【作者】 何林李；

【导师】 章林溪；

【作者基本信息】 浙江大学 ， 凝聚态物理学， 2010， 博士

【摘要】 将嵌段共聚物体系置于一定的外部环境下,如受限、参杂、外力场、及溶剂等,由于受外部各种因素的诱导作用,体系将表现出与本体状态下不同的相行为特征。人们可以通过控制共聚物与外部环境间的相关参数,制备出结构新颖、长程有序的纳米材料。在研究共聚物体系在受限、掺入杂质颗粒、剪切场、及选择性溶剂等条件下自组装的过程中,也使我们更理解了嵌段共聚物的相行为本质。尤其对于掺杂情况,各项同性的纳米球与高长径比的纳米棒对体系自组装行为的影响有着极大地区别。纳米棒附带的取向熵,引导我们从体系的熵与焓角度去更本质地理解一系列相结构的内在机制。本论文采用耗散粒子动力学(DPD)模拟方法研究了层状/柱状的两嵌段共聚物与杂质颗粒(纳米球、纳米棒)共混体系,剪切场下共聚物/纳米棒共混体系,及柔性链嫁接的纳米棒分子在溶剂环境中的自组装行为。1.层状的两嵌段共聚物与纳米球共混体系的自组装行为。为实现刚性的纳米球,引入了一种新的相互作用势。我们系统地研究了纳米球的浓度、半径、及球与高分子链间的相互作用等参数对共聚物相分离的影响,并得到了三种参数共同构建的结构相图,全面地反应了体系的相行为。纳米球的位置分布主导着体系从有序的层状结构到双连续相的转变。当纳米球全部分布于亲A相区时,共聚物的相分离决定了体系维持层状结构；由于刚性球的排斥体积效应,当部分球被迫分布于AB相分离的界面,或甚至B相区时,纳米球就会破坏层状结构,诱导体系向双连续相转变,一致与实验研究的结果。2.层状或柱状的两嵌段共聚物与纳米棒共混体系的自组装行为。其中棒与棒间的弱排斥作用,使棒有集聚行为的趋势。我们系统地考虑了棒的个数、长度、半径、及棒与高分子间的相互作用等因素对混合体系自组装的影响。一系列的构型及相转变都是在共聚物的相分离与纳米棒的聚集行为共同作用下的结果。当把具有显著物理或化学性质的纳米颗粒混入到高分子体系时,从熵和焓的角度可以更本质地理解体系的自组装,尤其是颗粒的相行为。从焓方面,纳米棒与各高分子链段间相互作用决定了棒的分布；从熵方面,棒的各向异性、相区域的空间约束,及高分子链的构象熵共同决定了棒的取向。3.层状或柱状的两嵌段共聚物与二元纳米棒共混体系的自组装行为。二元棒间的区别在于棒的长短,且棒间的强排斥作用阻止了棒的集聚行为。我们分别考虑了亲A棒与中性棒两种情况。在恒定棒总浓度的前提下,改变短棒/长棒的浓度比,不仅可以诱导体系自组装出一系列的相结构,还指导二元棒选择性地分布于高分子基体中。这种共聚物/二元纳米棒共混体系的自组装行为,主要取决于两方面的贡献：一是焓的作用,源于棒与各嵌段间的相互作用,支配着棒的分布,二是熵的作用,源于棒的各向异性,高分子链的构象熵,及相分离结构的空间限制。短棒和长棒识别性地分布在共聚物基体中的这一相行为,对于两嵌段共聚物(无论是对称还是非对称的)与二元纳米棒共混体是普适的。变化嵌段比值fa，区别只是诱导出不同的共混体相结构,而内在机理是相同的。4.剪切场作用下共聚物与纳米棒共混体的自组装行为。我们分别考虑了亲A纳米棒与中性纳米棒两种情况。通过控制纳米棒浓度,使共聚物/纳米棒体系的初始相结构始终为层状,在剪切流场作用下,不仅使体系层结构的取向发生了转变,还诱导了体系相构型的转变,且剪切场可以加速取向与相转变的过程。对于纯共聚物,剪切诱导的层取向在低剪切率下转向平行,而在高剪切率下转向垂直。对于纳米棒,在剪切场下棒间有分散趋势。体系最终的自组装结构取决于共聚物与纳米棒在剪切场下各自相行为间的竞争作用。5.溶剂诱导下的柔性链嫁接的纳米棒分子的自组装行为。我们考虑了三种嫁接方式(一端、两端、及中间嫁接)的纳米棒在三种溶剂环境下：纯棒选择溶剂,纯链选择溶剂,及两者混合溶剂,体系形成的集聚形态体结构有：柱状、六角柱状、双分子层、层/柱混合相、空心柱状、向列相,及液晶相。这些丰富构型的形成是由分子的拓扑结构、棒/链比,溶剂选择性,及混合溶剂含量等因素决定的。在纯溶剂下,由棒/链比所引起的相结构转变,也可以等效地通过改变混合溶剂的含量来得到。本质上,这类分子在溶剂中的自组装行为主要决定于链的自由伸展能,棒的液晶相行为,及体系的界面能等因素。更多还原

【Abstract】 Block copolymers under some external environments, such as confinement, nanoparticles, force field, solvent, and so on, due to the induction from the various external conditions, the systems exhibit different phase behaviors as to those in the bulk. It has been show that by controlling the related parameters, we can fabricate novel, and long-range order materials in nano-scale. The studies on the self-assembly of block copolymers suffer the inductions from confinement, nanoparticles, shear flow, and selective solvent, can promote the understanding of intrinsic characters of block copolymers segregation. Especially for nanoparticles, the effects of isotropic nanospheres and high aspect-ratio nanorods on the self-assembled behaviors have significant difference. Base on the additional orientational entropy of nanorods resulting from the particles’anisotropy, a consideration of enthalpic and entropic interactions can further exploite the inherent mechanism for driving these rich phase behaviors. In this dissertation, we use dissipative particle dynamics(DPD) method studied the self-assembly of the following systems:the mixtures of lamellar/cylindrical forming diblock copolymers(DBCPs) and nanoparticles(spheres, or rods), the mixtures of DBCPs and nanorods under shear flow, and the polymer tethered nanorods under selective solvent.1. The self-assembled phase behaviors of lamellar DBCPs and nanospheres mixtures. To ensure the rigidity characteristic of nanospheres, we introduce new interactive energies. We systematically study the effects of nanospheres volume fraction, radius, and the polymer-nanosphere interaction on the DBCPs microphase separation. As more, we get a phase diagram of copolymer nanocomposites in terms of these three parameters, which reflects the system phase behaviors comprehensively. The position distribution of nanospheres plays a decisive role in the phase transition from lamellar to bicontinuous morphology because of the strong excluded volume effects among nanospheres.2. The self-assembled phase behaviors of lamellar/cylindrical DBCPs and nanorods mixtures. The weak repulsions between nanorods drives the rods to aggregate. A series of parameters, such as nanorod number, length, radius, and the polymer-nanorod interaction, are introduced to analyse the cooperative phase behavior and novel morphologies of hybrids. The final phase structures of the mixtures result from the mutual inducement between mesophase-forming copolymers and NRs. When physically or chemically distinct nanoparticles are introduced into the polymer fluids, it is useful to understand the intrinsic characteristics of the composite self-assembly by considering the enthalpic and entropic interactions, especially for the nanoparticles’phase behaviors. On the one hand, the NRs distributions reveal a degree of enthalpically driven self-assembly, due to the attractions or repulsions among species. On the other hand, the NRs’ aggregates and orientations show a degree of entropically generated self-assembly, based on the competition between the inherent shape anisotropy of NRs and confinement of host phase separated domains.3. The self-assembled phase behaviors of lamellar/cylindrical DBCPs and binary nanorods mixtures. The binary NRs are identical in energy but different in lengths. The repulsions between nanorods avoid the aggregates among nanorods. We consider two cases of A-block preferential and neutral nanorods, respectively. Replacing the monodisperse NRs with an equal volume fraction of bidisperse NRs, and varying the ratio of short/long nanorod has prompted not only a series of phase transformations in the polymer microstructure but also, the creation of a uniform orientation, and a discriminative distribution of NRs. The inherent mechanism for driving such rich phase behaviors arises from the competition between enthalpic and entropic effects.4. The self-assembly of lamellar DBCPs/nanorods composites under shear flow. Both selective and nonselective nanorods are considered. To preserve lamellar morphology in the nanocomposites, the nanorods concentration is controlled to be not too high. Subjected to steady shear flow, there are not only the shear-induced the reorientations of lamellae, but also the shear-induced phase transitions. Moreover, enhancing shear rate can speed up the transition process of micophase structures. For the pure DBCPs case, the shear-induced lamellae adopt parallel alignment at low shear rates, while perpendicular at high shear rates. For the pure nanorods under shear, the nanorods trend to disperse. The final morphologies of nanocomposites depend on the interplay between DBCPs and nanorods under shear flow.5. The solvent-induced self-assembly of polymer-tethered nanorods (PTN). We focus on three types of PTN molecules(one end tethered, both ends tethered, middle tethered) under different solvent conditions:the pure rod-selective solventⅠ, the pure tether-selective solventⅡ, and theⅠ/Ⅱmixed solvent. The observed micellar structures include:cylinders, hexagonally cylinders, bilayer lamellae, lamellae/cylinder mixed phases, inverted hollow cylinders, nematic bundles, and ordered LC phases. These morphologies depend on the topology, rod/tether length ratio, solvent selectivity, and mixed solvent content. In pure solvent case, the morphologies and morphological transitions of PTN assemblies are affected by the rod/tether length ratio, which also can be induced by varying mixed solvent content in sequence. These self-assembled structures are formed by the competition between the stretching of tethers, liquid crystalline of rods, and interfacial energy.更多还原