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Atom Transfer Radical Polymerization And Electro-optical Properties Of Acrylate Polymers Containing Azobenzene Group

Posted on:2008-03-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:N J LiFull Text:PDF
GTID:1101360278966592Subject:Applied Chemistry
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In recent years, much attention has been devoted to polymeric materials with non-linear optical (NLO) properties for electro-optical and photorefractive applications because they have many advantages such as high NLO coefficient, rapid response speed and low cocurrent dielectric constant. In addition, they exhibit good mechanical properties, high chemical stability and excellent flexibility in fabrication. Azo (azobenzene) polymers have been paid great attention for their potential applications in nonlinear optical materials in recent decades and large numbers of azobenzene-containing side-chain NLO polymers have been designed and prepared. ATRP is one of the most attractive methods for the synthesis of novel and controlled architectures in rather straightforward operating conditions. In this way, we can obtain well-defined low molecular weight polymers with narrow polydispersity and the block polymers with desired units.In this thesis, we based on the azobenzene-containing polymers for NLO material. Series of (methyl) acrylates containing different substituted azobenzene side chain are design, synthesized and characterized. Azobenzen-containing polymers with different side chain and different molecular weight are prepared via atom transfer radical polymerization (ATRP) technique. The third-order NLO coefficients x(3) values and response times of the azo polymers are mearsured and the influence of the molecular structure and chain composition of the polymers to their NLO properties are studied.The work can be summarized as the following:(1) A series of azobenzene-containing monomers with different substituents are synthesized and polymerized via ATRP technique. The homopolymers and copolymers containing azobenzene side chain were obtained and their third-order NLO coefficients are measured by both degenerate four-wave mixing (DFWM) and Z-scan techniques. The influences of the molecular weight, the electric effect of the substituent on the end of azobenzene side chain to the third-order NLO properties are investigated.(2) Microwave irradiation (MI) is adopted in the ATRP of azobenzene-containing monomers in System I with different polymerization condition. The structures of the obtained polymers are characterized and those third-order NLO coefficients are measured to confirm the effect of MI in accelerating the polymerization and holding the NLO properties of azobenzene-containing polymers. (3) Based on the monomer structure of System I , the soft alkyl segments with different spacer length (n=2, 6) are introduced between the acrylate main chain and azobenzene group. The kinetics of the azo monomer containing different spacer lengths by ATRP are studied and the impacts of the spacer length on the ATRP of azobenzene-containing methacrylates and their third-order NLO properties are investigated.(4) One type of amino-substituted azobenzene group is designed and used to prepare a new series of azobenzene-containing acrylates. The predominance of the push-pull electric effect to the third-order NLO properties of azo polymers is confirmed. The diblock and triblock copolymers with azo monomer and MMA are prepared by ATRP in order to improve the solubility and film-forming capability. And the relation between the molecular structure and the third-order NLO property of polymers is analysized.(5) Attempts to extend the conjugate system based on azobenzene group are performed and the biazo-containing methacrylate is synthesized and polymerized via ATRP. The polymers with bisazo side chain have much better NLO property than monoazo polymers.The third-order NLO properties of all"the azobenzene-containing polymers with different units are measured. The relation between the structure and the NLO properties of azo polymers is studied and discussed at length, and some significative conclusions are obtained to be propitious to the synthesis of NLO polymeric materials containing azobenzene side chain.(1) A series of acrylate monomers containing different azobenzene groups are designed and synthesized. The electrical effect of the end group on the azobenzene to the activity of ATRP and the NLO properties of the obtained polymers is investigated. When the push-pull electric structure is formed in the azobenzene molecule, the azo monomer and its polymers show more excellent NLO properties than others.(2) The NLO coefficients of the azo homopolymers are related to their molecular weight, which is adjusted by reaction time of ATRP. The x(3)values of polymers are larger than their monomers and increase with their molecular weight.(3) Microwave irradiation is effective to enhance the ATRP of azo monomers and the reaction time can be shortened from lots days to about one hour. And the NLO properties of the azo polymers obtained under MI process are retained.(4) The introduction of the soft alkyl segments with different length can enhance the activities of the polymerization of azo monomers and the NLO properties of the obtained polymers containing different spacer length are increased.(5) The NLO properties of bisazo monomer and its polymers are enhanced with the extension of the conjugated azobenzene structure. The NLO refraction and NLO absorption of the bisazo polymer also can be changed with the composition of their chain units.In addition, the memory devices based on the azobenzene-containing polymers synthesized in this thesis are fabricated and their electric memory effects are investigated when I have the opportunity to go to National University of Singapore (NUS) for a short-term visit. The device exhibited a high ON/OFF current ratio of up to 106 at room temperature, a long retention time in both ON and OFF states, a switching time of 1 ms, and number of read cycles up to 108 with a read voltage of-1.0 V. Thus, the device based on the azo polymers are potential WORM memory device, which is distinct from the optical memory properties such as holographic and photorefractive data storage of azo polymers in the past report.All the monomers and polymers containing azobenzene side chains are potential NLO polymeric materials which have high third-order NLO coefficients ( up to 10-11 esu ) and rapid response time ( up to femtosecond magnitude).
Keywords/Search Tags:Azobenzene, (Methyl) acrylates, Atom Transfer Radical Polymerization (ATRP), Microwave Irradiation (MI), Third-order Nonlinear Optical (NLO), Electric Memory
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