【作者】 张新军；

【导师】 王维；

【作者基本信息】 南开大学 ， 高分子化学与物理， 2009， 博士

【摘要】 树枝化聚合物是一类侧链为树枝片的新型树形聚合物,近年来吸引了越来越多的研究兴趣,这与它们特殊的分子结构及其固有性质密不可分。利用树枝化基元的空间位阻效应,通过控制树枝化基元的结构、种类或代数,接枝率以及与聚合物主链之间的距离,从而实现对这类聚合物链的构型、柔顺性以及分子尺寸的控制。随着聚合物主链上连接的树枝化基元的尺寸(代数)的增大,相应聚合物的构象可以从无规线团转变成半刚性或刚性的圆柱体。因此对这一类大尺寸、高刚性聚合物链的构型及其柔顺性的研究将有助于对高分子化学及物理学科进行深入理解,而且对形成的柱状纳米分子表面或其内部进行官能团修饰与改性,也将是一项非常有意义的工作。目前科学界对其研究已经取得了可喜的进展,树枝化聚合物的研究作为一类从下而上(bottom-up)制备纳米分子材料的有效手段,相信在不久的将来,将会在生物医药、光电功能材料等领域有巨大的发展空间。然而到目前为止它们的合成仍然比较困难,通常采用的接枝法和大分子单体法都存在很大的局限性,如大大过量的投料,较长的反应周期,提纯步骤复杂甚至有些分离基本不可行,而且其产率也往往不高。因此,寻找一种相对简便的方法来合成这类聚合物具有重要的现实意义。超分子化学,是研究两种以上的化学物种通过非共价键的分子间作用力缔结而成具有特定结构和功能的超分子体系的科学,这些作用力包括离子键作用、氢键、亲水疏水作用、偶极一偶极作用、范德华力、π-π堆积作用和配位作用等。其中驱动力主要为离子键作用的自组装行为称作离子自组装,其优点是合成简单,结构完美稳定,得到的超分子结构中严格的1:1电荷计量比。聚电解质-表面活性剂复合物是离子自组装的完美应用,由于制备纯化简单,易于进行功能化,且能够形成高度有序的超分子结构而引起了人们的广泛关注。然而,由于采用的表面活性剂结构比较单一,由此制备的复合物在固态下形成的有序结构一般为层状相,而且其较差的溶解性也在一定程度上限制了对其溶液行为的研究。到目前为止,只有很少量的工作将复杂的侧链结构引入这个体系,人们对这一领域的了解还处于起步阶段。在本文中,我们将树枝化聚合物和聚电解质-表面活性剂复合物两种概念结合起来,提出了通过带电荷的树枝状双亲性分子与带有相反电荷的聚电解质通过离子键作用复合来制备树枝化聚合物的策略。制备过程中,发现三代树枝片的钾盐都可与聚电解质在水溶液中发生离子键复合作用,产物以沉淀形式析出,通过水洗再沉淀的纯化步骤即能得到高纯度的目标产物,三代复合物分别用PG1,PG2和PG3表示。实验工作中,主要采用核磁共振(~1H-NMR)、傅立叶变换红外光谱(FT-IR)、紫外可见(UV-vis)和元素分析(EA)等方法确认了复合物的分子结构,证实了通过超分子化学合成树枝化聚合物这一策略切实可行。与传统的基于共价键接枝的树枝化聚合物合成相比,本文采用的方法制备与纯化都非常简便,产率也较高,而且由于离子键和树枝状侧链的引入,制备的复合物有望在溶液中和固态下都具有与众不同的性质。首先研究了制备的复合物在溶液中采用的聚合物构象。在有机稀溶液中,静态光散射(SLS)证实PG1具有坍塌的无规线团的构象,而PG2和PG3则显现出了聚电解质性质;动态光散射(DLS)测得三代复合物都具有相同的流体力学半径,说明三者应该具有类似的无规线团的聚合物构象。原子力显微镜(AFM)下也观测到了球状聚集体的产生,我们认为它们与上述线团的产生有关。基于试验结果,我们提出了本文中制备的树枝化聚合物复合物在有机稀溶液中采用花式(flowerlike)线团的构象,即聚合物链形成回路围绕在电荷团簇的周围。这种构象中,团簇在聚合物稀溶液中起到了分子内交联剂的作用,基本上抵消了由位阻增大引起的聚合物骨架的伸展趋势。而团簇产生的驱动力是聚合物主链上残留的未中和电荷的疏溶剂作用。这种构象与传统的共价键接枝的树枝化聚合物有很大不同,可用于解释通过离子键作用制备的树枝化聚合物形成的超分子有序结构规整度都较低的实验事实。进一步的工作中,我们研究了制备的树枝化聚合物在溶液中的组装行为。由于离子键的引入,它们对体系中的离子强度非常敏感,可在外加盐的诱导下进行自组装。光散射(LS)实验结合电镜(EM)共同证实了这一性质,我们发现,随着体系盐度的增加复合物首先会聚集形成多层胶束,其半径在40纳米左右;进一步增加盐的浓度会使胶束向半径为100纳米左右的薄层囊泡转变;随后盐度的进一步增加使溶液中产生宏观可见沉淀,并伴随着微米级巨型囊泡的出现。我们认为,这是具有较强组装能力的树枝片分子以聚电解质为模板进行的自组装,它的产生是由于盐的加入压制了溶液中离子的熵效应,从而促进了相分离的进行。而聚集体形貌的不同,则是由于离子键极化面积的不同而引起的聚合物重复单元的分子形状不同造成的。聚合物的这种环境响应性是这类复合物的一个重要性质。而在固态下,热失重(TGA)和差热扫描(DSC)实验显示,制备的复合物热稳定性较高,说明团簇结构和树枝片分子中的苯环在稳定复合物时起到了很重要的作用。广角X光衍射(WXRD)和小角X光散射(SAXS)证实三代复合物均可在固态下形成有序的超分子结构,PG1为层状结构,在偏光显微镜(POM)下可观察到其形成的液晶织构。而随着代数增加树枝片分子体积增大,第二和三代复合物形成结构的有序性变差,POM下未观察到液晶现象,但SAXS显示PG2在经历从层状相到柱状相的转变,而PG3则进行了典型的六方柱状堆积。这种超分子结构的变化是由非极性树枝片分子随代数增加导致的体积增大引起的,与经典的嵌段共聚物的相分离理论完全一致。在最后一章,我们利用树枝片分子合成了具有不同分子形状且较为刚性的两种树枝状双亲性分子。TEM和AFM共同表明,两种分子在选择性溶剂己烷中都能够自发形成反式单层囊泡,但囊泡的尺寸有很显著的差别,分子几何形状被认为是造成这种差异的主要原因。双亲性分子在形成双层膜(bilayer)时的能量变化由弯曲能量决定,这导致具有不同曲率半径的分子会形成不同大小甚至不同形貌的聚集体。这一工作表明,可以通过改变分子形状来达到操控自组装体形貌和大小的目的,这对于自组装(self-assembly)以及指导组装(direct-assembly)都具有重要的理论和现实意义。更多还原

【Abstract】 Dendronized polymers, with dendrons as the side chains, are a new kind of dendritic polymers and have been attracting more and more research interesting due to their unique molecular structures and inherent properties. Basing on the steric hindrance effect, one can control their chain conformation and flexibility by tuning the structures, types and generation of dendritic pendants, dendronization degree as well as the distance between the dendritic side chains and backbones. As the volume of dendritic pendants increases, the backbone conformation can be stretched, from random coils to semi-rigid or rigid column. Because of these factors, the studies on their chain conformation and flexibility are of fundamental importance in understanding polymer chemistry and physics. Moreover, the modification in the surface and interior of the molecular nano-columns is also a promising subject. So far some achievements have been made to these molecules, one can believe that in the near future, as a bottom-up mean of building nano-molecular materials, dendronized polymers will be applied in many fields, such as modification in the surface of nano-objects, nano-catalysts, photoelectric functional materials and biochemical materials and so on. However, the synthesis of this kind of polymers is still a challenge. The mostly used grafting strategy and macromonomer strategy have their own disadvantages, such as low yield, long reaction period, and strict stoichmetric ratio and so on. Even worse, sometimes the separation of target molecules from side-products is impossible. Thus one rather simple synthesis strategy for dendronized polymers is demanded to promote the studies on them and to expand their applications.Supramolecular chemistry refers to the area of chemistry beyond the molecules focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components. While traditional chemistry focuses on the covalent bond, supramolecular chemistry examines the weaker and reversible noncovalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi-pi interactions and electrostatic effects. The study of non-covalent interactions is crucial to understanding many biological processes from cell structure to vision that rely on these forces for structure and function. The self-organization with the employment of Coulombic interaction is called ionic self-assembly (ISA) and has been used in the preparation of polyelectrolyte-surfactant complexes in the recent decades. This strategy has some advantages, such as facile preparation and separation, well-defined and stable result structures, and 1:1 stoichimetric ratio of anion and cation and so on. However, the surfactants adopted in this system are merely single- and double-tailed, and the limited structure of surfactants results in that the highly ordered supramolecular structures are usually of lamellar phase, which greatly confined the applications and studies on them. So far, very little works involved other complex structures in this system.Basing on the review on dendronized polymers and comb-shaped supramolecular polymers, we propose in this paper to prepare dendronized polymers via complexing ionic Frechet-type dendritic amphiphiles with oppositely charged polyelectrolyte through ionic interaction. The result first-, second- and third-generation dendronized polymers are labled as PG1, PG2 and PG3, respectively. Their chemical structures were confirmed by nuclear magnetic resonance (~1H-NMR), Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-vis) and elemental analysis (EA). According to EA results, the grafting degrees were also obtained as 83%, 74% and 64% for PG1, PG2 and PG3, respectively. This decrease is attributed to the steric hindrance caused by increasing volume of dendrons as the generation increases.The chain conformation of these complexes in dilute organic solutions is also investigated, revealing the effects of dendrons generation. Light scattering (LS) and atomic force microscopy (AFM) reveal that in dilute THF solution, all the three complexes take a similar flowerlike conformation with the formation of multiplets, which is much different with the conformation change of conventional covalently linked dendronized polymers. This chain conformation can be also used to explain that the low order of supramolecular structures formed by ionic interaction based dendronized polymers.Aggregation is another important property of these dendronized polymers complexes. Dynamic light scattering (DLS) showed addition of salts (LiCl) to complexes solution can lead to the conversion from individual chains to multi-layer reversed micelles then to thin membrane reversed vesicles. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and AFM combined with static light scattering (SLS) were employed to confirm the structure of these self-assemblies. The formed vesicles exhibited high stability upon dilution, and the polyelectrolyte properties of them were also observerd. In higher salinity, macroscopic precipation can occur, accompanying the appearance of huge vesicles with diameters of several micrometers.The thermal properties of resulting dendronized polymers in bulk were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), showing that these polymers have high thermal resistance and the crystallinity property of alkyl chains were changed due to the localization of dendrons. Small angel X-ray scattering (SAXS) as well as X-ray diffraction (XRD) both showed that the ordered supramolecular structure could be formed in bulk, lamellar phase by PG1 and columnar phase by PG3.In the last chapter, the relationship between molecular geometry and self-assemblies is explored based on two synthesized dendritic amphiphiles. We experimentally observed that the volume change of solvophilic segment can strongly affect the size of formed aggregates, and some theoretical explanations are also displayed. This work demonstrates the possibility of tuning the morphology and size of supramolecular structures by tailoring the molecular shape.更多还原