Research On The Modification Of Crystallization And Reactive Blending Toughening For Poly (L-lactic Acid)

Poly(L-lactic acid)(PLLA),as a kind of bio-renewable and completely biodegradable polyester,is derived from renewable plant resources and accords with the requirement of the circular economy concept and sustainable development strategy.In other hand,it has good mechanical properties and excellent processing performance,so it has been considered as one of the most potential environmental-friendly materials among so many green polymer materials.Unfortunately,the slow crystallization rate and inherent brittleness of PLLA are still the major drawbacks that prevent it from further practical applications as engineering material.Therefore,this research is based on the crystallization modification and toughening problems of PLLA.On the one hand,a nucleating agent-N,N'-bis(phenylcarbamoyl)hexamethylenediamine(NA6)was synthesized to prepare PLLA/NA6 blends and the crystallization properties and the crystal morphology were researched.On the other hand,a novel reactive blending method was introduced to prepare biodegradable and super-tough PLLA matrix composites.The toughening mechanism was further studied by instrumented impact tester and analyzing the morphology structure.The effects of molecular weight of PCL,crosslinking degree,and matrix crystalline state on the toughness of PLLA were researched as well.The main conclusions are displayed as follows:1.Neat PLLA shows a crystallization peak temperature(Tc)at around 93.8 ?,while the Tc values shift gradually to 118.6 ? with the increasing content of NA6 for the PLLA/NA6 blends,and the crystalline degree increases from 12.0% to 65.5%,while the isothermal crystallization rate at 130 ? is increased by up to 14.5 times than that of neat PLLA.Besides,the half-crystallization time is reduced from 25.8 min to 1.8 min.Moreover,the size of PLLA spherulites becomes smaller and the nucleating effect gets more significantly with the increasing of NA6 content.It indicates that the improvement on the crystallization behavior of PLLA is achieved by introducing the nucleation agent of NA6.2.Super-tough and high-strength PLLA composite was prepared by in situ reactive blending of PLLA with the CPU formation in PLLA matrix.The super-tough PLLA blend with the most excellent toughness of 55.0kJ/m2 is obtained by optimizing the blending ratio of PCL(molecular weight of 1000 g/mol)at 20 wt% to form the CPU phase dispersed in PLLA matrix.Compared with neat PLLA,the elongation at break is also increased from 5% to 139.4%,indicating a transformation from the brittle to ductile fractures.However,for the PLLA/LPU20 blend,the notched Charpy impact strength is far lower than that of PLLA/CPU20 blend,so the formation of CPU crosslinking network makes great effect on the toughness property of PLLA blends.DMA analysis indicates some compatibility between PLLA and PU.The rheological properties reflects a transition from liquid-like to gel-like behavior for PLLA/CPU20 blend,conjecturing the existence of a threshold CPU content for the co-continuous network formation.The analysis of instrumented impact tester and morphology structure imply that the strong interfacial adhesion can enhance the ability of resistance to crack propagation.The interfacial adhesion is enhanced to a suitable strength,resulting in construction of a unique “bicontinuous phase structure” which triggers massive plastic deformation of PLLA matrix.3.The molecular weight of PCL has obvious effect on the toughness of PLLA/CPU20 blends.The highest impact strength with 62.4kJ/m2 is obtained when the molecular weight of PCL is 1000 g/mol,then the toughness declines with the increasing of PCL molecular weight.However,the hard segment of formed CPU with lower molecular weight of PCL occupies the higher content,so the prepared blend becomes stiffer,which is bad for the stress transfer and release,resulting in brittle fracture.4.Crosslinking degree also has effect on the toughness of PLLA/CPU20 blends.The impact strength increases first and then decreases with the increasing of crosslinking degree.The samples maintain at high impact strength about 60 kJ/m2 when the crosslinking degree is in the scope of 1~10%.However,the hard segment of formed CPU with higher crosslinking degree occupies the higher content,so the prepared blend becomes stiffer,and the ability of resistance to crack propagation and triggering crazing becomes weak,resulting in brittle fracture.5.The tensile strength is improved but the elongation at break declines when the matrix crystallinity of the blend sample is enhanced.The toughness with high crystallinity is improved when the nucleating agent content exceeds 0.4 wt%.The heat resistance increases obviously with high storage modulus(above 100 MPa)within the scope of 60~120?.The area of shear yielding deformation triggered by elastomer enlarges with increasing crystallinity of the matrix,and then the more excellent toughness can be achieved compared with that of the samples with lower crystallinity in the matrix.