archives@tulane.edu / A block copolymer (BCP) consists of two or more covalently-bound chemically distinct homopolymer blocks. These macromolecules have emerging applications in energy storage, membrane separations, and nanolithography stemming from their propensity to self-assemble into regular nanoscale structures. For a pure BCP, this self-assembly is dictated entirely by the polymer’s degree of polymerization (N), chemistry (χ), composition (f), and chain architecture. Blending together different types of these polymers provides a simpler, synthesis-free method for tuning nanoscale morphology and feature size. This dissertation describes the use of dissipative particle dynamic (DPD) simulation to develop a fundamental understanding of phase behavior in BCP/homopolymer and cyclic-linear BCP blends. Block copolymer-homopolymer blends offer a simple method for tuning nanostructure sizes to meet application-specific demands. We systematically investigated morphology and feature size in ternary blends of symmetric linear copolymers and their constituent homopolymers (A-b-B/A/B), finding a close match between simulation results and known experimental behavior. Having established DPD simulation as a valid model, we used the simulation results to explore the relationship between polymer chain length, molecular packing, and the degree of lamellar swelling with homopolymer addition in ternary blends. A consensus of theoretical and simulation work suggests that cyclic BCPs form features up to 40% smaller than their linear analogues - while also exhibiting superior thin film stability and assembly dynamics – making them intriguing candidates for nanolithography. However, the complex syntheses required to produce these molecules mean that a need for pure cyclic BCPs would present a challenge to large-scale manufacturing. Thus, we aspired to understand the self-assembly of cyclic-linear copolymer blends. We first combined DPD simulation results and strong segregation theory to develop a scaling prediction for neat BCP feature size based on experimentally-tunable parameters (χ, N, f, and polymer architecture). The resulting Revised Scaling Law quantitatively predicts domain spacings over a wide range of BCP chain lengths, segregation strengths, and compositions and offers an explanation for the significant discrepancies between prior theoretical predictions and experimental results of cyclic BCP feature size. Next, we investigated the dimensions and interfacial roughness of nanofeatures formed by cyclic-linear BCP blends. For mixtures of symmetric cyclic and linear polymers of equivalent N, up to 10% synthetic impurity had minimal impact on cyclic BCP feature dimensions. On the other hand, we found that adding small amounts of cyclic BCP was an effective method for “fine-tuning” linear BCP feature sizes. We also analyzed our simulated blend domain spacings in the context of the revised scaling law, and found deviations between simulation and theory that arose from molecular-level packing motifs not included in theory. Finally, we investigated the impact of blending polymer architectures on the BCP order-disorder transition, finding that a mismatch in molecular size and architecture can strongly inhibit ordering. These insights into blend self-assembly will assist experimentalists in the rational design of BCP materials for advanced nanolithography applications. / 1 / Amy Dubetz Goodson

Attachment of particles to one another due to action of certain inter-particle forces is called as particle agglomeration. It has applications ranging from efficient capture of ultra-fine particles generated in coal-burning boilers to effective discharge of aerosol sprays. Aerosol sprays have their application in asthma relievers, coatings, cleaning agents, air fresheners, personal care products and insecticides. There are several factors that cause particle agglomeration and based on the application, agglomeration or de-agglomeration is desired. These various factors associated with agglomeration include van derWaals forces, capillary forces, electrostatic double-layer forces, effects of turbulence, gravity and brownian motion. It is therefore essential to understand the underlying agglomeration mechanisms involved. It is difficult to perform experiments to quantify certain effects of the inter-particle forces and hence we turn to numerical simulations as an alternative. Simulations can be performed using the various numerical simulation techniques such as molecular dynamics, discrete element method, dissipative particle dynamics or other probabilistic simulation techniques. The main objective of this thesis is to study the geometric characteristics of particle agglomerates using dissipative particle dynamics. In this thesis, agglomeration is simulated using the features of dissipative particle dynamics as the simulation technique. Forces of attraction from the literature are used to modify the form of the conservative force. Agglomeration is simulated and the characteristics of the result ing agglomerates are quantified. Simulations were performed on a sizeable number of particles and we observe agglomeration behavior. A study of the agglomerates resulting from the different types of attractive forces is performed to characterize them methodically. Also as a part of this thesis, a novel, dynamic particle simulation technique was developed by interfacing MATLAB and our computational C program.

Electrostatic interactions between charged species play a prominent role in determining structures and states of physical system, leading to important technological and biological applications. In coarse-grained simulations, accurate description of electrostatic interactions is crucial in addressing physical phenomena at larger spatial and longer temporal scales. In this thesis, we implement ENUF method, an abbreviation for Ewald summation based on non-uniform fast Fourier transform technique, into dissipative particle dynamics (DPD) scheme. With determined suitable parameters, the computational complexity of ENUF-DPD method is approximately described as O(N logN). The ENUF-DPD method is further validated by investigating dependence of polyelectrolyte conformations on charge fraction of polyelectrolyte and counterion valency of added salts, and studying of specific binding structures of dendrimers on amphiphilic membranes. In coarse-grained simulations, electrostatic interactions are either explicitly calculated with suitable methods, or implicitly included in effective potentials. The effect of treatment fashion of electrostatic interactions on phase behavior of [BMIM][PF6] ionic liquid (IL) is systematically investigated. Our systematic analyses show that electrostatic interactions should be incorporated explicitly in development of effective potentials, as well as in coarse-grained simulations to improve reliability of simulation results. Detailed image of microscopic structures and orientations of [BMIM][PF6] at graphene and vacuum interfaces are investigated by using atomistic simulations. Imidazolium rings and alkyl side chains of [BMIM] lie preferentially flat on graphene surface. At IL-vacuum interface, ionic groups pack closely together to form polar domains, leaving alkyl side chains populated at interface and imparting hydrophobic character. With the increase of IL filmthickness, orientations of [BMIM] change gradually from dominant flat distributions along graphene surface to orientations where imidazolium rings are either parallel or perpendicular to IL-vacuum interface with tilted angles. The interfacial spatial ionic structural heterogeneity formed by ionic groups also contributes to heterogeneous dynamics in interfacial regions.

In this study, the nanocomposite of graphene and PMMA at the different volume fractions was investigated by molecular dynamics and dissipative particle dynamics simulations. The MD simulation can be performed to simulate the nanocomposite system at different weight fractions to obtain the different repulsive parameters. After obtaining the repulsive parameters, the DPD simulation can be utilized to study the equilibrium phase of graphene and PMMA nanocomposite. From our result, all equilibrium phases at different volume fractions are cluster. However, it is difficult to enhance the property for nanocomposite material due to the aggregated graphene (cluster). Hence, we change the interaction repulsive parameters to stand for the different degrees of functionalized graphene. When the interaction repulsive parameter is smaller than 80, the equilibrium phase is dispersion. In addition, the different number of functionalized garphene bead per graphene was studied, and results show that the equilibrium phase is dispersion when all graphene beads per graphene are functionalized.

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In this thesis, molecular dynamics and dissipative particle dynamics simulation methods are adopted to investigate the effects of volume fraction (1:1; 1:4; 1:6; 1:14; 1:20), repulsive interaction parameter (aij) and chain length on the microstructure of (5,5) carbon-nanotube (CNT)/polyethylene (PE) mixture. In order to obtain the information of microstructure for different simulation conditions, we used the radius of gyration and orientational order parameter to explore the polymer conformation. It is found that micro-structures will be very different when different repulsive interaction parameters and volume fractions are used.

An aqueous solution of electrolytes can be modelled simplistically as charged hard spheresdispersed in a dielectric continuum. We review various classical theories for hard sphere systems including the Percus-Yevick theory, the mean spherical approximation, the Debye-Hückel theory and the hyper-netted chain theory, and we compare the predictions of the theories with simulation results. The statistical associating fluid theory (SAFT) has proved to be accurate for neutral polymers. It is modified to cope with charged polyelectrolyte systems. A chain term for the charged reference fluid is introduced into the theory. Some well-established results are reproduced in this study and we also introduce new terms and discuss their effects. The results show that the SAFT is semi-quantitatively correct in predicting the phase behaviour of polyelectrolytes. The electrostatic attraction between unlike charged particles at low temperature is very strong. The short-range attractions between unlike pairs are treated via an association theory while the remaining interactions are handled by hypernetted chain theory. This method works quite well with multiple associating sites. The phase prediction for the size and charge symmetric restricted primitive model is quantitatively correct as compared with simulation results. Furthermore, it also gives semi-quantitatively correct predictions for the phase behaviour of size- and charge-asymmetric cases. Dissipative particle dynamics (DPD) is a powerful simulation technique for mesoscopic systems. Molecules with specific shapes (rods and spheres) are simulated using this technique.By tuning the density of the system, some liquid crystal phase transitions can be observed.The properties of spider silk fibroin are also modelled by DPD, indicating a possible route offorming spider silk.

Isoporous membranes manufactured from diblock copolymer are successfully produced at laboratory scale under controlled conditions. Because of the complex phenomena involved, membrane preparation requires trial and error methodologies to find the optimal conditions, leading to a considerable demand of resources. Experimental insights demonstrate that the self-assembly of the block copolymers in solution has an effect on the final membrane structure. Nevertheless, the complete understanding of these multi-scale phenomena is elusive. Herein we use the coarse-grained method Dissipative Particle Dynamics to study the self-assembly of block copolymers that are used for the preparation of the membranes. To simulate representative time and length scales, we introduce a framework for model reduction of polymer chain representations for dissipative particle dynamics, which preserves the properties governing the phase equilibria. We reduce the number of degrees of freedom by accounting for the correlation between beads in fine-grained models via power laws and the consistent scaling of the simulation parameters. The coarse-graining models are consistent with the experimental evidence, showing a morphological transition of the aggregates as the polymer concentration and solvent affinity change. We show that hexagonal packing of the micelles can occur in solution within different windows of polymer concentration depending on the solvent affinity. However, the shape and size dispersion of the micelles determine the characteristic arrangement. We describe the order of crew-cut micelles using a rigid-sphere approximation and propose different phase parameters that characterize the emergence of monodisperse-spherical micelles in solution. Additionally, we investigate the effect of blending asymmetric diblock copolymers (AB/AC) over the properties of the membranes. We observe that the co-assembly mechanism localizes the AC molecules at the interface of A and B domains, and induces the swelling of the B-rich domains. The B-C interactions control the curvature of the assemblies in these blends. Finally, we study the self-assembly triblock copolymers used for membranes fabrication. We show that the polymer concentration, the block-copolymer composition, and the swelling of the micelle are responsible for the formation of elongated micelles in the casting solution. The formation of nanoporous membranes arises from the network-like packing of those micelles.

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