PLA Stereocomplexation Effects On The Structure And Properties Of Polymeric Materials

Poly(lactic acid)(PLA)has two enantiomers,poly(L-lactic acid)(PLLA)and poly(D-lactic acid)(PDLA).PLA-Stereocomplex crystal(PLA-SC)will be formed between the two enantiomers due to multiple hydrogen bonds.The special interaction between the two enantiomers of polylactic acid provides a new way for polymer modification(toughening,crosslinking and compatibilization,etc.).In this thesis,the stereocomplexation effect of L-polylactic acid and D-polylactic acid was innovatively constructed in the modification of polymer materials,and the physical crosslinking,high-efficiency toughening and non-covalent compatibilization of non-polylactic acid-based immiscible blends were realized.The main research contents and conclusions are as follows:(1)Investigation on the construction and property of“Stereocomplexed crosslinking network”:Benefiting from the advantages of SC such as high melting point,high modulus,large rigidity and enhancing effect on composite material,PLLA/PDLA stereocomplexation was introduced into the crosslinking of thermoplastic elastomer to form a crosslinking network as the physical crosslinking point.Firstly,two kinds of graft copolymers were obtained by grafting the chain of PLLA and PDLA to the molecular chain of POE-co-GMA in advance through reactive blending.Then,POE-g-PLLA and POE-g-PDLA were melt-blended,and the SC crystal was formed by the hydrogen bond interaction between PDLA and PLLA molecular chains,while the POE crosslinked network was constructed.The influence of SC as a physical crosslinking point on the micro-structure and macro-performance of the crosslinked network was systematically studied.The results show that PLA-SC,as a physical crosslinking point,can endow the crosslinked network with higher strength,modulus and better heat resistance.In addition,the physical crosslinking network based on SC balances“plasticity-toughness”to some extent,and the elastic recovery rate of the network crosslinked by chemical bonds is equivalent to that of the network crosslinked by chemical bonds under small applied strain(<50%);Besides,due to the reversibility of non-covalent interactions,when the processing temperature is higher than the melting temperature of SC,the crosslinking network can be destroyed,and the melt viscosity decreases significantly,which is beneficial to high-temperature secondary processing.(2)Investigation on toughening of PLLA by dual effects of EGMA micro-crosslinking and stereocomplexation:Firstly,ethylene-methyl acrylate-glycidyl methacrylate terpolymer(EGMA),PDLA and dicumyl peroxide(DCP)were melt blended by“one-pot method”,and the reactive micro-crosslinked elastomer grafted with PDLA(EGMA_C-PDLA)was prepared by in-situ ring-opening reaction between the carboxyl group of PLA and the epoxy group on EGMA molecular chain during processing.Then,EGMA_C-PDLA was melt blended with PLLA to obtain PLLA/EGMA_C-PDLA polymer alloy.The effects of PDLA addition on the mechanical properties,crystallization behavior and micro-structure of PLA/EGMA composites were systematically investigated.Accordingly,PLA composites with high notched impact toughness(7.3 kJ/m~2)and“rigidness-toughness balance”were achieved with low rubber content(5 wt%).Furthermore,the toughening mechanism of micro-crosslinking and interfacial SC(i-SC)was discussed.The results show that EGMA_C-PDLA with PDLA can form an interface stereocomplex crystal with PLLA due to hydrogen bonding,while the unreacted epoxy groups on the elastomer can still undergo in-situ ring-opening reaction with PLLA to improve the interfacial adhesion.This structural model with dual functions of micro-crosslinking and i-SC can effectively toughen PLLA and keep the modulus and rigidity of the matrix.It is the first report to efficiently toughen PLA matrix composites by reactive micro-crosslinked elastomer and i-SC.(3)Investigation on the construction of“Quasi-Block/Graft Copolymer”by interfacial stereocomplex crystal and its non-covalent compatibilization for immiscible polymer blends:Interfacial compatibilization is acknowledged to be the most effective approach to improve interfacial strength between the thermodynamically immiscible components of polymer blends.At present,interfacial compatibilization is mainly realized by grafting or block copolymer formed by covalent bond connection,but whether“non-covalent bond force”can also realize interfacial compatibilization has not been reported.Herein,a new concept of“quasi-block/graft copolymer”is proposed to compatibilize the immiscible polyolefin elastomer(POE)and poly(styrene-acrylonitrile-glycidyl methacrylate)(SAG)blends.The quasi-block/graft copolymer was achieved by the stereocomplex crystals(SC)of PLLA and PDLA moieties that reactively grafted on the main chains of the SAG and POE.By taking advantage of the interfacial compatibilization through SC,the microphase morphology of the blend can be adjusted and the mechanical properties can be improved:firstly,the curvature of the phase interface decreases significantly,facilitating co-continuous morphology on account of the“rigid”interfacial layers;secondly,the tensile strength and modulus of the blend are obviously improved;thirdly,the original phase structure of the blend can be well maintained during long-term annealing due to the high melting point of the interfacial SC.This strategy of compatibilizing immiscible blends based on the hydrogen bonding force of the stereocomplex opens an avenue for interfacial compatibilization.