Toughening And Reinforcing By Formation Of Shell-like Layered Microstructure In Polylactide

Poly(lactide) (PLA) is a biodegradable aliphatic polyester derived from renewable resources that has makes it attractive from an environmental and sustainable standpoint. Polylactide could become a competitive alternative to traditional commodity plastics for everyday applications. Unfortunately, broad substitution of polylactide is mainly thwarted by brittle behavior under impact loads. Many attempts to toughen PLA were proposed during last decade, including blending and copolymerizing. However, while enhancing the toughness of PLA by architectures and components methods, the elastic modulus and tensile strength drop drastically.At the same time, the solid-state deformation of thermoplastics has been considered the right way to change the microstructure in polymer processing. The solid-state deformation of polymers induces the alignment of the molecular chains along the deformation direction and achieves an increase in the strength and modulus. Most of the researches focused on structure and properties under high-stain deformation, however, little attention was paid to structure and properties under low-stain deformation.In the paper, we presented that by applying pressure in the solid state (at moderate temperatures), PLA and it’s blend and copolymer aligned hard segments and re-orient crystalline lamellae in a flow field (pressure-induced flow (PIF) processing) resulting in a nacre-like micro-layered morphology of the bulk materials, including PLA, PLA/PEG blend and PLA-PCL multiblock copolymer, and the resulting materials possessed improved mechanical properties.1. Effect of PIF processing on the structure and properties of PLAThe layered microstructure was formed through PIF processing, as well as the tie molecular chains in amorphous phase bridged the layers. By controlling crystallization conditions, pressure, temperature and duration of PIF processing, the layered morphology of hard and soft domains can be tuned.The tensile strength of sample processed at 110℃, 250MPa for 5min increased by 70% than that of original sample, tensile modulus increased by 24%, impact strength increased from 17 kJ/m~2 to 41 kJ/m~2, bending strength increased by 2.5 folds and modulus increased by 120%. The impact strength enhanced to 61 kJ/m~2 again after PIF sample annealed for 90min at 100℃.POM and SEM micrographs showed the deformation of spherulite in the pressure field, the lamellae composed of layered and linked by tie molecules aligning along the flowing orientation. MDSC curves showed that the increase of lamellae thickness enhanced the melt point. MDSC and DMA illustrated that the Tg increased compared with the original sample as the molecular chains was confined between lamellae. Furthermore, XRD results indicated the molecular chains oriented along the direction of flowing.NaOH aqueous solution went through amorphous phase easily since layered microstructure was neatly arranged by lamellae, which accelerated the degradation of PLA.2. Effect of PIF processing on the structure and properties of PLA/PEG blendThe mechanical properties improved greatly after PLA/PEG blend processed by PIF. Impact strength increased by 24 times than that of original sample, tensile strength increased by 2 times, tensile elongation increased from 3.3 percent to 22.7 percent; bending strength increased by 240%, bending modulus increased by 2 times. The impact strength of samples annealed for 90min at 100℃enhanced to 50 kJ/m~2.SEM and POM micrographs showed the spherulite formed, and lamellae slipped each other in the pressure field, at the same time, the amorphous phase was stretched and formed microfiber structure. The microstructures of blend were investigated by MDSC, DMA and XRD. It shown that the molecular chains were extended under interaction of shearing between lamellae, which leaded to increase of orientation andTg.As the crystal phase aligned along the flowing direction, the layered structure accelerated the degradation of PLA/PEG blend.3. Synthesis of PLA-PCL multiblock copolymer and structure and properties after PIF processingA series of PLA-PCL triblock copolymers were synthesized by ring opening polymerization of lactide, initiated by the hydroxyl terminal groups of the PCL chain, catalyzed by the stannous octanoate. Followed by the chain extension of these PLA-PCL-PLA triblocks, using hexamethylene diisocyanate (HDI) in treated chloroform solution, the corresponding PLA-PCL multiblock copolymers were synthsized. The original opaque polymer powder was transformed into transparent samples after PIF processing and the samples shown preferable mechanical properties.The effect of PCL contents, processing temperature and processing pressure on compression ratio of copolymers, tensile properties and impact properties of samples were studied. The results showed that tensile strength decreased as the increase of PCL content increase and processing temperature; the effect of PCL content, processing temperature and pressure on impact properties presented a trend of ascending on the beginning then descending.XRD showed that the copolymers were orientated along the flow direction under pressure, the orientation degree increased as the increase of pressure. SEM micrographs showed that the copolymers formed a new layered structure. It was concluded that the layered structure contributed to the promotion of mechanical properties of copolymers. The times of PIF processing had a slight impact on molecular weight and distribution.4. Formation of layered structure and failure mechanismA theory of low-strain deformation of two-phase morphology in semicrystalline polymer was present. The layered structure was built after the spherulits and lamellae deformation, and the force interaction between lamellae enhanced after the molecular chain of amorphous phase was extended. The layered microstructure mimicked nacre structure enhanced mechanical properties based on the increase of route of fracture and fracture work. An analysis was conducted between impact strength and compression rate of sample PIF processing, and the conclusion of analysis was similar with experimental results.