受限空间中嵌段共聚物自组装行为的模拟退火研究

【作者】 于彬；

【导师】 史安昌； 李宝会；

【作者基本信息】 南开大学 ， 凝聚态物理， 2009， 博士

【摘要】 由于嵌段共聚物可以自组装形成丰富的微观结构,以及这些微观结构潜在的应用价值,所以吸引了科学家们的广泛兴趣。对于双嵌段共聚物,实验和理论研究已经使人们对其体相行为有了很好的认识。在熔体状态下,双嵌段共聚物可以自组装形成包括层状相、六角排列柱状相、体心立方球状相和双连通Gyroid相的一系列稳定有序结构。实践中,嵌段共聚物在受限环境中的自组装通常被用于制备长程有序结构。受限效应包括结构受挫程度、受限导致体系熵的减少以及共聚物—受限表面相互作用,强烈地影响共聚物自组装过程,并且会形成具有潜在应用价值的新结构。本论文使用模拟退火方法,系统地研究了嵌段共聚物在受限环境中的自组装行为。旨在预言其形态结构随着众多因素的演化规律;揭示受限环境中复杂形态形成的原因,以及不同形态之间转变的机制;指导实验有目的地调控受限环境、共聚物组分、相互作用等参数,筛选合适的共聚物体系,从而可得到具有特定功能和新颖纳米级有序结构的自组装材料,以满足特定的需要。在第一章中,我们对本文所涉及的嵌段共聚物研究背景进行了回顾,并简要介绍了采用的计算机模拟方法。在其后的章节中,对我们的研究工作做了逐一论述。在第二章中,我们研究了双嵌段共聚物熔体的相行为,获得了体相相图。进而,我们分别确定了体相中各有序结构的周期,为进一步对其受限体系的研究做好准备。另外,我们研究了两种体积分数不同而体相都形成柱状相的双嵌段共聚物受限于两平行板间的自组装形态,阐明了体积分数对受限环境中自组装形态的影响。在第三章中,我们系统地研究了体相分别形成层状相、柱状相和Gvroid相的双嵌段共聚物受限在圆柱形纳米孔内的自组装形态。考察了孔径和孔壁作用对自组装形态和共聚物链构象的影响;预测了多种新结构;并揭示了复杂形态的形成原因以及不同形态之间的转变机制。对体相形成层状相的双嵌段共聚物,我们发现在中性孔壁下共聚物自组装形成垂直于孔壁的垂直层结构,在强选择性孔壁下形成一系列的同心层结构,而在弱选择性孔壁下形成平行于孔轴的平行层、螺旋和链节等新奇结构。并且,我们提出了一种简单模型用来描述对称双嵌段共聚物形成的同心层结构的厚度随孔径的变化。模型和模拟结果符合得很好。对体相形成柱状相的双嵌段共聚物,在不同孔径的孔内我们观察到了一系列体相不存在的螺旋和层叠圆环等新奇结构。并且发现其自组装形态由孔径和体相周期的比值D/L₀所决定,反映了受限环境中结构受挫程度对自组装形态的重要影响。对体相形成Gyroid相的双嵌段共聚物,在不同孔径、不同孔壁作用和不同体积分数的体系中,我们观察到了垂直层、平行层、同心层、同心孔层等多种结构。我们的模拟结果和已有的实验以及理论结果符合得很好。在第四章中,我们研究了柱状受限纳米孔边界几何形状对共聚物自组装形态的影响。对于对称双嵌段共聚物,研究中涉及的纳米孔截面包括正三角形、正方形和椭圆形。在截面为正方形和正三角形的纳米孔中,我们预测随着孔壁作用的增加,可出现从垂直层到平行层到同心层或三叉层的一系列形态变化。在截面为椭圆形的弱选择性纳米孔中,我们预测平行层结构仅在椭圆短轴的长度与体相周期匹配时出现。对体相形成柱状相的双嵌段共聚物,除了以上几种截面外,还涉及长方形、正六边形和正八边形截面的纳米孔。我们发现当孔的对称性很低时,共聚物主要形成柱状结构;而当孔的对称性较高时,共聚物形成了多螺旋、层叠圆环等新奇结构。我们的模拟结果说明,可以通过构建受限纳米孔的几何形状来控制共聚物自组装形态及其对称性。在第五章中,我们系统地研究了双嵌段共聚物受限在球状纳米孔内的自组装行为。对体相形成层状相的双嵌段共聚物,我们发现随着孔壁表面的选择性逐渐由中性变为弱选择性最后到强选择性,共聚物依次形成垂直层、螺旋或内嵌的补丁结构、同心球层结构。我们提出了一种新的模型,与第三章的模型相比,它可以更好地描述同心球层和同心柱层结构每一层厚度随孔径的变化。对体相形成柱状相的双嵌段共聚物,我们研究了两个体系,分别为A₂B₁₀和A₃B₉体系。我们发现前者在球状孔内自组装形成一系列在同心壳层内卷绕的柱状结构;而后者由于A嵌段的体积分数较大,在球状孔内自组装形成一系列的同心孔层结构。另外模拟结果表明,体相形成体心立方球状相的双嵌段共聚物,在球状纳米孔中可自组装形成一系列分散的球状结构。第二、三章和本章的研究结果共同揭示了受限效应的强烈程度对自组装形态有着巨大的影响。在第六章中,我们研究了体相形成三色层结构的线形ABC三嵌段共聚物受限在柱状和球状纳米孔内的自组装形态。研究中发现在对A嵌段具有强选择性的柱状纳米孔中,随着孔径的增加,共聚物自组装形成包括同心层和垂直层组合而成的双周期层等一系列新的层状结构。另外在小孔径下,随着嵌段之间作用强度的减弱,共聚物逐渐趋于形成同心层结构。对于球状纳米孔,我们系统地研究了孔径大小、孔壁作用和共聚物体积分数对自组装形态的影响。模拟结果表明,在中性孔壁或当孔壁对两端A、C嵌段同时具有吸引作用时,共聚物始终形成表面带有圆形补丁的球状结构。而且补丁的数目和大小可以通过改变孔径的大小以及共聚物和孔壁的相互作用进行调节。更多还原

【Abstract】 The self-assembly of block copolymers has attracted much scientific interest due to the formation of rich microstructures and potential applications of these microstructures. For the simplest case of diblock copolymers which are linear polymers composed of two different subchains, their bulk phase behavior is well known from both experiments and theories. A variety of ordered bulk phases, including lamellae, hexagonally packed cylinders, body-centered-cubic spheres, and a bicontinuous network structure called Gyroid are observed for diblock copolymers. In practice, confinement is frequently employed to induce long range ordering of structures resulting from self-assembly of block copolymers. Confinement effects, including the degree of structural frustration, confinement-induced entropy loss and surface-polymer interaction, influence the self-assembly process and can generate novel microstructures that may have potential novel applications.In this thesis, we systematically investigated the self-assembly behaviors of block copolymers under confinement using simulated annealing technique. The purposes of this thesis include predicting the underlying rules of self-assembled structures with the variation of various parameters, understanding the origin of the confinement-induced morphologies, elucidating the mechanism for the morphological transitions between novel structures and finally providing guidelines to design rich microstructures in experiments.In chapter one, we briefly introduced the background of block copolymer researches and computer simulation methods involved in the study. In the following chapters, we presented our researches on block copolymer melts in the bulk, confined between two parallel surfaces, and confined in cylindrical and spherical nanopores in detail.In chapter two, firstly, we systematically investigated the self-assembly of diblock copolymer melts in the bulk. Then, we calculated the characteristic periods for the various bulk equilibrium structures, which is crucial for the further studies of diblock copolymers under confinement. Furthermore, we studied the self-assembly of two kinds of cylinder-forming diblock copolymers with different volume fraction confined between two parallel surfaces. As a conclusion, we elucidated the influence of volume fraction on the self-assembled morphologies under confinementIn chapter three, we systematically investigated the self-assembled morphologies of diblock copolymers confined in cylindrical nanopores. The pore diameter and surface preference are systematically varied to examine their effects on the self-assembled morphologies and the chain conformations. A rich variety of novel structures under the two-dimensional confinement has been revealed. The morphological transitions between different structures are elucidated. For bulk lamella-forming diblock copolymers, perpendicular lamellae and concentric lamellae spontaneously form under the neutral and strongly preferential surface, respectively. When the surface preference is relatively weak, novel structures such as parallel lamellae, helices and catenarian structures are observed. A simple model is proposed for the concentric-cylindrical lamellae, which gives a reasonable description of the layer thickness. Good agreement between the computed value and the model predictions are obtained. For bulk cylinder-forming diblock copolymers, a series of novel structures such as helices and stacked toroids are identified which cannot form in bulk block copolymers. The formation of these novel structures is correlated with the ratio between the pore diameter and the bulk period, reflection the effects of confinement-induced structural frustration to the self-assembly. For bulk Gyroid-forming diblock copolymers, a series of novel morphologies including perpendicular, parallel, concentric and concentric perforated lamellae are observed with varying the pore diameter, surface preference and the volume fraction. Our simulation results are consistent with the available experiments.In chapter four, the self-assembly of diblock copolymers confined in channels of various shaped cross sections is studied. For the symmetric diblock copolymers, the cross sections of the confining channels are of different shapes including regular triangle, square and ellipse. For square and regularly triangular cross sections, a generic morphological transition sequence, from perpendicular lamellae to parallel lamellae to concentric lamellae or trifurcate lamellae is predicted with increasing the surface preference. For elliptic cross sections with weakly preferential surfaces, parallel lamellae occur when the ratio between the short axis length and the characteristic periods is close to an integer. For the bulk cylinder-forming diblock copolymers, the cross sections of the confining channels also include rectangules, regular hexagons and regular octagons besides the above mentioned shapes. Multiple packed cylinders and more complex structures such as helices or stacked toroids spontaneously form for low-symmetry and high-symmetry cross sections, respectively. Furthermore, the symmetry and domain spacing of these structures can be altered by the shape and size of the confining pore. Our studies indicate that the self-assembled morphologies of copolymers and their symmetry can be manipulated by imposing different shaped nanopores.In chapter five, we systematically investigated the self-assembly of diblock copolymers confined into spherical nanopores. For the bulk lamella-forming diblock copolymers, the observed sequence of stable structures is from perpendicular lamellae to helices or embedded structures and finally to concentric-spherical lamellae as the strength of the surface preference is increased gradually from neutral to weakly preferential and finally to strongly preferential. A new model is proposed which can describe the layer thichness for the simulated concentric-spherical and concentric-cylindrical lamellae better than the model proposed in chapter three. For the bulk cylinder-forming diblock copolymers, two systems of A₂B₁₀ and A₃B₉ are considered. In the former case, the minority blocks form curved cylinders packing into concentric layer in spherical pores. However, concentric-perforated lamellae are observed in the latter case. This different is attributed to the larger volume fraction of the minority blocks in the latter case. Furthermore, for the bulk body-centered-cubic sphere-forming diblock copolymers, a series of structures with dispersed spheres spontaneously form in confined spacing. Our studies elucidate that the self-assembled morphologies under these spherical-pore confinement are more complex than those under cylindrical-pore or under plane surface confinement.In chapter six, we systematically investigated the self-assembly of lamella-forming linear ABC triblock copolymers confined into cylindrical and spherical nanopores. In the cylindrical nanopores which prefer to the A-blocks, a series of novel lamellar morphologies are observed with increaing the pore diameter. And the results show the copolymers prefer forming concentric-lamellar structures with decreasing the interactions between different blocks when the pore diameter is relatively small. In the spherical nanopores, the pore diameter, surface preference and the volume fraction of the copolymers are systematically varied to examine their effects on the self-assembled morphologies. When the pore-surface is neutral or preferential for both of the terminal blocks, a variety of novel patchy structures spontaneously form. And the results show the number and the size of the patches in each structure can be tuned through varying the pore diameter and the strength of the surface preference.更多还原