The Effect Of Interfacial Stereocomplexation On The Crystallization Kinetics Of Nanoparticles And Toughener Modified Polylactide

Polylactide(PLA)is a new green and environmentally friendly material that is expected to replace petroleum-based polymers.It has a wide range of raw materials and its mechanical properties are comparable to engineering plastics.It is also biodegradable and biocompatible,and has been widely used in all aspects of life.However,PLA is brittle and has poor toughness.At the same time,the slower crystallization rate and lower crystallinity of PLA lead to poor heat resistance of its related products.These shortcomings limit the further development of PLA.Filling with rigid fillers can significantly improve the mechanical strength and heat resistance of PLA,and blending with elastomers can significantly improve the toughness of PLA.Previous studies have found that after filler compounding and toughening modification,the crystallization of polylactide matrix has a close influence on the mechanical properties,heat resistance,and degradation of the material.This thesis takes toughened polylactide(PLLA)as the object,studying the effect of stereocomplex crystals(sc-PLA)constructed at the interface of different modifiers on the nucleation,crystal form characteristics and crystallization kinetics of matrix crystals,discerning the difference and internal mechanism of sc-PLA nucleation ability at different rigid interfaces and improving crystallization kinetics to provide new technical means and theoretical support for the future development of polylactide materials.First,PLLA and D-polylactide(PDLA)with the same content and chain length were grafted on the surface of nanocellulose(CNC),and the crystallization behavior of the composite modified PLLA was studied.The results show that: compared with CNC-g-D,CNC-g-L nano-filler is more conducive to increasing the crystallization rate of PLLA.Both microscopic observation and quantitative analysis found that the nucleation density and growth rate of PLLA/CNC-g-L were higher than those of PLLA/CNC-g-D.Although the interface sc-PLA has a certain nucleation effect,it causes the viscosity of the PLLA/CNC-g-D melt to increase and restricts the movement and diffusion of PLLA.The CNC-g-L,as a heterogeneous nucleation point,promotes nucleation while restricting the diffusion of PLLA is weaker.Secondly,the fully bio-based polyurethane elastomer(L-PU)containing PLLA was blended with PLLA,and the crystalline morphology and crystallization kinetics of toughened PLLA with different blending ratios of L-PU were studied.The results show that PLLA is partially compatible with L-PU,and the phase morphology of the blended material is "sea-island structure".During non-isothermal crystallization,L-PU does not promote the crystallization of PLLA melt,but can accelerate the cold crystallization of PLLA,due to the segmental diffusion promotion effect of L-PU.During the isothermal crystallization,L-PU has a weaker crystallization promotion effect on PLLA.The POM photomicrograph results show that highly flexible L-PU can accelerate the growth of PLLA spherulites,but the nucleation effect is not obvious.Hoffman-Lauritzen analysis shows that with the increase of L-PU content,the nucleation energy barrier of the blended material first decreases and then increases,which is related to the size change of the dispersed phase.Finally,in the fully bio-based polyurethane(D-PU)containing PDLA toughened PLA system,by constructing the interface sc-PLA,the influence of the interface sc-PLA on the crystalline morphology and crystallization kinetics of toughened PLLA was studied.The results showed that: the interface sc-PLA significantly improved the compatibility between PLLA and D-PU,the dispersed phase was deformed and gradually presented a "co-continuous" phase morphology.During non-isothermal crystallization,the addition of D-PU can promote the melt crystallization and cold crystallization of PLLA at the same time,due to the formation of interface sc-PLA.During the isothermal crystallization,the half crystallization time of PLLA/D-PU blended samples is significantly reduced compared with pure PLLA.The half crystallization time of PLLA/D-PU30 blended samples at 120 ℃ is more than 4 times less than that of pure PLLA,indicating the introduction of sc-PLA greatly promoted the crystallization of PLLA.The interface scPLA significantly improves the nucleation ability of PLLA.The nucleation density of the PLLA/D-PU30 blend material when crystallized at 120 ℃ is two orders of magnitude higher than that of pure PLLA.