significantly higher than those prepared by the twin-screw extruder under the same component conditions.For the PLA/ nylon12(PLA/PA12)blends with viscosity ratio of 4.19,typical two-phase structure was observed for the blends prepared by the twin screw extruder and the eccentric rotor extruder,and there is an obvious phase interface between PA12 and PLA matrix.The results showed that different processing flow fields have little effect on the phase morphology and crystallization behavior of PLA/PA12 blends,which is mainly due to the extremely incompatibility between PA12 and PLA,hindering the effective force transfer between the two phases.Therefore,ethylene acrylate glycidyl methacrylate terpolymer(EGMA)was introduced as the compatibilizer of PLA/PA12 in this paper.PLA/PA12/EGMA ternary blends were prepared by the twin screw extruder and the eccentric rotor extruder respectively.The phase morphology study showed that no obvious two-phase interface can be observed for the PLA/PA12/EGMA ternary blend prepared by twin-screw extrude,indicating that the addition of EGMA significantly improved the interfacial compatibility between PLA and PA12 phase.Different from the PLA/PA12/EGMA ternary blend prepared by twin-screw extrude,the dispersed phase of PLA/PA12/EGMA ternary blend prepared by the eccentric rotor extruder presents a fibrous orientation structure,and the two-phase interface is also fuzzy.Due to the fiber morphology of PA12 phase,PLA/PA12/EGMA ternary blends prepared by the eccentric rotor extruder have better mechanical properties.