Browsing by Subject "self-assembly"
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Item The Colloidal Glass Transition Under Confinement(2018-05) Zhang, BoUnderstanding the nature of the glass transition is one of the most challenging problems in condensed matter physics. Although ubiquitous and technically important, glasses still elude a universally accepted theoretical description. Here, we use colloidal particles as hard-sphere models and experimentally study particle dynamics of colloidal suspensions under different confinements near the glass transition. In three dimension (3D), we design a colloidal system, where particles are confined inside spherical cavities with an amorphous layer of particles pinned at the boundary. Using this novel system, we capture the amorphous-order particle clusters proposed in the framework of the random first-order transition (RFOT) theory and demonstrate the development of a static correlation near the glass transition. Moreover, by investigating the dynamics of spherically confined samples, we reveal a profound influence of the static correlation on the relaxation of colloidal liquids. In analogy to glass-forming liquids with randomly pinned particles, we propose a simple relation for the change of configurational entropy of confined colloidal liquids, which quantitatively explains our experimental findings and illustrates a divergent static length scale during the colloidal glass transition. In two dimension (2D), we prepare quasi-2D confined colloidal liquids with optical tweezers. We confirm the existence of a divergent static length in quasi-2D liquids. We further use the confinement as a tool to probe the Mermin-Wagner long-wavelength fluctuations. We find that the fluctuations have a logarithmic dependence on the system size in quasi-2D when the system approaches to the glass transition. Ellipsoidal and rodlike particles are also used to directly compare the translational and rotational dy- namics. We show a decoupling between translational and rotational dynamics and the decoupling is not affected by the confinement. What’s more, constant values of critical volume fractions are observed regardless of types of particle aspect ratios, measurement methods, fitting functions, and values of structural factors. Lastly, we have also conduct an experimental study on the 1D dynamic self-assembly of charged colloidal particles in microfluidic flows. Using high-speed confocal microscopy, we systematically investigate the influence of flow rates, electrostatics and particle poly- dispersity on the observed string structures. By studying the detailed dynamics of stable flow-driven particle pairs, we quantitatively characterize interparticle interac- tions. Based on the results, we construct a simple model that explains the intriguing non-equilibrium self-assembly process. Our study shows that the colloidal strings arise from a delicate balance between attractive hydrodynamic coupling and repulsive electro- static interaction between particles. Finally, we demonstrate that, with the assistance of transverse electric fields, a similar mechanism also leads to the formation of 2D colloidal walls. Our study provides key experimental evidences to support the development of RFOT theory to better understand the glass transition in both 3D and 2D. The fundamental differences of particle dynamics between 3D and 2D are also studied. In addition to providing experimental results for assessing general glass transition theories and par- ticle self-assembly, our studies also provide new insights into the dynamics of confined colloidal liquids and may shed light on the behavior of atomic/molecular liquids under nano-confinements.Item Data for Surface Relief Terraces in Double Gyroid-Forming Polystyrene-block-Polylactide Thin Films(2023-09-28) Yang, Szu-Ming; Oh, Jinwoo; Magruder, Benjamin R; Kim, HeeJoong; Dorfman, Kevin D; Mahanthappa, Mahesh K; Ellison, Christopher J; cellison@umn.edu; Ellison, Christopher J; University of Minnesota Department of Chemical Engineering and Materials ScienceThis study describes the thin film self-assembly behavior of a polystyrene-block-polylactide (SL-G) diblock copolymer, which undergoes melt self-assembly in bulk into a double gyroid (DG) network phase with a cubic unit cell parameter a = 52.7 nm. Scanning electron microscopy (SEM) and grazing-incidence small-angle X-ray scattering (GISAXS) reveal that thermally annealing 140–198 nm thick copolymer films on SiO2 substrates below the morphological order-to-disorder transition temperature yields polydomain DG structures, in which the (422) planes are oriented parallel to the surface. Bright-field optical microscopy (OM) and atomic force microscopy (AFM) analyses further reveal the film thickness-dependent formation of topographical terraces, including islands, holes, and bicontinuous features. The occurrence of these features sensitively depends on the incommensurability of the as-prepared film thickness and the (211)-interplanar spacing (d211) of the DG unit cell. While the steps heights between adjacent terraces exhibiting characteristic “double wave” patterns of the DG (422) planes coincide with d211, previously unreported transition zones between adjacent terraces are observed wherein “boomerang” and “droplet” patterns are observed. These intermediate patterns follow the expected sequence of adjacent termination planes of the bulk DG unit cell along the [211] direction, as confirmed by comparisons with self-consistent mean-field theory calculations.Item Molecular Simulation and Design of High-χ Low-N Block Oligomers for Control of Self-Assembly(2022-02) Shen, ZhengyuanMulti-component oligomer systems are exciting candidates for nanostructured functional materials, due to the wide variety of their self-assembled morphologies with extremely small feature size. However, experimentally screening through the vast design space of molecular architectures can be extremely laborious. Therefore, guidance from predictive modeling is essential to reduce the synthetic effort. This dissertation discusses the predictive design of self-assembling block oligomer systems using molecular simulations, and the development of computer vision models for automated morphology detection for simulation trajectories. Work presented in this thesis creates a roadmap for efficient computational screening of shape-filling molecules, thus accelerating the design and discovery of nanostructured functional materials. First, with the aid of experimentally-validated force fields, molecular dynamics simulations were exploited to design: 1) a series of symmetric triblock oligomers that can self-assemble into ordered nanostructures with sub-1 nm domains and full domain pitches as small as 1.2 nm, 2) Blends of a lamellar-forming diblock oligomer and a cylinder-forming miktoarm star triblock oligomer leading to stable gyroid networks over a large composition window. Similarities and distinctions between the self-assembly phase behavior of these block oligomers and block polymers are discussed. Second, existing simulation data were used to train deep learning models based on three-dimensional point clouds and voxel grids. The pretrained neural networks can readily detect equilibrium morphologies, and also give rich insights of emerging patterns throughout new simulations with different system sizes and molecular dimensions.Item Nanoporous and Functionalized Polymer Thermosets by Polymerization-Induced Microphase Separation in Bulk, Dilution, and Suspension(2021-10) Peterson, ColinThe microphase separation of diblock polymers allows for excellent control over the nanostructuring of polymer-based materials. Polymers are also readily functionalized and chemically manipulated to alter their chemical properties. Therefore, block polymers represent an important tool in the preparation of precision nanostructured functional materials. Polymerization-induced microphase separation (PIMS) is a convenient and powerful strategy towards the development of such materials. In PIMS, the diblock polymer is simultaneously grown while one block is crosslinked. This captures a non-equilibrium percolating morphology. In this thesis, the morphology is used as a host for photochromic dyes, diluted with solvent to increase the possible porosity, and prepared in suspension to give uniform mesoporous beads.Chapter 1 is a brief overview of key topics relevant to the entire thesis. Chapter 2 describes the incorporation of photochromic dye molecules into a variety of materials from liquid solvent to rigid polymer. PIMS thermosets were created using a liquid-like polycaprolactone derivative and crosslinked polymethylmethacrylate. The liquid-like domains provide an environment for the dye where fast structural relaxation allows for fast dye decoloration while being encased in a rigid matrix. Chapter 3 shifts focus to porous PIMS derivatives. In particular, the effect on the pore size distribution of diluting the monomer solution with solvent to create an organogel is explored. Chapter 4 presents a new synthetic method to prepare beads from PIMS thermosets by performing the chain-growth and cross-linking steps in aqueous suspension. The size of the particles is tuned independently from the size of the pores. Also, functionality is incorporated into the pore walls using a diblock precursor. Chapter 5 provides general conclusions and possible future directions for research relating to disordered diblock thermoset materials.Item Plasmon Hybridization In Self-Assembled 3D Graphene-Based Metamaterials(2020-04) Agarwal, KritiThree-dimensional (3D) photonic geometries are attractive for developing novel coupled optical modes that cannot exist in the two-dimensional (2D) nano and microfabrication world. In this thesis, the various optical properties that can be induced as a result of 3D architecture are designed, fabricated, and characterized. Even for the well-established resonance in split-ring resonator-based metamaterials, the addition of the multiple planes of symmetric coupling or decoupling induce isotropic and anisotropic resonances for applications such as ultra-sensitive molecular analysis with two-fold advantage of frequency and amplitude monitoring for small concentrations and low on-chip power inclinometers with nanodegree sensitivity, respectively. The limited spatial coverage of the plasmon-enhanced near-field in 2D graphene ribbons presents a major hurdle in practical applications. The ability to transform 2D materials into 3D structures while preserving their unique inherent properties offers enticing opportunities for the development of diverse applications for next-generation micro/nanodevices. Diverse self-assembled 3D graphene architectures are explored here that induce hybridized plasmon modes by simultaneous in-plane and out-of-plane coupling to overcome the limited coverage in 2D ribbons. While 2D graphene can only demonstrate in-plane bi-directional coupling through the edges, 3D architectures benefit from fully symmetric 360° coupling at the apex of pyramidal graphene, orthogonal four-directional coupling in cubic graphene, and uniform cross-sectional radial coupling in tubular graphene. The 3D coupled vertices, edges, surfaces, and volumes induce corresponding enhancement modes that are highly dependent on their shape and dimensions. While most of this work strives to achieve multiple coupled planes of symmetry, the same ideas are also applied to achieve multiple 3D graphene geometries that break mirror symmetry across multiple planes. The asymmetric graphene induces giant optical activity (chirality) that has remained previously unrealized due to the 2D nature of graphene. The chirality induced within the 3D graphene chiral helixes is also a strong function of the geometrical parameters that are analyzed using a machine-learning-based multivariate regression approach to determine the 3D geometry with the strongest chirality. The hybrid modes introduced through the 3D couplings amplify the limited plasmon response in 2D ribbons to deliver non-diffusion-limited sensors, high-efficiency fuel cells, and extreme propagation length optical interconnects.Item Structure and Thermodynamics of Neutral and Charged Block Copolymer-Based Materials(2022-08) Zhang, BoNext-generation materials are often required to exhibit two (or more) orthogonal properties simultaneously. One example is polymer electrolytes, as both facile ion transport and mechanical robustness are desired. However, these orthogonal properties are hard to achieve in single-component systems, because ion transport usually requires high chain mobility while high chain rigidity or low chain mobility is desired for mechanical stability. One way to overcome this challenge is to develop co-continuous nanostructured materials, such that one domain provides ion transport while the other imparts mechanical robustness. A promising predictable and tunable co-continuous structure is the bicontinuous microemulsion from ternary blends of an AB diblock copolymer and the corresponding A and B homopolymers. However, the structure and thermodynamics of such ternary mixtures are not fully elucidated, even in the limit of neutral ternary blends. Moreover, little is known about ion-containing ternary blends. Therefore, the focus of this thesis work is to understand the fundamental phase behavior of these systems and to ultimately provide insight into the rational design of functional materials. In Chapter 2, we investigate the phase behavior of neutral ternary blends comprising a linear diblock copolymer and the corresponding homopolymers. The impacts of block copolymer compositional asymmetry on ordered, disordered, and macrophase-separated regions of the ternary phase prism are discussed. In Chapter 3, we expand the research to ternary mixtures involving a bottlebrush diblock copolymer and the corresponding linear homopolymers. The overall phase behavior closely resembles that of linear ternary mixtures, except for an unconventional spatial distribution of the homopolymers. Chapters 4 and 5 focus on the self-assembly of charged diblock copolymers, serving as the starting point for the investigation of charged ternary blend phase behavior. Chapter 4 details the phase behavior of a series of symmetric charged diblock copolymers, where the effective interaction parameter was found to increase linearly with the increase in charge fraction. Chapter 5 extends the work to a different model system with a relatively nonpolar charged block. A tilted, “chimney”-like order-disorder transition boundary was observed. However, the composition windows of the ordered phases remain nearly unchanged. Overall, the findings from this thesis research provide valuable insight into the structure and thermodynamics of neutral and charged polymer mixtures, and will inform the rational design of nanostructured polymer electrolytes with tunable structure and properties.Item Structure and Thermodynamics of Salt-Doped Polymer Blends(2020-08) XIE, SHUYIA central challenge in designing novel polymeric materials is to find a broadly applicable strategy to systematically tailor microstructures in order to simultaneously optimize two or more orthogonal properties. For example, polymeric materials with high ion transport and mechanical stiffness are highly desired in water treatment membranes, ion battery electrolytes, fuel cell membranes etc. To achieve this goal, at least two components with distinct properties are usually needed, and a co-continuous microstructure, where one domain is responsible for ion transport while the other imparts mechanical strength, is favored. In this thesis, we propose that the salt-doped A/B/AB ternary system that consists of A and B homopolymer and a corresponding AB diblock copolymer is an attractive platform in accessing the bicontinuous microemulsion (BμE), where the introduction of salt improves the ionic conductivity. To help design such materials with desired properties, the influence of salt ions on the thermodynamics of mixing and phase behaviors needs to be elucidated. We start with one limit of the salt-doped ternary system that no copolymer is added. In Chapter 2, we investigated the phase behavior of LiTFSI-doped poly(ethylene-alt-propylene)/poly(ethylene oxide) (PEP/PEO) and polystyrene/poly(ethylene oxide) (PS/PEO) binary blends and observed a significant reduction of miscibility and an asymmetric phase diagram. Chapter 3 details the symmetric isopleth phase diagram of LiTFSI-doped PS/PEO/PS-b-PEO ternary blends, where a robust and wide BμE channel has been found. Chapter 4 extends the research reported in Chapter 3 to off-symmetric isopleths, where an unexpected C15 Laves phase has been observed, and isothermal phase diagrams have also been mapped out. Finally, Chapter 5 describes the influence of salts on the single-chain dimensions of PEO melts by small-angle neutron scattering and the difficulties in data analysis. Throughout the whole thesis, the main goal is to comprehensively understand salt-polymer interactions and explore the change of phase behavior compared to the salt-free system, which may help to prepare polymer electrolytes with tunable structures and properties.Item Supporting Data for "From Order to Disorder: Computational Design of Triblock Amphiphiles with 1 nm Domains"(2020-07-06) Shen, Zhengyuan; Chen, Jingyi L; Vernadskaia, Viktoriia; Ertem, S Piril; Mahanthappa, Mahesh K; Hillmyer, Marc A; Reineke, Theresa M; Lodge, Timothy P; Siepmann, J Ilja; siepmann@umn.edu; Siepmann, J Ilja; Materials Research Science & Engineering Center (MRSEC)Data including input/output and restart files for all the systems, analysis codes (python, fortran, cpp), and figures in the paper "From Order to Disorder: Computational Design of Triblock Amphiphiles with 1 nm Domains." Sample molecular dynamics trajectories pieces are provided due to the extremely long simulation trajectories.Item Understanding self-assembly and flow heterogeneities in poloxamer wormlike micelles(2023-06) McCauley, PatrickWormlike micelles (WLMs) are elongated, self-assembled structures formed from amphiphilic molecules in solution. Although the structure of WLMs resembles that of polymers, WLMs are differentiated by their ability to break and recombine at rest and in response to deformations. This unique property has led to their ubiquitous use in a variety of applications such as consumer products and oilfield recovery. Additionally, entangled WLMs exhibit unique nonlinear flow behavior such as the formation of shear bands and other flow heterogeneities, which have garnered considerable scientific interest. While often formulated using small molecule ionic surfactants, WLMs can also form in solutions of amphiphilic block polymers. The most well-studied of these are poloxamers, ABA block polymers formed from two polyethylene oxide (PEO) end blocks and one polypropylene oxide (PPO) midblock. Poloxamers have the potential to form WLMs with a range of rheological properties due to the tunability block composition. The self-assembly of poloxamers into spherical micelles is well characterized, but the formation of poloxamer WLMs is poorly understood, leaving this class of WLMs underutilized. The first goal of this thesis is to formulate guidelines for varying poloxamer composition, temperature, salt type, and salt concentration to induce the formation of WLMs and tune their rheological properties. Small-angle neutron scattering, light transmittance measurements, and linear and nonlinear rheology were performed to characterize the temperature-induced rod formation, local micelle structure, and bulk mechanical properties of a large variety of poloxamer WLM formulations. This characterization revealed that the local microstructure of poloxamer WLMs is fairly insensitive to the poloxamer block composition, molecular weight, and the presence of salts, but the rheological properties varied greatly among formulations. Higher viscosity solutions were produced in poloxamers with higher molecular weights, lower PEO content, and added sodium chloride. Leveraging the insights from this self-assembly characterization, the second goal of this thesis is to study the nonlinear flow behavior of a highly elastic, high-viscosity WLM solution formed using poloxamers. These WLMs exhibited rheological behaviors typically observed in gels, including a yield stress and viscoelastic aging. Combined shear rheology and particle tracking velocimetry (rheo-PTV) measurements revealed this solution of WLMs formed shear bands in startup flows, accompanied by elastic instabilities and wall slip. The mechanism of shear-band formation was unlike any WLMs studied previously and more closely resembled the mechanism in yield stress fluids. Exploring the evolution of shear bands in this WLM gel with rheology similar to both canonical viscoelastic WLMs and yield stress fluids provided new insights on shear banding in both of these complex fluids. Shear bands in poloxamer WLMs are frequently accompanied by wall slip, which is also prevalent in nonlinear flow studies of many other WLMs. The third goal of this thesis is to explore the impact of wall slip on the spatiotemporal evolution of the flow field during shear banding. Cylindrical Couette flows of WLMs were simulated with the German-Cook-Beris (GCB) model and two phenomenological slip boundary conditions. Introducing wall slip was shown to delay the onset of shear banding and reduce the width of the high-shear-rate band, consistent with experiments. During shear band formation, the evolution of the flow field was sensitive to the form of the slip boundary condition; flow reversal prior to shear-band formation was enhanced with shear-rate-dependent wall slip and diminished with shear-stress-dependent wall slip. These results demonstrated that the qualitative agreement between shear-banding models of WLMs and experiments can be improved by incorporating wall slip into shear-banding simulations. The final goal of this thesis is to re-examine the peculiar shear-band formation in poloxamer WLM gels and verify the proposed yield-stress-driven shear banding mechanism. A new technique was developed to characterize flow heterogeneity that combines cessation of flow protocols with rheo-PTV. To successfully implement this technique, new methods to analyze rheo-PTV data were developed, which improved the calculation of the velocity by fitting entire particle trajectories described in cylindrical coordinates. Before performing experiments, theoretical flow problems were analyzed to demonstrate that fluid retraction accompanies stress relaxation in viscoelastic fluids with flow heterogeneity. The proposed evolution of flow heterogeneity in poloxamer WLM gels was confirmed by measuring fluid retraction in cessation of flow. This technique also revealed another poloxamer WLM formulation developed shear bands via the canonical shear-band formation mechanism. These findings demonstrate the utility of cessation of flow combined with PTV to characterize flow heterogeneity in viscoelastic complex fluids. Overall, this thesis combines computational and experimental approaches to gain a deeper understanding of the self-assembly and nonlinear flow behavior of WLMs formed from poloxamers. Fundamental insights are also uncovered about nonionic surfactant self-assembly, wall slip, shear banding, and flow heterogeneity in complex fluids in general. The unique rheology and highly tunable self-assembly of poloxamer WLMs make these solutions ideal candidates for future investigations about shear banding.