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The Crystallization Behavior Of PPO(MePEG)-b-PLLA Copolymers

Posted on:2011-03-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z F WeiFull Text:PDF
GTID:1101360305497362Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
A series of linear crystalline-crystalline and triarm amorphous-crystalline diblock copolymers with various molecular weight were synthesized by ring-opening polymerization of L-lactide (L-LA) using monomethoxy poly(ethylene glycol) (MePEG) or low unsaturated poly(propylene oxide) (PPO) as macromolecular initiator. The crystallization processes and crystallization kinetics of the PLLA blocks were investigated under different confined condition formed by crystalline PEG or amorphous PPO of copolymers samples. Based on the polymer nucleation-growth theory, the crystallization behaviors of PLLA block was investigated under the confinement condition formed by triarm amorphous PPO, in which the complicated crystallization behaviors was discussed. From the viewpoint of the condensed structure of polymers, thermodynamics and crystallization kinetics, the relationships between the structure and properties of novel PLLA matrix copolymers functional materials were investigated. The newly environmentally friendly and biodegradable shape memory materials (SMP) of PPO-b-PLLA poly (ether-ester) were developed. The properties of poly (ether-ester) SMP were investigated from the aspect of experiment results. The main contents of the thesis are as follows:Linear MePEG-b-PLLA copolymers with various PLLA block lengths were synthesized by ring-opening polymerization of L-lactide in the presence of MePEG macroinitiator, using Sn(Oct)2 as catalyst. The condensed structure formed with the crystallization of one block or two blocks can be obtained by controlling the experimental temperatures. The crystals of the PLLA blocks under crystalline PEG corresponded to a form crystal with the orthorhombic unit cell, which is as same as that of PLLA homopolymer. However, the lateral sizes of the PLLA crystals were much smaller than that of PLLA homopolymer. The entanglements between MePEG-b-PLLA copolymers chains decreased by preparation the bilayer films. The lateral sizes of the PLLA crystals from the bilayer films were much larger than values of the bulk samples.For the samples confined by configuration of polymer chains, triarm PPO-b-PLLA copolymers with different compositions were synthesized by ring-opening polymerization of L-lactide in the presence of PPO macroinitiator, using Sn(Oct)2 as catalyst. Meanwhile, triarm PLLA homopolymer and linear PPO-b-PLLA copolymers were also synthesized for experiments. The crystallization behaviors from glass states and the crystal structure of PPO-b-PLLA copolymers were investigated. The results indicated that the segment structure had obvious effect on the cold crystallization behaviors of PPO-b-PLLA copolymers. The cold crystallization peak temperatures, melting temperatures and melting enthalpy of triarm PPO-b-PLLA copolymers with fixed PPO block length decreased with decreasing the PLLA block length. Meanwhile, the melting temperatures, melting enthalpy of triarm PPO-b-PLLA copolymers with fixed PLLA block length increased with increasing the PPO block length. In addition, crystals formed in the cold crystallization behavior corresponding to the a form crystals with the orthorhombic unit cell.The effects of the segment structure and the block lengths on the crystallization kinetics and thermodynamics of PPO-b-PLLA copolymers were investigated. The effect of various PLLA block lengths on the crystallization kinetics of triarm PPO-b-PLLA copolymers was investigated and analyzed with nucleation and growth theory by Lauritzen-Hoffman. The crystallization kinetics of triarm copolymers indicated that the regime transition temperature from regime II to regime III depended on the compositions and various block length of copolymers. The transition temperatures of different samples were in the range of 111℃-125℃, respectively. The calculated values of isothermal crystallization activation energy, nucleation constant and the fold surface free energy of triarm copolymers were higher than that of triarm PLLA homopolymer, which suggested the complication of the crystallization behavior in the polymers with star shape structure. The result implies that the amorphous PPO block not only had effect on the plasticization but also the confined effect on the nucleation and crystals growth of triarm copolymers. Furthermore, nonisothermal crystallization behaviors and subsequent melting behavior of triarm and linear of PPO-b-PLLA copolymers were also investigated. Nonisothermal crystallization kinetics was analyzed with Jeriorny, Ozawa and Mo method, respectively. The values of nonisothermal activation energy of triarm PPO-b-PLLA copolymers were obtained by Arrhenius equation and exhibited the same trend with the analysis of melt isothermal crystallization, which results further demonstrated the isothermal crystallization experiments and conclusions.In the morphological researches, the spherulite morphologies of triarm PPO-b-PLLA copolymers were observed by polarized optical microscopy (POM) with hot-stage. Different PLLA block lengths and the crystallization temperature had obvious effect on the crystals morphologies of triarm PPO-b-PLLA copolymers. The spherulites with concentric extinction banded textures were observed under various supercooling degrees. The clusters of the lamella in the banded spherulites were observed using atomic forced microscope (AFM). The protrude part of the banded spherulites was built up with fiber-orientation lamella, while the depressed part built up with the stacks of lamellae with screw dislocations. From such phenomenon, it can be speculated that the formation of the banded spherulites were attributed to different orientation of the lamella. The spherulites after etching were observed with scanning electron microscope (SEM), which growth obeyed the mechanisms of "branching" model. In addition, the crystals morphologies of triarm PPO-b-PLLA copolymer in the ultra thin films under geometrical confined environment were investigated by AFM. With the film thickness of 1μm, the dendrite can be observed, of which the crystal growth was controlled by diffusion mechanism. Moreover, the morphology of the dendrite strongly depended on the crystallization temperatures. The crystals of sunflower-shape and hexagonal-shape were observed at 105℃and 110℃, respectively. As the thickness of the film decreased to 200 nm, the crystal morphology exhibited the dendritic shape, which was built up with the stacks of micro-bind lamella. As the thickness of the film decreased to 100 nm, crystals with oriented lamellas can be observed and exhibited "island" distributing. The results indicated the distribution and the size of crystals in the thin films under low dimensional confinement.As one of the novel biodegradable diblock copolymers materials, the shape memory (SMP) materials of crosslinked PPO-b-PLLA copolymers were synthesized using Sn(oct)2 as a catalytic and isocyanate (TDI) as a crosslink agent. The effect of the crystallization on the shape memory parameters was investigated, which results elucidated the relationships between the structure and properties of such SMP materials. A series of novel biodegradable poly(ether-ester) SMP materials with different molecular configuration and shape memory properties can be obtained by adjusting the crystallization process and the crystal structures of PLLA segments.
Keywords/Search Tags:triarm configuration, diblock copolymer, crystallization, structure, shape memory
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