嵌段共聚物体系中缺陷产生和消亡的机理研究

Defects play an irreplaceable role in the solid crystals, and thus attract considerable interest. The periodic nanostructures self-assembled by block copolymers can be generally regarded as mesocrystals. As a classical type of soft condensed matter systems, distinct defects are usually formed in the mesocrystals of block copolymers. Deep understanding to the creation and annihilation mechanisms of these defects is crucial to promote the application of the mesocrystals. In this project, to unveil the creation and annihilation mechanisms of defects from the aspects of both thermodynamics and kinetics, we apply the self-consistent field theory to calculate the free energy of defective structures and couple the self-consistent field theory with the string method to identify the minimal free-energy path of the evolution or annihilation of defects. We mainly consider two categories of defects. One category is the topological defects occurred in the perpendicular lamellae or in the single-layer lying cylinders, and another is the 5-fold and 7-fold defects formed in the two-dimensional hexagonal mesocrystals such as the perpendicular hexagonal cylinders and the one-layer hexagonal spheres. We focus on the impact of a few important characteristics of block copolymer systems on the stability of defects, including the polydispersity of molecular weights or block compositions, chain architectures, constitutes of blend, and external conditions. The main goal is to theoretically predict the conditions for the formation of defect-free mesocrystals or stable defects, respectively. In addition, we also aim to build up a comprehensive understanding on the creation and annihilation mechanisms of defects in the soft mesocrystals, and thus to broaden the knowledge on the defects occurred in the entire condensed matter systems.

缺陷在固体晶体中起着不可或缺的作用，吸引了特别的研究兴趣。嵌段共聚物形成的周期性结构可以被视为广义的“介观晶体”，作为典型的软物质体系，这些介观晶体中存在形态各异的缺陷，理解缺陷的产生和消亡机理对推动介观晶体的应用至关重要。在该项目中，拟使用自洽场理论计算缺陷的自由能，结合自洽场理论和弦方法计算缺陷演化和消亡的最小自由能路径，分别从热力学和动力学上揭示缺陷的产生和消亡机理。主要考虑两类缺陷，一类是在垂直层状或者单层平行柱状介观晶体中形成的拓扑缺陷，另一类是在二维六角介观晶体中形成的5重和7重缺陷。聚焦在嵌段共聚物体系的多个重要特性对缺陷稳定性的影响，包括分子量和组分的多分散性、分子链构型、共混体系的组成以及外加条件。主要的研究目的是理论预测分别形成无缺陷介观晶体和稳定缺陷的条件，为应用提供理论指导。此外，获取对软介观晶体中缺陷的系统理解，拓宽人们对凝聚态体系中缺陷结构的认识。

本项目聚焦嵌段共聚物链结构对层状结构中拓扑缺陷稳定性的影响，结合自洽场理论和弦方法研究了不同嵌段共聚物分子体系中缺陷的演化路径；设计了沟槽-台阶间隔的周期性图案化衬底，并从热力学平衡态和动力学两个方面研究了该图案化衬底引导下的嵌段共聚物自组装；研究了正多边形受限下的异相成核分相过程；设计了多种嵌段共聚物分子，研究了它们的自组装行为。取得了以下的研究成果：（1）提出了等效分凝强度的概念，揭示了链拓扑结构对缺陷演化动力学的影响，发现对称AnBn星型嵌段共聚物在相同等效分凝强度的情况下层状结构的周期小，典型位错缺陷的稳定性差且不容易被形成；组分对称但链结构不对称的ABn嵌段共聚物体系中典型位错缺陷的稳定性差，但是断裂缺陷变成了亚稳态，在强分凝条件下可以长时间稳定存在。（2）设计了台阶-凹槽衬底，通过热力学平衡态和动力学研究均证实该图案化衬底可以引导非对称嵌段共聚物形成大尺度有序的点-线混合图案，当衬底图案的周期是嵌段共聚物结构周期的多倍时依然可以形成无缺陷结构。（3）采用TDGL理论模拟方法在正方形、五边形和八边形中预测了一系列由5-7缺陷对构成的图案，而在三角形和六边形中通过“角诱导成核”和“边诱导成核”分别获得了完美和包含少量缺陷的六角图案。（4）设计了一系列嵌段共聚物体系，自洽场理论预测了一系列非经典相结构和相变行为，包括体积分数大于0.5和甚至接近0.7的球状相、低配位的简单立方和金刚石球状相等。这些研究结果不仅加深了对嵌段共聚物自组装机理的认识，也拓宽了其应用。