Multiscale Simulation Study On The Complex Phase Behavior Of Soft Matter Systems

Because of the long chain characteristics of polyolefin systems, the physical properties of which are multiscaled on both length and time scales. On different scales, there have been a series of well developed simulation techniques, such as molecular dynamics (MD) and Monte-Carlo (MC) on the microscopic scale, dissipative particle dynamics (DPD), lattice Boltzmann method (LBM), dynamic density functional theory (DDFT), and field-theoretic polymer simulation (FTPS) on the mesoscale, finite element method (FEM) based on fluid mechanics on the macroscopic scale. These simulation methods are all focused on a specific scale, therefore it is very urgent and signifcant to propose a multiscale simulation method, with which we can simulate the polyolefin systems on a larger time and length scale and keep enough microscopic information simultaneously.Moreover, multiscale simulation study on the hierarchical self-assembly of block copolymers is also an important work. Recently, a perspective paper appeared in Macromolecules with the title"Research in macromolecular science: challenges and opportunities for the next decade", indicates that complex polymer systems from block copolymers can be used as a route to fabrication of nanoscale objects. In the solution, block copolymers can firstly self-assemble into various micelles with the core-corona structure. By changing the concentration and the structure of block copolymers, we can obtain the micelles with anisotropic interactions and shapes, such as disklike micelles and rodlike micelles. These anisotropic micelles can further self-assemble into more advanced structures. But, we don't know too much about mechanism and the controlling factors of the hierarchical self-assembly of block copolymers. Computer simulation can be used as a powful tool to study these processes. Furthermore, to date, there have been no simulation techniques and studies available regarding self-assembly of these anisotropic micelles into more advanced structures. Hence, a larger scale simulation is also very important for studying hierarchical self-assembly of soft anisotropic particles.The thesis is dedicated to develop the multiscale simulation methods of polyolefin systems: including bridging the gap between the microscale and the mesoscale and between the mesoscale and the macroscale simulation techniques. We also predict the spinodal curve of iPP/PEOC blend by molecular dynamics simulations combined with Sanchez-Lacombe lattice fluid theory (SLLFT). Moreover, we develop a novel simulation technique to study the ordered phase behavior in bulk and hierarchical self-assembly in dilute solution for soft anisotropic particles.Bridging the gap between the microscale and the mesoscale and between the mesoscale and the macroscale simulation techniques:(1) Connection between microscale and mesoscale simulations. a, Microscale and mesoscale simulations are connected by mapping the RDF of the coarse-grained iPP or PEOC bead from the Lowe-Andersen temperature controlling method (LA) onto the target one from detailed molecular dynamics simulation. We update the coarse-grained potentials according to the method as in RMC. The procedure is performed iteratively until the difference between two RDFs is within a designated small value. This numerical potential can be applied in larger systems, and the properties of polyolefin systems on the mesoscale can be obtained. Moreover, in order to test the coarse-grained potentials, we simulated the miscibility of the blend of iPP/PEOC. b, After simulating the mesoscale polyolefin systems using LA with the numerical coarse-grained potential, we can further fill the beads with atomistic details, i.e., do a fine-graining. By fixing bond lengths and bond angles, the atoms are regrown according to a scheme similar to Rosenbluth sampling. Because the system is already in equilibrium, only tens of picoseconds of MD run is needed to alleviate the local tension arising from the chain regrowing procedure. In this way one may fast equilibrate the system by mesoscale simulation and still keep its atomistic details in a fine-graining step.(2) Connection between the mesoscale and macroscale simulations. We apply the obtained coase-grained potential in the non-equilibrium LA simulation to derive the shear viscosity of polyolefin systems. The dependence of the shear viscosity on the shear rate derived from meso-scale simulation can be input in the finite element simulation. Then we can get the informations about the velocity and the pressure distributions of polyolefin systems by finite element calculating of the momentum equation. So the mesoscale and macroscale simulations of polyolefin systems can be connected. It should be noted that the accuracy of the dependence of the shear viscosity on the shear rate is crucial to the connection between the mesoscale and macroscale simulations.(3) Predicting the spinodal curve of iPP/PEOC blend. We adopt MD simulations to calculate the SLLFT EOS characteristic parameters for iPP and PEOC, respectively. Especially, a Boltzmann fitting of the relation between T* and the simulation temperature is proposed, which further helps us to obtain the characteristic parameter T* in the high-temperature limit. With these EOS parameters, we predict the PVT data for iPP and PEOC. Furthermore, a recursive method is proposed to obtain the characteristic interaction energy parameterε1*2between iPP and PEOC. This method, which does not require fitting to the experimental phase equilibrium data, suggests an alternative way to predict the phase diagrams that are not easily obtained in experiments. As an example, in the framework of SLLFT, the spinodal curve of the iPP/PEOC blend is predicted for the low molecular weights that are used in the simulations.Multiscale simulation of the hierarchical self-assembly of block copolymers:(1) Ordered packing of soft disklike particles. A novel mesoscopic simulation method is adopted to study the ordered packing of the anisotropic disklike particles with a soft repulsive interaction, which possesses a modified anisotropic conservative force type used in dissipative particle dynamics. With the modified potential, we perform mesoscopic simulations to study the ordered packing of soft disklike particles, which represent the core-corona disklike micelles, the core-shell-corona disklike micelles, and so on. This mesoscopic simulation technique can be used to study the systems over greater length and time scales which are not accessible in traditional molecular dynamics (MD) simulations. We examine the influence of the shape of the particles, the angular width of the repulsion, and the strength of the repulsion on the packing structures. Specifically, an ordered hexagonal columnar structure is obtained in our simulations. Our study demonstrates that an anisotropic repulsive potential between soft discoidal particles is sufficient to produce a relatively ordered hexagonal columnar structure.(2) Hierarchical self-assembly of soft disklike particles in dilute solution. The effective interactions between soft disklike particles may be slightly attractive in the relatively long range arising from hydrophobic or hydrogen bonding interactions. Therefore, we develop a novel mesoscale simulation model that can reflect the interaction nature between soft disklike particles in a simple way. Hierarchical self-assembly of soft disklike particles is successfully investigated with the aid of the simulation model. The soft disklike particles can represent the core-corona micelles with stable structures and the similar kinds of particles. The cooperative driving factors of hierarchical self-assembly of soft disklike particles are the weak non-covalent attraction and phase separation. The weak attraction allows disklike particles to self-assemble into one-dimensional threads. The phase separation does not break the one-dimensional thread structures, but brings the threads together to form hexagonal bundle structure. This new bundle structure can serve as a starting point to form further advanced materials. Therefore, our mesoscopic simulation strategy may represent a useful route to study the hierarchical self-assembly of soft anisotropic systems and design novel and complex structures.(3) Phase behavior of soft rodlike particles. We propose a simple single-site purely-repulsive anisotropic soft-core model by introducing an anisotropic factor into the conservative potential in DPD, with which we can study the phase behavior of soft disklike particles. By examining the influences of the strength of the repulsion and the shape of soft rodlike paritlces on the phase behavior, we observe the nematic, smectic-A and smectic-B phases in the simulations. Therefore, a proper choice of the repulsion strength and the shape of soft rodlike particles is very important to observe different phase behavior. This novel mesoscopic simulation model can be successfully used to study the phase behavior of soft rodlike particles. Our study demonstrates that an anisotropic purely-repulsive potential between soft rodlike particles is suffcient to produce the nematic, smectic-A, and smectic-B phases.Developing new multiscale simulation techniques are very urgent and important in soft condensed matters. There are a lot of problems to be settled. For example, our proposed potential can only reflect the interactions between soft anisotropic particles qualitatively. Can we obtain the potential by calculating the potential of mean force between soft anisotropic particles, which can reflect the interactions between them quantitatively? By applying a shear field to these systems, we will examine the influence of an external field on the hierarchical self-assembly structures of soft anisotropic particles. Moreover, in order to simulate soft anisotropic systems more effectively, we will further optimize our parallel simulation codes.