【作者】 尤莉艳；

【导师】 吕中元；

【作者基本信息】 吉林大学 ， 物理化学， 2009， 博士

【摘要】 本论文研究中开发非平衡态耗散粒子动力学模拟方法,实现了稳态剪切,振荡剪切以及顶盖驱动流场等一系列流场。利用非平衡态耗散粒子动力学方法,分别对环形二嵌段共聚物、线型二嵌段共聚物在剪切场的作用下的相行为,嵌段共聚物胶束在顶盖驱动流场下的形貌变化进行了研究。主要内容包括:研究了本体中环形二嵌段共聚物的穿孔的层状相在简单剪切流场的作用下的相行为和流变学性质。确认了两种典型的相转变过程:(1)从穿孔层状相到层状相的转化,(2)从穿孔层状相到六方堆积柱状相的转化。发现剪切速率不仅对穿孔层状相最终形成的相态,起决定性的作用;而且还对最终形成的相的形貌有巨大的影响。通过序参量随时间的演化过程发现,剪切可以促进体系的演化,加快其达到稳定的状态。剪切粘度随着剪切速率的增加而减小,但是在剪切导致的柱状相出现时,有一个局部的升高。利用非平衡态耗散粒子动力学方法研究了线型二嵌段共聚物层状相在振荡剪切作用下的相行为,发现剪切导致的层状相形貌是剪切振幅和频率共同作用的综合结果。我们在同一振幅的条件下,比较频率对取向选择的影响,确认了取向对频率的依赖关系。另外,我们还通过计算在相同振幅、不同频率下体系序参量来研究剪切导致的层状相相转变的动力学过程。在模拟中,剪切可以引起无序相到层状相的相转变。我们发现剪切可以使相转变临界点(χN)ODT降低。我们还研究了复合剪切粘度和复合模量在振荡剪切中的变化。利用非平衡态耗散粒子动力学模拟方法,研究了溶液中线性二嵌段共聚物胶束在顶盖驱动流场的条件下的形态变化,研究了顶盖驱动流场对其相行为的影响。我们发现:在顶盖驱动流场和限制性的墙的共同作用下,线性两嵌段共聚物胶束的形貌和平衡态时的形貌有很大的区别。此外,限制性的墙与高分子链段之间的相互作用对嵌段共聚物聚集体的形貌有不可忽视的影响。更多还原

【Abstract】 A key challenge in nanotechnology is to design and synthesize the soft materials with simple block copolymers, which are now widely used in industry and daily life for the sake of their novel self-assembly behavior and the periodic physical properties on nanoscale. As a fertile source of soft materials, block copolymers can self-assemble into various ordered structures in both melts and solutions. These stable and metastable microstructures are always under different external fields during industrial processing. It actually relies on the ability to couple an external bias field to some molecular or supermolecular features, and thus achieve directional control over the microstructures. Shearing, as one of the most efficient techniques of symmetry breaking, had been widely used to obtain (or choose) perfect long-range ordered microstructures in block copolymer systems.Computer simulation method and experiments can be considered as the two convenient means while doing scientific researches. We use computer simulation to do scientific researches in this dissertation, because it is one of the powerful techniques to visualize the above-mentioned physical processes directly. It helps us understand and explore the laws inherent in these natural phenomena. We carry out the dissipative particle dynamics (DPD) simulations in order to study the topics mentioned above in detail, respectively. The DPD method includes soft interaction potential, where all the particles interact with each other through three pairwise forces: a conservative force, a dissipative force, and a random force. Compared with traditional molecular dynamics (MD) simulation method, the integration time step can be larger than that in MD. The time scale in DPD simulation can be at milliseconds. We can unite some molecules or polymer segments into one DPD bead due to the soft repulsion potential, thus the DPD model can be used to study the systems at micron length scale. Therefore, DPD is the simulation method that is based upon the mesoscopic scale at both length scale and time scale. The pairwise interactions in the DPD model also result in the momentum of the system being conserved. It is proper to adopt DPD to simulate the microphase separation of block copolymers, because hydrodynamic interaction (HI) is another characteristic in DPD model. DPD is one of the powerful tools to model the dynamic process of complex fluids. At present, DPD has been widely used in the research fields of Chemistry, Physics, Biology, and also Materials Science.In our study, DPD method is used to study the microphase behavior of diblock copolymers which is far away from equilibrium state, such as the cyclic diblock copolymers under steady shear and the linear diblock copolymers subjected to the oscillatory shear. The polymer micelle in the lid-driven flow is also studied comprehensively by carrying out DPD simulations. The main results are as follows:The dissipative particle dynamics simulation technique is used to study the microphase transitions of perforated lamellae of cyclic diblock copolymers under steady shear. The perforated lamellae are transformed to perfect lamellae, and the layer normal is aligned to the direction parallel to the gradient of the velocity under weak shear, whereas they undergo a phase transition to form perfect lamellae whose normal is aligned to the direction perpendicular to the gradient of the velocity due to strong shear. Subjected to the moderate shear, the perforated lamellae are transformed to hexagonally ordered cylinders. By examining the microphase morphologies in the shearing process, we find shear thinning in general, which is reflected by the reorientation of the lamellae, and shear-induced thickening when hexagonally ordered cylinders appear. The calculated shear viscosity basically decreases with increasing shear rate but shows a local maximum at the shear rate that induces hexagonally ordered cylinders.The phase morphologies of symmetric linear diblock copolymers subjected to the oscillatory shear are investigated with the aid of dissipative particle dynamics simulations. The frequency dependent reorientations of the lamellar phase (LAM) have been identified. We find that the parallel orientation of LAM, i.e., the lamellar normal is parallel to the velocity gradient appears at high shear frequency, whereas the perpendicular orientation of LAM the lamellar normal being perpendicular to the velocity gradient takes place at low shear frequency. In both of the cases, the reorientations undergo similar processes: the original LAM phase prepared in equilibrium breaks down rapidly, and it takes a very long time for the perfectly oriented LAM being reformed. Moreover, the shear-induced isotropic to lamellar phase transitions are observed when the oscillatory shear amplitude is large enough. It indicates that the shear amplitude plays a dominant role in the order-disorder transition. The viscosity and the modulus of the melt are found to be dependent on the shear amplitude and the shear frequency in a complex way.The morphology variations of the micelles under lid-driven flow are studied via dissipative particle dynamics simulations. The morphologies of the micelles under flow are different from that in equilibrium. The weak lid-driven flow has hardly changed the morphologies of micelles. We find the worm-like micelles under moderate lid-driven flow. In the strong lid-driven flow, smaller micelles are observed. The properties of the walls are also a dominant factor influencing the micelle morphologies.更多还原