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Controllable Synthesis and Self-Assembly of Stimuli-Rresponsive Multiarm Copolymers Based on Hyperbranched Polymers

Author: HongHaiYan
Tutor: YanDeYue；ZhouYongFeng
School: Shanghai Jiaotong University
Course: Polymer Chemistry and Physics
Keywords: hyperbranched polymers atom transfer radical polymerization stimuli-responsive self-assembly vesicles micelles porous network fission adhesion
CLC: O631.3
Type: PhD thesis
Year: 2008
Downloads: 360
Quote: 2
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Abstract

Hyperbranched polymers are a novel kind of three dimensional torispherical irregular macromolecules possessing highly branched architectures, many inner cavities and a large amount of terminal functional groups. Due to their unique molecular structures and properties, hyperbranched polymers have become the hot topics in many research fields. Up to now, great progresses have been made in the synthesis, characterization, modification, and application of hyperbranched polymers. However, many problems still need to be resolved, such as the exploration of the unknown features, the explanation of new phenomena and the spread of the application fields. In this dissertation, based on the summarization of previous research works of hyperbranched polymers, some creative investigations are conducted in the modification and self-assembly of hyperbranched polymers. Two novel kinds of functional star copolymers based on hyperbranched polyethers with stimuli responsibilities were synthesized successfully, their stimuli responsibilities and self-assembly behaviors in solution were investigated detailedly, their potential use in drug delivery and the stabilization of gold nanoparticles were explored. The detailed main results are shown as follows.1. The controll synthesis and self-assembly of HBPO-g-PDMAEMA in water;The PDMAEMA chains were grafted to HBPO surface by graft-from method, through atom transfer radical polymerization. By altering the amount of DMAEMA monomer, the DParm of HBPO-g-PDMAEMA can be controlled easily. And HBPO-g-PDMAEMAs with DParm varied from 5 to 77 were synthesized successfully. The HBPO-g-PDMAEMAs can form aggregates in water as confirmed by pyrene and Nile Red probe fluorescence spectrometry, and the CMC value of HBPO-g-PDMAEMAs rises with the increase of PDMAEMA arm length (DParm values). These star copolymers can self-assembly into small unimolecular micelles with particle size of approximately 10 nm and large spherical micelles with particle size of approximately 100 nm simultaneously. With the increase of the PDMAEMA chain length, the dimension of the large spherical micelles decreased, but the size of the small unimolecular micelles changes slightly. The solution-state NMR studies show that the aggegates hold a core-shell structure, the hydrophobic HBPOs aggregate together to form the solid-like core while the hydrophilic PDMAEMA arms form the shell. TEM measurements have provided direct evidence that the large micelles are multimolecular aggregates with the basic building units of unimolecular micelles. Then a self-assembly mechanism named as multimicellar aggregates was presented, it is the first demonstration of the self-assembly mechanism for the large multimolecular micelles generated from the solution self-assembly of hyperbranched copolymers.2. The stimuli-responsibilities of HBPO-g-PDMAEMA copolymer and potential use in drug delivery;Aqueous HBPO-g-PDMAEMA solution is sensitive to ionic strength, pH value and temperature. The DLS studies for HBPO-g-PDMAEMA solution with different amount of NaCl shows that large aggregates can be formed when the amount of NaCl is large enough. The DLS and CMC stuides for HBPO-g-PDMAEMA solution with different pH value shows that the aggregate behavior happened with the increase of pH value, and the CMC at pH 1.97 water is larger than that at neutral water. HBPO-g-PDMAEMA copolymers with narrow molecular weight distribution have reverse temperature responsibilities. The quantitative variable temperature NMR studies show that the PDMAEMA arms dehydrated by heating, so the MMAs aggregate together to form large aggregates as detected by the DLS analysis, driving by the increased hydrophobic interactions between PDMAEMA shells. Based on these studies, a proposed mechanism of heating-induced aggregation was presented. But the thermosensitivity of copolymers with broad molecular weight distribution are quite different from the above ones. Several heating experiments including transmttance and DLS mesurements show that they need more time to achieve the stable thermodynamics state. It is supposed that the broad molecular weight distribution plays an important role in the heating-history-influenced thermosensitivities. The selective arrangements of the longer and the shorter PDMAEMA chains are thought to be the main reason. The drug loading and temperature-dependent release properties of HBPO-g-PDMAEMA micelles were also investigated by using indomethacin as a model drug. The indomethacin-loaded micelles display a rapid drug release at temperature around LCST.3. The controllable synthesis of HBPO-g-PDEAEMA star copolymers and the self-assembly behavior in mixture of dioxane and water;The HBPO-g-PDEAEMAs with different length of PDEAEMA arms were also synthesized successfully by ATRP methods. The transmittance vs water content studies show that POPE3 sample with DParm of 11 can aggregate in various mixture of solvent, such as dioxane/water, THF/water, DMF/water, acetone/water and ethanol/water. The Cryo-TEM results show that the self-assembly behavior of POPE3 in dioxane/water mixture is very complicated. Solvent-induced multistage morphology transitions were observed. As the increase of water content, the morphology of the aggregates changed from vesicles to unimolecular micelles and vesicles, then to MMAs and vesicles, then to MMAs, then to hollow spheres, the wall of the hollow spheres narrowed and then they aggregated together to form large objects as the further increase of water content. Moreover, the solution state NMR studies show that this morphology transitions are induced by the multistage change of the solubility of PDEAEMA arms. In addition, the fluorescence studies on DNS-labelled HBPO-g-PDEAEMA with DParm of 11 also provided strong evidence for the morphology transition.4. The stimuli-responsibilities of HBPO-g-PDEAEMA and their use in autoreduction and stablization of gold nanoparticles;HBPO-g-PDEAEMAs have certain pH responsibility. As the increase of the pH value of solution, aggregation happen. At low pH, core-shell micelles including unimolecular micelles and large micelles are formed. As the increase of pH value, the aggregates transfered into vesicles., and further increase of pH value induced the precipitation of the aggregates. By using HBPO-g-PDEAEMA as the dioxidizer and stablizer, stable gold naopaticles were synthesized successfully in water and organic solvent. When using HBPO-g-PDEAEMA aqueous solution, pH value is the main parameter which has big influences on the synthesis of gold nanoparticles. At low pH, the dioxide of AuCl4- can not be achieved. Effective preparations were carried out at pH value larger than 6.15, N/Au molar ratio larger than 10. And when using HBPO-g-PDEAEMA solution in dioxane-riched dioxane/water mixture, gold nanoparticles can also be synthesied successfully. The N/Au molar ratio and water content in dioxane/water mixture are parameters which give effects on the resultant gold nanoparticles.5. The solvent-evaporation induced behavior of the aggregates formed by HBPO-g-PDEAEMA in dioxane/water mixture.Firstly, an asymmetrical budding and fission transition of single-component HBPO-g-PDEAEMA vesicle induced by the solvent evaporation, and a very special bilayer-to-monolayer molecular movement was found in the process. Secondly, the adhesion behaviors of the hollow structures were investigated by TEM and cryo-TEM. At last, by using the hollow spheres as the templet, irreglar porous networks with pores of 170 nm and walls of 15 nm were prepared by the cryo-fix of hollow spheres and the in-situ evaporation of enwrapped solvents. And large irreglar porous networks with porous of 800 nm and walls of 280 nm were prepared by the spontaneous aggregation of the self-assembly objects induced by natural solvent evaporation.

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