Multilevel Aggregated Structure And Properties Of Poly(Lactic Acid)and Its Copolymers In Solution Tuned By Crystallization

The multilevel aggregates at different levels on macroscopic,mesoscopic and microscopic scales in solution,such as polymer single crystal,shish-kebab,microsphere,micelles and gels,have attracted great attentions due to their potential applications in industrial catalyst,biomedical engineering and nanotechnology.The formation of these multilevel aggregates is influenced by the synergetic effects of the interaction of solvent,interaction of interface,and the interchain interaction.Meanwhile,the morphology and properties of aggregates are also greatly affected by crystalline structure of polymers.However,the mechanism for the formation of multilevel aggragates in solution driven by crystallization is still unclear.Therefore,it is important to focuse on the formation,evolution and regulation of crystallization-driven multilevel aggregates,which is greatly helpful for the design of functional polymer materials.As a typical crystalline polymer.poly(lactic acid)(PLA)is biodegradable and biocompatible,which has been widely used for various biomedical applications.PLA has defferent stereoisomers,i.e.,poly(L-lactic acid)(PLLA)and poly(D-lactic acid)(PDLA).Stereocomplexation(SC)crystallization can be formed when PLLA and PDLA mix or copolymerize.which affords the materials better mechanical properties,higher melting temperature and better hydrolytic resistance than the homocrystalline(HC)materials.Therefore,well-defined aggregates with excellent properties can be obtained when SC of PLLA and PDLA is used as the driving force in solution aggregation.First,we prepared the polymer single crystal and shish kebab in dilute solution.The polymer single crystal was obtained by slow cooling method and the formation of hierarchically ordered PLA nanofiber shish kebabs was prepared by combining electrospinning and controlled polymer crystallization.The resultant morphology of hierarchical structure was influenced by the stereostructure of PLA.The formation mechanism was also investigated.Nanofibers were prepared by electrospning high molecular weight(HMW)PLA,which was used as the shish,and a secondary low molecular weight polymer(HC-PLA or SC-PLA)was decorated on the nanofiber by an incubation method to form kebab lamella.For HC-PLLA or HC-PDLA,scanning eletron microscopy(SEM)experiment showed that the toroid kebab completely wrapped around the nanofiber along the nanofiber axis.For SC-PLA,the kebabs with smaller size were uniformly located along the entire nanofibers and completely wrapped around the nanofibers.The formation process of toroid kebabs in time sequence was conducted to investigate the formation mechanism of the nanofiber shish kebab,which followed soft epitaxy-governed crystal growth.The PLA chain conformation in SC-PLA was the same as that of the SC-PLA fiber,which led to the crystallographic matching.Meanwhile,the PLA chain conformation in SC-PLA nanofiber and PLLA or PDLA polymer was totally different,so the nucleanting capability of PLA homopolymer was lower than the SC-PLA.Second we investigate the formation of lamellae aggragates in higher concentration solution.The PLA particles with diversified morphology were obtained through the precipitation(or solvent replacement)method.Morphology of assembled particles was strongly influenced by stereo structure of polymers,supramolecular units,and molecular weight of polymers.Both the homocrystalline flower-shaped and stereocomplexed sphere particles could be attained by varing the contents of PLLA and PDLA supramolecular polymers(SMP).The morphology reflected the interplay of liquid-liquid phase separation and nucleation of polymer crystals in solution.A series of hydroxyl-terminated telechelic and 3-arm star-shaped PLLA and PDLA precursors with different molecular weight were prepared by ring opening polymerization of L-lactide(or D-lactide)using 1,6-hexanediol and trimethylolpropane as the initiator.Afterward,the terminal hydroxyl of PLA was converted to UPy by reacting with excess of UPy-NCO.HC crystallization of UPy-functionalized supramolecular PLA yield the flower-shaped particle in precipitation;yet SC crystallization of PLA supramolecular copolymers led to the formation of sphere particles.Flower-shaped particles could be transformed into the sphere particles by varing the mixing ratio of PLLA and PDLA contents.The attained PLA particles showed controlled degradation rates,drug loading content(DLC),and drug release behavior,which strongly depended on the particle morphology and crystalline structure.The spherical morphology and stereocomplexed structure afforded the racemic SC-SMP particles higher DLC,slower degradation,and drug release rates due to the more compact chain packing.Finally hydrophilic oligo(ethylene glycol)methyl ether methacrylate(OEGMA)or poly(ethylene glycol)(PEG)was copolymerized with PLA to prepare amphiphilic copolymers.The structure and properties of copolymer aggregates were greatly controlled by the PLA crystallization.Random graft copolymers containing OEGMA and PLLA or PDLA as the enantiomeric copolymer pair was successfully synthesized by atom transfer radical polymerization.The amphiphilic copolymers self-assembled into micelles in aqueous solution that exhibited a cloud point temperature(TcP).Tcp could be tuned only by varing the mixing ratio of L-and D-configured copolymers,which controlled the degree of crystallization.With the enhancement of SC,Tcp decreased.Based on the analysis of synchrotron radiation small angle X-ray scattering(SAXS)results,SC crystallization of the hydrophobic block led to formation of micelles with a larger size,bigger core and higher aggregation number,which facilitated the aggregation between micelles above Tcp due to improved intermicellar attractions.The SC crystallization in the micelle core also improved the separation efficacy of thermoresponsive copolymers for the removal of hydrophobic pollutants from water.Multiple hydrogen bonds were also incorporated into the PLA copolymers to prepare the stimuli-responsive supramolecular micelles by combing the reversibility of hydrogen bonds and crystalline structure of PLA chain.PLA/PEG supramolecular amphiphilic diblock copolymers were prepared based on the thymine/diaminotriazine(THY/DAT)complenentary hydrogen bonding interactions between PLA-THY and PEG-DAT.Compared with the conventional covalently-bonded diblock copolymer micelles,supramolecular micelles had larger size.The micelles were sensitive to external stimuli such as pH and ions and they underwent fast aggregation in the acidic and salted solutions due to the disassociation of complementary hydrogen bonds.The stereocomplexed micelles also exhibited larger DLC and slower drug release rate than the homocrystalline ones.