A Dissipative Particle Dynamics Study Of Block Copolymer In Non-Equilibrium State

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.