The Study Of Self-Assembly Behavior Of Poly(L-lactide)-Poly(Ethylene Glycol)-Poly(L-lactide) Triblock Copolymer

Molecular solution of an amphiphilic block copolymer may act as oil phase by dispersing into an aqueous micellar system of small-molecular surfactant,forming oil-in-water(O/W)emulsion droplets.And amphiphilic block copolymers can self-assembly by different kinds of methods.In this paper,Poly(L-lactide)-polyoxyethylene-poly(L-lactide)(PLLA-PEG-PLLA)were synthesized with different EO/LA ratios by ring-opening polymerization of L-lactide using PEG and Sn(Oct)2 as the macroinitiator and catalyst,respectively.They were PLLA20PEG227PLLA20,PLLA53PEG227PLLA53,PLLA109PEG227PLLA109,End PLLA53PEG454PLLA53.For the preparation of copolymer emulsion,PLLA-PEG-PLLA was first dissolved in tetrahydrofuran(THF)and then added into aqueous micellar solution of nonaethylene glycol monododecyl ether(AEO-9),forming the relatively stable emulsion system after 30 min stirring.In the PLLA-PEG-PLLA emulsion system,the amphiphilic backbones themselves experienced solution-based self-assembly to form inverted core-corona aggregates within an oil-phase domain along with the gradually increasing of the concentration of PLLA-PEG-PLLA molecules caused by the volatilizing of THF.For the core-corona structure,PEG segments formed the core and PLLA segments formed the corona.More importantly,anisotropic coalescence of adjacent O/W emulsion droplets could be occurred accompanying with the further volatilizing of THF,then both the hydrophilic block crystallization of core-forming PEGs and the hydrophobic chain stretch of corona-forming PLLAs induced the formation of rodlike intermediates with an average diameter of 300-800 nm.Finally,with the repetition of these processes and oriented alignment of crystallized PEG segments,copolymers fibers could be formed with an average diameter of～2.0 pm。For this paper,the formation mechanism of copolymer fibers was studied by different characterization methods.The copolymers were characterized by 1H NMR,GPC and FT-IR.The copolymer fibers could be observed by SEM and TEM test.Crystallization properties and segments interaction could be achieved by XRD,POM and FT-IR tests.The variation of melting points of copolymers and copolymer fibers were observed by DSC.Furthermore,these novel copolymer fibers could be formed by the emulsification/solvent evaporation method,and it would provide an access to aqueous solution crystallization of these copolymers hydrophilic segments.