Browsing by Subject "Self-assembly"

Now showing 1 - 14 of 14
  • Thumbnail Image
    Item
    Amphiphilic polymers: crystallization-assisted Self-assembly and applications in pharmaceutical formulation
    (2013-04) Yin, Ligeng
    Amphiphilic polymers are macromolecules that simultaneously contain hydrophobic and hydrophilic components. These molecules not only attract much attention in academic research but also are important materials in industry. Application areas include detergency, oil field, paints, agriculture, food, cosmetics, and pharmaceutics. This dissertation highlights my efforts since the November of 2007 on three separate systems of amphiphilic polymers, which addresses both the fundamental self-assembly behavior in solution and applications in pharmaceutical formulation. Chapter 2 describes the self-assembled micelles in water that contain semicrystalline polyethylene (PE) as the core-forming material. Poly(N,N-dimethylacrylamide)-polyethylene (AE) diblock copolymers were chosen as the model system. An AE diblock copolymer with relatively low PE composition resulted in micelles with oblate ellipsoidal cores in water, in which crystalline PE existed as flat disks at the center and rubbery PE resided on both sides. In contrast, a control sample with a rubbery polyolefin as the hydrophobic component resulted in micelles with spherical cores in water. The morphology transition was ascribed to the crystallization of PE. The heat-assisted direct dissolution for sample preparation was identified as a stepwise "micellization-crystallization" procedure. In addition, the morphology of the aggregates exhibited much dependence on the composition of AE copolymers, and wormlike micelles and bilayered vesicles were obtained from samples with relatively high PE compositions. Chapter 3 demonstrates the precise synthesis of glucose-containing diblock terpolymers from a combination of anionic and reversible addition-fragmentation chain-transfer (RAFT) polymerizations. The resulting micelles exhibited excellent stability in several biologically-relevant media under in vitro conditions, including 100% fetal bovine serum. These particles may find applications as serum-stable nanocarriers of hydrophobic drugs for intravenous administration. Chapter 4 presents the development of novel cellulose derivatives as matrices in amorphous solid dispersions for improving the bioavailability of poorly water-soluble drugs in oral administration. Hydroxypropyl methylcellulose (HPMC) was modified with monosubstituted succinic anhydrides using facile anhydride chemistry, and the resulting materials simultaneously contained hydrophobic, hydrophilic, and pH-responsive moieties. Several HPMC esters of substituted succinates exhibited more effective crystallization inhibition of phenytoin under in vitro conditions than a commercial hydroxypropyl methylcellulose acetate succinate (HPMCAS). (341 words)
  • Thumbnail Image
    Item
    Complexation of DNA with Polycationic Micelles
    (2018-08) Jiang, Yaming
    Interpolyelectrolyte complexation is a ubiquitous phenomenon that plays vital roles in biological systems and in design of responsive materials. However, precise control of polyelectrolyte complexes (PEC) has been challenging, particular for DNA-polycation complexes designed for gene delivery applications. Incorporating polyionic micelles is a promising strategy to tune PEC properties, but has been under-utilized in designing polymeric gene delivery vehicles. Herein, cationic micelles self-assembled from amphiphilic block polymers are complexed with double stranded DNA. The structure, composition, and stability of the resulting "micelleplexes" are characterized to probe the fundamental physics that govern the formation and properties of micelleplexes. With cationic AB+ micelles, complexation of linear semiflexible DNA and flexible poly(styrenesulfonate) were compared and the influence of polyanion chain flexibility was extracted and discussed. DNA length was found to strongly influence the size, composition, and colloidal stability of micelleplexes, whereas DNA topology (linear or circular supercoiled) has minimal influence. To improve the colloidal stability and reduce the size of micelleplexes that are composed of multiple micelles connected by bridging DNA chains, AB+C micelles with hydrophilic nonionic outer coronas of varying length were designed. The addition of the outer nonionic corona dramatically improves the colloidal stability of micelleplexes over a much wider charge ratio, and the outer corona length closely correlates to micelleplex size, zeta potential, and the average number of micelles per micelleplex. In addition, AB+C micelleplexes adopt a beads-on-a-string structure that resembles the organization of DNA in chromatin. Lastly, structure, composition, and stability of micelleplexes were closely compared with those of another typically studied family of DNA complexes, "polyplexes", which form between DNA and cationic homopolymers or AB+ diblock copolymers with a hydrophilic nonionic A block. Compared to the polyplexes, micelleplexes showed more than a 4-fold increase in gene transfection efficiency, which was attributed to the high positive charge content of micelleplexes.
  • Thumbnail Image
    Item
    Data for Boundary Frustration in Double-Gyroid Thin Films
    (2024-02-29) Magruder, Benjamin R; Morse, David C; Ellison, Christopher J; Dorfman, Kevin D; dorfman@umn.edu; Dorfman, Kevin D; Dorfman Group, UMN CEMS
    We have used self-consistent field theory to predict the morphology and preferred orientation of the double-gyroid phase in thin films of AB diblock polymers. A manuscript has been submitted containing this data, and is expected to appear shortly. The data were generated using the C++ version of the open-source software PSCF (https://pscf.cems.umn.edu/). All input and output files from PSCF used to generate the data in the paper are included in this dataset, as well as the code used to process the data and generate the figures.
  • No Thumbnail Available
    Item
    Data for Superlattice by charged block copolymer self-assembly
    (2019-04-09) Shim, Jimin; Bates, Frank S; Lodge, Timothy P; lodge@umn.edu; Lodge, Timothy P; Bates Research Group; Lodge Research Group
    These files contain data along with the associated output from instrumentation supporting all results reported in "Superlattice by charged block copolymer self-assembly" by Shim et. al. We report the discovery of an intriguing superlattice morphology from compositionally symmetric charged block copolymers, poly[(oligo(ethylene glycol) methyl ether methacrylate–co–oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]–b–polystyrene (POEGMA–PS). These materials are conveniently prepared by sequential reversible addition–fragmentation chain transfer (RAFT) polymerization, followed by introduction of charged groups, in a manner that allows for systematic variation of the molecular structure in general, and the charge content in particular. POEGMA–PS self-assembles into a superlattice lamellar morphology, a previously unknown class of diblock nanostructures, but strikingly similar to oxygen-deficient perovskite derivatives, when the fraction of charged groups in the POEGMA block is about 5–25%. The charge fraction in the POEGMA block, and the tethering of the ionic groups, both play critical roles in driving the formation of the superlattice. This study highlights the accessibility of novel morphologies by introducing charges in a controlled manner.
  • Thumbnail Image
    Item
    Manipulating colloids and surfactants as co-templates for porous nanostructures and nanocomposites.
    (2010-02) Li, Fan
    Templating is a general and efficient strategy for creating nanostructured, particularly nanoporous materials. Two commonly employed classes of templates are colloidal crystals and surfactants. Colloidal crystals typically have an opal-like structure and have been used to produce macroporous (>50 nm pores) solids; surfactants generate various mesoporous structures (2−50 nm pores) as a result of their versatile phase behavior. One aim of this study is to combine colloidal crystals and surfactants to realize simultaneous templating at two length scales. A series of hierarchically structured porous silica samples were synthesized under different synthetic conditions, comprehensive TEM characterization was conducted to reveal the detailed hierarchical porous structures, and simulation was performed to correlate the structures to the surfactant phase behavior within the colloidal crystal confinement. The dual templating approach was further extended to synthesize functional materials with composite porous architectures, in which functional cores were embedded in a hierarchically porous framework for optical ionsensing application. A second aim of this study is to develop a template-based strategy for sculpting nanoparticles of desired shapes and sizes. Owing to the ordered structure and symmetry of the template, a templating-disassembly process was found to produce uniform, nanometer-level, multipodal particles. This method is applicable to a variety of compositions, including oxides, phosphates and carbon, and it could further lead in-situ organization of particles following a self-reassembly process. In addition, through a coupled passivation-disassembly process, site-specific functionalization was achieved to modify only the tips of the multipods with a range of functional groups, and therefore to enable their directional bonding to other colloidal particles. (256 words)
  • Thumbnail Image
    Item
    Multiscale modeling and analysis of microtubule self-assembly dynamics
    (2014-08) Castle, Brian Thomas
    Microtubules are dynamic biopolymers that self-assemble from individual subunits of αβ-tubulin. Self-assembly dynamics are characterized by stochastic switching between extended phases of growth and shortening, termed dynamic instability. Cellular processes, including the chromosome segregation during mitosis and the proper partitioning of intracellular proteins, are dependent on the dynamic nature of microtubule assembly, which facilitates rapid reorganization and efficient exploration of cellular volume. Microtubule-targeting chemotherapeutic agents, used to treat a wide range of cancer types, bind directly to tubulin subunits and suppress dynamic instability, ultimately impeding the capacity to complete cellular processes. Microscale length changes observed during dynamic instability are the net-effect of the addition and loss of individual subunits, dictated by the interdimer molecular interactions. Therefore, a multiscale approach is necessary to extrapolate submolecular level effects of microtubule-targeting agents to dynamic instability. The work presented in this dissertation integrates multiscale computational modeling and experimental observations with the goal of better understanding the functional mechanisms of microtubule-targeting agents. First, we develop a computational model for the association and dissociation of tubulin subunits, in which the interdimer interaction potentials are specifically simulated. Simulation results indicate that the local polymer end structure sterically inhibits subunit association as much as an order of magnitude. Additionally, the model informs how microtubule-targeting agents could alter assembly dynamics through the properties of the interdimer interactions. Second, the mechanisms of kinetic stabilization by microtubule-targeting agents are tested and constrained by combining predictions from a computational model for microtubule self-assembly and experimental observations in mammalian cells. We find that assembly- and disassembly-promoting agents induce kinetic stabilization via separate mechanisms. One is a true kinetic stabilization, in which the kinetic rates of subunit addition and loss are reduced 10- to 100-fold, while the other is a pseudo-kinetic stabilization, dependent upon mass action of tubulin subunits between polymer and solution. Overall, this work advances our knowledge of the basic physical principles underlying multistranded polymer self-assembly and can inform the future design and development of more effective and tolerable microtubule-targeting drugs.
  • Thumbnail Image
    Item
    Self-assembled nanotube/nanoparticle biosensors.
    (2010-05) Lee, Dongjin
    The self-assembled carbon nanotube (CNT) and indium oxide nanoparticle (INP) multilayers are presented for the applications to electrochemical pH and biological sensing. The excellent electrochemical properties of the nanomaterial thin film made of layer-by-layer self assembly is exploited to design and fabricate sensors targeted for a facile and low-cost application. The pH-sensitive conductance of the self-assembled CNT/INP chemoresistor and ion-sensitive field-effect transistor (ISFET) is studied, and its shift mechanisms are elucidated. There are two primary factors influencing the conductance of the semiconducting nanomaterial thin film: the direct protonation/deprotonation and the proximal ion effect. The CNT chemoresistor experiences the conductance change due to the molecular protonation/deprotonation of carboxylic groups. The effect of proximal ions demonstrates conventional semiconductor theory, where the pH increase corresponds to negative shift in gate voltage resulting in a higher conductance in p-type CNTs. The additional silica nanoparticle (SNP) layer adjusts the pH-sensitive conductance behavior, particularly from nonlinear to linear response, which is beneficial to the implementation of pH sensors. Indeed, the electrochemical properties of nanomaterial thin film are tunable by exploiting a different type of the nanomaterial, surface chemistry, and structure. Glucose biosensors and immunosensors are designed and implemented based on the conductance shift mechanisms explored. The sensitivity of CNT chemoresistor and ISFET glucose sensors is 10.8 and 18-45 μA/mM, respectively, on a linear range of 0-10 mM with a response time of a few minutes. An INP chemoresistor sensor array is designed to address variant electrical properties of the nanomaterial films, allowing the statistical analysis of data with one-shot of sample delivery. The INP immunoglobulin G (IgG) ISFET sensor exhibits a resolution of 40 pg/ml, and the CNT conductometric H1N1 swine influenza virus (SIV) sensor demonstrates a detection limit of 180 viruses TCID50/ml with a specificity to non-SIVs. The nanomaterial thin film electrochemical transducers are proven to be a potent candidate for the next-generation of the chemical and biological sensors possessing a high sensitivity and resolution. Due to a facile implementation and operation, nanomaterial biosensors could be used for domestic and clinical diagnosis, point-of-care detection, and a sensing component in lab-on-a-chip systems.
  • Thumbnail Image
    Item
    Self-assembling Phosphoramidate Pronucleotides: Enzymatic Regulation and Application Towards Therapeutic Delivery
    (2019-08) West, Harrison
    Prior characterizations of the nucleoside phosphoramidate moiety have centered upon the ability of amine containing side-chains to mask the negative charge inherent to monophosphorylated nucleosides for the purpose of enhancing their passive movement across biological membranes. When used for the intracellular delivery of therapeutic nucleoside monophosphate analogs, these molecules are referred to as phosphoramidate “ProTides” and represent an important class of antiviral and anticancer prodrugs. The primary aim of this thesis to build upon these works and present the nucleoside phosphoramidate moiety as a multifunctional regulator of molecular self-assembly. Appendage of nucleoside phosphoramidates to molecules such as self-assembling peptides was shown to modulate the self-assembling properties of the molecules through alteration of the non-covalent interactions between individual monomers and nanostructure assemblies. Additionally, the nucleoside phosphoramidate moiety was found to impart enzyme responsive qualities. Histidine triad nucleotide binding proteins (Hints), an enzyme class that possesses phosphoramidase activity, were found to regulate the assembly of nucleoside phosphoramidate bearing nanostructures by inducing ionic interaction mediated crosslinking after enzymatic hydrolysis. Chemical modification of self-assembling peptides with nucleoside phosphoramidates bearing non-natural and therapeutic nucleosides was also achieved to effect the first ever demonstrated self-assembling phosphoramidate ProTides as one-component nanomedicines. The developed formulations are currently under investigation for localized delivery of cancer chemotherapeutic prodrugs
  • Thumbnail Image
    Item
    Self-assembly of block copolymers in thin films
    (2013-08) Kim, Sangwon
    The self-assembly of block copolymers in thin films has been a subject of recent studies from both academic and industrial perspectives. One of its potential applications is nanolithography; block copolymers can function as novel mask materials intended for fabrication of small features, not easily realizable by current optical lithography. This dissertation addresses several fundamental issues associated with thin-film block copolymers. The bulk and interfacial wetting properties of partially epoxidized poly(styrene-b-isoprene) diblock copolymers, denoted as PS-PEI, were studied while varying the degree of the chemical modification. The incorporation of the random copolymer architecture induced decoupling between the bulk and the thin-film thermodynamics. The tunable surface wetting, a consequence of the partial modification, permitted control over the orientation of the domains in thin films. The morphologies of thin-film block copolymers were investigated using two different boundary conditions that involve one neutral interface and one preferential interface. The neutralities at the free surface and the underlying substrate were attained independently by the partial epoxidation in PS-PEI and the composition adjustment of random copolymer mats, respectively. For both boundary conditions, thin-film block copolymers formed an island/hole motif, characterized by 0.5 L0 step heights (L0: bulk lamellar periodicity). The thin-film behavior of PS-PEI block copolymers with random copolymer architecture was examined as the segregation strength (χN) was adjusted systematically across the order-disorder transition. Unlike in the bulk, the random copolymer architecture did not generate abnormal behavior in thin-film thermodynamics compared to plain linear architecture. With decreasing segregation strength, the thin-film system exhibited fluctuation-pervaded morphologies prior to reaching a disordered state. An agreement was found between the order-disorder transition temperatures in three dimensions (bulk) and in two dimensions (thin film). Lastly, the bulk properties and the thin-film structures of lamellae-forming poly(styrene-b-isoprene-b-methyl methacrylate) (SIM) triblock copolymers were studied. The thin-film morphology exhibited the dependence on the size of the poly(isoprene) (PI) middle block. While perpendicular lamellae were observed for the thin-film SIM block copolymer with a small PI volume fraction, complex behavior was observed for the sample with a large PI volume fraction.
  • Thumbnail Image
    Item
    Self-assembly of Block Polymers: Self-Consistent Field Theory and Monte-Carlo Simulations
    (2018-05) Arora, Akash
    Block polymers are a class of soft materials that self-assemble at mesoscopic length scales to form a wide variety of ordered structures. The resulting nanostructures have been instrumental in the development of several advanced technologies such as separation membranes and photonic crystals. This thesis focuses on three diverse problems that use self-consistent field theory (SCFT) and Monte Carlo simulations to study the fundamental phase behavior of block polymers. In recent years, several experimental studies have witnessed the formation of complex low-symmetry structures, commonly referred to as Frank–Kasper phases, contain- ing particles of disparate sizes arranged in multiple coordination environments. The first problem of this thesis focuses on examining the stability of various Frank–Kasper phases in AB diblock copolymers. Using SCFT, we computed the free energies of a host of Frank–Kasper phases and observed that the associated free energies differ only marginally (10−3kBT), leading to a rugged free energy surface with many local minima that may be accessible via different nucleation pathways. We have highlighted the significance of these theoretical predictions in the context of two new Frank–Kasper phases discovered experimentally in poly(isoprene)-b-poly(lactide) diblock copolymers. During the course of this project, we have also made a few advancements in the numerical framework of SCFT. Specifically, we have developed a physically informed and robust approach that uses information from experiments to create guess structures of the ordered phases that are required to perform the SCFT calculations. Additionally, we have developed an improved version of the Anderson-mixing iteration algorithm that increases the computational efficiency by at least 5-10 times compared to the previous version. The second problem focuses on studying the phase behavior of three different multiblock polymers, ABC triblocks, ABCA tetrablocks, and ABAC tetrablocks. In each of the cases, we have performed extensive SCFT calculations and compared the resulting predictions with experimental results to further the understanding of experimentally observed morphologies. While investigating the phase behavior of multiblock polymers, we observed that an accurate temperature-dependence of all the involved Flory–Huggins χ parameters is crucial for making any reliable predictions using SCFT. In this context, we have studied the sensitivity of the phase behavior of a specific ABAC-type multiblock, poly(styrene)-b-poly(isoprene)-b-poly(styrene)-b-poly(ethylene oxide) tetrablock terpolymer, towards the set of required χ parameters (χIS,χSO,χIO). In the third problem, we have studied the order–disorder transition of short lamellae- forming diblock copolymers using Monte Carlo simulations. We have developed a systematic approach to accurately estimate the domain spacing of the lamellar structure, and thereby remove the incommensurability and finite-size effects in lattice simulations. This enabled us to precisely determine the order–disorder transition value for short symmetric diblock copolymers.
  • Thumbnail Image
    Item
    Structure and Chain Exchange Kinetics of Block Copolymer Micelles in Selective Solvents
    (2017-08) Ma, Yuanchi
    Block copolymers can self-assemble into various structures, such as micelles and vesicles. Previous studies have shown that single chain exchange is the main mechanism for block copolymer micelles to achieve equilibrium. In this study, a new lower critical micelle temperature (LCMT) system, poly(methyl methacrylate)-block-poly(n-butyl methacrylate) in two room temperature ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was developed, and its chain exchange kinetics were investigated using time-resolved small-angle neutron scattering (TR-SANS). In order to probe the effect of the core block length, the corona block length and the solvent selectivity on the chain exchange rate, we synthesized two series of protonated and deuterated copolymers, one with identical core block length and one with identical corona block length, as well as systematically varied the Flory-Huggins interaction parameter χ by tuning the ratio of the two ionic liquids in the solvent. Notably, the results show that the solvent selectivity has a remarkable effect on the chain exchange rate, and therefore we proposed a more elaborate function of χ for the energy barrier of chain expulsion, which is rationalized by a calculation in the spirit of Flory−Huggins theory. Besides the kinetic study, complementary dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) experiments were also conducted to investigate the structure of micelles. Particular emphasis was placed on elucidating the scaling relationship between the micelle core radii and the degree of polymerization of the core block in the copolymers.
  • Thumbnail Image
    Item
    Structure and Dynamics of Compositionally Asymmetric Diblock Copolymers
    (2018-09) Lewis, Ronald
    Diblock copolymers are among the simplest amphiphilic molecules, and thus provide a model platform for understanding self-assembly in soft matter. The research presented in this work is broadly focused on the interplay between structure and dynamics in particle-forming diblock copolymer melts, motivated by a recent rise in the number of reports describing complex phase formation in these materials. Analogous complex, low-symmetry structures have been observed in hard materials, such as metals and metal alloys, pointing to the existence of underlying universalities within condensed matter physics. In this work, thermal processing methods commonly employed on hard materials are applied to short, compositionally asymmetric poly(1,4-isoprene)-block-poly(±-lactide) (IL) diblock copolymers. Two disordered IL samples exhibiting characteristic spherical micelle fluctuations above the order-disorder transition (ODT) were quenched in liquid nitrogen and reheated to target temperatures. This processing method lead to the formation of unconventional, low symmetry phases that were not otherwise formed by direct cooling from the disordered state, bearing similarities to metallurgy. However, unlike metals, the ordered states below the ODT imprinted particle densities onto the samples that persisted in the disordered state. This remarkable feature is a manifestation of the fluctuating disordered fluid in self-assembling soft materials. A recent report showed that conformational asymmetry in diblock copolymers, or the difference in space-filling capability between each block, is a key factor in complex phase formation. However, the most important parameter in polymer physics is the length of the polymer chain, which in a diblock copolymer may be represented by the invariant block degree of polymerization N ̅_b. In this work, the role of chain length on complex phase formation is investigated by probing the behavior of asymmetric poly(styrene)-block-poly(1,4-butadiene) (SB; N ̅_b ≈ 80) diblock copolymers. This system was devoid of complex phases, in contrast to previous results for short asymmetric poly(ethylethylene)-block-poly(±-lactide) (EL; N ̅_b ≈ 800) diblock copolymers with approximately the same conformational asymmetry. Differences in phase behavior associated with packing and entanglement theory and resulted in calculation of a universal crossover parameter, N ̅_x ≈ 400. In the case of SB, where N ̅_b > N ̅_x, asymmetry in space-filling capabilities are less important and the system exhibits phase behavior analogous to mean-field predictions. Conversely, the N ̅_b < N ̅_x regime places emphasis on conformational asymmetry and presumably other molecular factors that stabilize complex structures. In a diblock copolymer system, a dynamic constraint is imposed upon complex phase formation as mass (chain) exchange between particles is required to accommodate the multiple discrete particle shapes and sizes comprising these structures. In this work, the dynamics associated with particles below the ODT is investigated using dynamic mechanical spectroscopy (DMS) and X-ray photon correlation spectroscopy (XPCS). In the supercooled liquid prior to ordering, DMS and XPCS measurements conducted on a BCC-forming SB diblock copolymer revealed that particle dynamics are dependent on the ergodicity temperature, above which particle rearrangements are mediated by ergodic chain dynamics and below which non-ergodic ‘frozen’ particle motion becomes dominant. Additionally, a new analytical framework was developed to investigate the time evolution of particle dynamics via XPCS, which uncovered a wealth of dynamic features including time-resolved relaxation time and speed distributions associated with particles in grains.
  • Thumbnail Image
    Item
    Thermodynamics of Micellar Lyotropic Liquid Crystals
    (2020-06) Jayaraman, Ashish
    Aqueous lyotropic liquid crystals (LLCs) comprise a class of ordered morphologies formed by self-assembly of amphiphiles in water. LLCs assume a variety of concentration- and temperature-dependent structures including lamellae (bilayers), bicontinuous networks, hexagonally-packed cylindrical micelles, and spherical micelles packed on a lattice. Typically, LLC sphere packings include high-symmetry body-centered cubic (BCC), face-centered cubic (FCC), and hexagonally closest-packed (HCP) structures. Recently, a giant tetragonal σ phase containing 30 micelles of five different sizes was discovered in the aqueous LLC self-assembly of dianionic alkylphosphonate surfactants. The σ phase belongs to a class of tetrahedrally close-packed structures called Frank-Kasper (FK) phases, which possess ≥ 7 particles of two or more types situated at 12-, 14-, 15-, or 16-fold coordination environments in low-symmetry unit cells. Ubiquitous in intermetallic alloys, FK phases have been recapitulated in other soft materials including dendritic thermotropic liquid crystals, giant-shape surfactants, and block polymers. The observation of these complex morphologies across different soft material classes stabilized by varying non-covalent interactions begs the question of universality in the principles that govern FK phase formation. The formation of the σ phase in LLCs of ionic surfactants was rationalized based on maximizing counterion-mediated intermicellar cohesion, while minimizing expensive local variations in headgroup-counterion solvation. However, molecular design principles guiding FK phase selection in LLCs are lacking. FK phases are periodic approximants of dodecagonal quasicrystals (DDQCs), structures which possess 12-fold rotational symmetry yet lack translational symmetry. DDQCs have been observed in self-assembled micelles of neat, neutral amphiphiles in regions of phase space adjacent to FK morphologies. However, quasiperiodic ordering of micelles in LLC self-assembly is surprisingly unknown given the pervasiveness of the periodic approximants. This thesis elucidates the amphiphile structural motifs that stabilize FK phases and related DDQCs in aqueous LLCs. We first establish the molecular design criteria for the formation of σ phases in ionic amphiphiles by investigating the LLC phase behavior of alkylmalonate dianionic surfactant analogous to the alkylphosphonate amphiphiles. FK phase formation was observed to depend on the nature of the counterions and length of the alkyl tail. Using real-space electron density reconstructions, we find that the preference for local micellar symmetry in the σ phase is dictated by the extent of headgroup-counterion association. We next report the formation of a well-ordered DDQC in oil-swollen micelles of alkylphosphonate surfactants, and we use high-resolution small-angle X-ray scattering data to determine the space group symmetry of this quasiperiodic structure. The formation of the DDQC was contingent on the sample-processing protocols employed, indicating the metastability of this mesophase. We further illustrate the non-specific nature of FK phase formation in soft materials by the discovery of a σ phase on self-assembly of hydrated non-ionic polyethylene-block-poly(ethylene oxide) surfactants. For the hydroxyl terminated surfactant, access to the σ phase depends on sample thermal history, indicating its metastability with respect to the A15 structure. Finally, the hydroxyl end-group of the amphiphile was synthetically modified with ionic and strongly H-bonding moieties. We find that strongly interacting terminal groups provide increased temperature- and composition-windows of σ phase stability. Moreover, cationically-terminated oligomers surprisingly self-assembled into a DDQC. These findings are rationalized based on the drive to minimize local variations in intramicellar chain-chain interactions, while maximizing intermicellar cohesion. These fundamental studies of the thermodynamics of micellar morphologies in solvated amphiphiles provide insights into the general underlying principles which stabilize these complex packings of soft reconfigurable particles.
  • Thumbnail Image
    Item
    Three-dimensional self-assembled photonic crystal waveguide.
    (2010-12) Baek, Kang-Hyun
    Photonic crystals (PCs), two- or three-dimensionally periodic, artificial, and dielectric structures, have a specific forbidden band for electromagnetic waves, referred to as photonic bandgap (PBG). The PBG is analogous to the electronic bandgap in natural crystal structures with periodic atomic arrangement. A well-defined and embedded planar, line, or point defect within the PCs causes a break in its structural periodicity, and introduces a state in the PBG for light localization. It offers various applications in integrated optics and photonics including optical filters, sharp bending light guides and very low threshold lasers. Using nanofabrication processes, PCs of the 2-D slab-type and 3-D layer-by-layer structures have been investigated widely. Alternatively, simple and low-cost self-assembled PCs with full 3-D PBG, inverse opals, have been suggested. A template with face centered cubic closed packed structure, opal, may initially be built by self-assembly of colloidal spheres, and is selectively removed after infiltrating high refractive index materials into the interstitials of spheres. In this dissertation, the optical waveguides utilizing the 3-D self-assembled PCs are discussed. The waveguides were fabricated by microfabrication technology. For high-quality colloidal silica spheres and PCs, reliable synthesis, self-assembly, and characterization techniques were developed. Its theoretical and experimental demonstrations are provided and correlated. They suggest that the self-assembled PCs with PBG are feasible for the applications in integrated optics and photonics.