【作者】 李娜君；

【导师】 路建美；

【作者基本信息】 苏州大学 ， 应用化学， 2007， 博士

【摘要】 本论文以研究偶氮苯聚合物材料的三阶非线性光学性能为出发点,设计、合成和表征了一系列带有不同取代基团和不同偶氮键个数的(甲基)丙烯酸酯类偶氮单体,并通过ARTP的聚合方法合成了不同分子量和不同分子链段组成的偶氮聚合物。对得到的偶氮聚合物进行三阶非线性光学性能的测试,首次较系统地研究偶氮聚合物分子结构与其三阶非线性光学性能之间的关系。论文主要进行了以下几个方面的工作:(1)合成了一组具有不同取代基和主链的偶氮单体(体系I),通过ATRP方法实现可控聚合,合成了不同分子量和不同链段组成的偶氮均聚物及其与MMA的共聚物。分别用四波混频和z-扫描的方法对样品三阶非线性光学系数进行了测定,讨论了聚合物分子量、聚合物中偶氮苯末端取代基的电子效应等因素对聚合物三阶非线性光学性能的影响。(2)采用微波辐照的加热方式研究了体系I中合成的偶氮单体的ATRP。对两个单体分别在不同聚合条件下的ATRP作了尝试,对所得聚合产物进行了结构的表征和三阶非线性光学系数的测定,考查微波辐射对偶氮聚合反应的促进作用以及对所得聚合物性能的影响。(3)在体系I单体的基础上,在丙烯酸酯主链与偶氮苯基团之间引入了不同长度的烷基链-(CH2)n(n=2,6),合成了一组含不同柔性间隔基和末端取代基的偶氮单体,考查了柔性间隔基的长度对偶氮单体的聚合活性及其聚合物性能的影响。(4)体系II设计了一个氨基型的偶氮苯基团,合成了取代基不同的一组氨基型偶氮单体,考查了取代基为-NO2时推一拉电子结构(push-pull,D-π-A)的形成对聚合物三阶非线性光学性能的贡献。采用ATRP的方法合成了偶氮均聚物以及二嵌段和三嵌段共聚物,一方面调节聚合物链段的柔性以提高偶氮聚合物的溶解性和成膜性,同时对聚合物的微观结构及其三阶非线性性能之间的关系进行研究。(5)增加偶氮苯基团共轭链中-N=N-的个数,合成含有双偶氮苯基团的单体,并聚合得到含有双偶氮侧链的聚合物,并与单偶氮体系进行对比,考查偶氮共轭链长度的增加对聚合物三阶非线性光学性能的影响。论文通过以上几个方面对不同结构的偶氮聚合物的三阶非线性光学性能的测试,研究和分析了偶氮聚合物的结构与其三阶非线性光学性能的关系,得到了一些规律性的结论,为制备性能优良的偶氮非线性光学材料提供了一定的理论依据。(1)合成了一系列含不同末端取代基偶氮苯基团的丙烯酸酯类单体,并通过ATRP的方法实现了聚合,考查了取代基电子效应造成的ATRP聚合活性以及偶氮聚合物三阶非线性光学性能的差异。结果发现,末端取代基为硝基的偶氮单体活性较低,但当硝基的强吸电子效应与强供电子的氨基分别处于偶氮苯基团的两端时,分子体系中形成的推-拉电子结构将显著增强分子的极化程度,在受激发光照射下分子具有更大的跃迁偶极距,从而使得聚合物具有较大的三阶非线性极化率。另外,由于取代基造成各个偶氮单体及其聚合物的线性透过率的差异,末端取代基为硝基的单体及其聚合物由于硝基的存在使得其受532 nm的纳秒激发光照射后产生的基态吸收截面较大,线性透过率较低,非线性折射和非线性吸收存在部分热效应的贡献。(2)我们采用ATRP的方法通过调节投料比和聚合时间得到了一系列不同分子量的偶氮聚合物。结果发现,偶氮单体聚合后,其三阶非线性极化率提高,并且聚合物的分子量越大,其三阶非线性极化率也越大。(3)微波辐射技术一直是我们课题组多年来用于聚合反应的一项有利工具。本论文所合成的偶氮单体在常规加热条件下进行ATRP的时间较长,转化率较低,因此我们采用了微波辐射这一技术实现偶氮单体的ATRP,考查微波辐射对偶氮单体的ATRP体系有无促进作用。实验结果表明,采用微波辐射加热的方法可以使该体系偶氮单体的ATRP速率迅速提高,聚合可控并且符合ATRP的活性特征。微波辐射并没有破坏聚合物的分子结构,聚合物的性能也未受影响。但随着辐照时间的进一步延长,“过热效应”的产生使ATRP反应失去可控性,部分高分子量的聚合物发生了分解,聚合物的平均分子量下降。因此,控制适当的微波辐射时间,我们可以快速有效地合成得到具有较大三阶非线性光学效应的偶氮聚合物材料。(4)实验中合成了含不同长度烷基链的偶氮单体及其ATRP均聚物以及新偶氮单体与MMA的无规共聚物以及嵌段共聚物,得到不同链段组成的侧链型偶氮聚合物。结果发现,在偶氮侧链中引入烷基链之后,其空间位阻的下降和自由度的提高使得新偶氮单体的ATRP活性也有明显的提高,单体及其聚合物的三阶非线性极化系数也都相应增加了。并且所得聚合物的溶解性和成膜性也有相应的提高。Z-扫描结果显示,烷基链的引入和共聚物的链段组成及其偶氮组分的含量并未改变非线性吸收和非线性折射的类型,所有单体及其聚合物仍然保持反饱和吸收和自散焦特性。(5)我们设计并合成了一类含有氨基型偶氮苯结构的偶氮单体,除了引入硝基,还分别引入了电子效应不同的甲氧基和溴取代基,将它们与含硝基的单体进行比较,考查推-拉电子效应对聚合物的三阶非线性光学效应所产生的贡献。结果发现,体系中含有推-拉电子结构的硝基单体及其聚合物因分子体系内部形成较强的分子极化,其三阶非线性光学系数明显要比其它偶氮单体和聚合物来得高。另外,本论文中具有推-拉型电子结构的硝基单体在532 nm激光照射下产生的激发态吸收截面小于基态吸收截面,其非线性吸收为饱和吸收。用RB3LYP/6-31G(d)方法对三个含氨基型偶氮苯基团的单体小分子进行几何优化和频率计算,得到的分子三阶非线性极化率的结果也与实验的测试结果呈现相同的变化规律。(6)我们从偶氮聚合物侧链上的偶氮苯基团入手,增加偶氮键的个数,合成了双偶氮聚合物。结果发现,虽然双偶氮单体及其聚合物的溶解性较差,但其三阶非线性系数的测试结果与我们预期的一样,双偶氮侧链的引入使得聚合物的三阶非线性效应有了数量级上的提高。对双偶氮共聚和嵌段聚合物的研究结果表明,不同链段序列的双偶氮嵌段聚合物表现出了不同的非线性吸收和非线性折射特性,聚合物中双偶氮组分含量的适当减少将导致双偶氮聚合物的非线性吸收由饱和吸收转变为反饱和吸收,其非线性折射由自散焦转化为自聚焦特性。(7)我们还合成了偶氮单体与甲基丙烯酸甲酯的共聚和嵌段聚合物,一方面通过引入MMA的柔性链段改善偶氮聚合物的溶解性和成膜性,另一方面由于偶氮单体与MMA活性的不同以及聚合条件的不同而得到了一系列不同分子量和不同链段组成的偶氮聚合物,以调节偶氮聚合物作为非线性材料所须的综合性能。在研究中我们发现,当偶氮单体与MMA的摩尔投料比为1:1时所得的共聚物既保持了偶氮聚合物较高的三阶非线性极化系数,其溶解性和成膜性也得到了进一步的改善。偶氮高分子链嵌入了MMA的柔性链段之后,不仅溶解性和成膜性得到改善,并且保持了较高的三阶非线性光学性能以及化学稳定性,并且,嵌段聚合物的结构较为清晰明了,其合成比无规共聚物具有更好的可控性。(8)另外,我们还利用前往新加坡国立大学短期学习交流的机会,将本论文中合成的偶氮聚合物PNAMA和:PMANAzo带去进行了它们在电学存储器件(Electric Memory devices)方面的研究。分别将偶氮聚合物薄膜制备成由ITO/Polymer/Al组成的M/I/M型三明治结构的存储器件,研究了偶氮聚合物在电学存储(Electric memorv)方面的性能,取得了令人振奋的结果。通过对偶氮聚合物器件的J-V特性曲线以及稳定性的测试发现,此两个偶氮聚合物都具有WORM(Write-only read-many-times)型的电学存储性能,并且具有较好的稳定性。本论文所合成的偶氮单体及其聚合物均具有较大的三阶非线性极化率系数(x(3))值达l0-10esu)和较快的响应时间(飞秒级响应),是一类具有潜力的非线性聚合物材料。更多还原

【Abstract】 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).更多还原