Preparation Of Poly(Lactic Acid)-Based Composites And Regulation Mechanism Of Its Crystallization And Mechanical Properties

Poly(lactic acid)is a thermoplastic bio-based polymer with monomers of various natural origins such as sugar beet,sugar cane and corn.On the one hand,poly(lactic acid)has become one of the most promising bio-based polymers due to its excellent mechanical strength,biodegradability and adaptability to a variety of existing molding processes.However,defects such as slow crystallization rate,low heat resistance,and brittleness limit the scope of poly(lactic acid)applications.On the other hand,poly(lactic acid),as a semi-crystalline polyester,has a variety of crystal structures and crystal morphologies.Therefore,in order to broaden the application scope of poly(lactic acid),it is of great scientific significance and application value to regulate the crystalline and mechanical properties of poly(lactic acid)by constructing composites.In the second chapter of this thesis,we designed two kinds of nano-fillers to improve the crystallization of poly(L-lactic acid)(PLLA)from the perspective of regulating the crystalline properties of poly(lactic acid).One is poly(lactic acid)stereocomplex fibers(Sc-fibers)prepared by electrospinning technique,and the other is cellulose nanocrystals grafted with PLLA on the surface(g-CNCs).The two nano-fillers can be uniformly dispersed in the PLLA matrix and promote the crystallization of PLLA,and the crystallization-promotion effect of g-CNCs is more significant.However,in the case of simultaneous compounding of g-CNCs and Sc-fibers in PLLA,the crystallizationpromoting effect of g-CNCs would be inhibited to some extent.Further research revealed that this is because the poly(D-lactic acid)(PDLA)in Sc-fibers will pre-form the stereocomplex crystal form of polylactic acid on the surface of g-CNCs.The nucleation of g-CNCs is weakened by the chain conformational mismatching between the stereocomplex crystal form and the α-homogeneous crystal form of the PLLA matrix.The PLLA crystals induced by Sc-fibers and g-CNCs were different in terms of crystal structure and crystal morphology.Finally,Sc-fibers and g-CNCs were also found to improve the mechanical properties of PLLA,and the relationship between the structure and properties of the composites was analyzed.In the third chapter of this thesis,we propose an idea to prepare poly(lactic acid)toughening filler using reversible covalent bond from the perspective of improving the toughness of poly(lactic acid).The reversible covalent bond are capable of reversible breakage and reorganization under the action of certain external stimuli.With the introduction of reversible covalent bonds in the plasticizing filler,the plasticizing filler is able to form a reversible cross-linked structure,which improves the toughness of polylactic acid and endows reprocessability that general cross-linked materials do not have.Based on this idea,we designed a reversible covalent bond cross-linked network(PEG-DA)with polyethylene glycol(PEG)as the main chain to improve the toughness of PLLA,and demonstrated the feasibility.The reversible cross-linked of PEG-DA is achieved by the [4+2] cycloaddition reaction between the furan-modified PEG and 4,4’-Bismaleimidodiphenylmethane.Our research demonstrated that the PEG-DA filler has excellent compatibility with PLLA,and its cross-linked structure remains well temperature responsiveness and thermal reversibility after dispersion in PLLA matrix.In addition,the introduction of reversible covalent bonds can effectively improve the loss of mechanical strength of PLLA when the plasticity is improved,resulting in a significant increase in the toughness of PLLA.