Browsing by Author "Keith E. Gubbins, Committee Member"

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    Computer Simulation of the Formation of Mechanically-Assembled Monolayers and Heteropolymers with Adjustable Monomer Sequences
    (2009-12-07) Strickland, Lawrence Anderson; Carol K. Hall, Committee Chair; Jan Genzer, Committee Co-Chair; Orlin D. Velev, Committee Member; Keith E. Gubbins, Committee Member; Maurice Balik, Committee Member
    This thesis describes a computational investigation, using discontinuous molecular dynamics simulation, of the formation and properties of two types of self assembled polymer structures: mechanically-assembled monolayers (MAMs) and heteropolymers with adjustable monomer sequences (HAMs). MAMs are described in part 1 and HAMS are described in part 2. MAMs in good solvent were created by grafting polymers composed of 5 to 100 hard-sphere monomers to surfaces at low density and then compressing the surface laterally at varying rates. Data for brush thickness and end-monomer density were collected as a function of surface density; this data corresponded well with theoretical predictions and simulation results performed by others. Brush thickness for all chain lengths could be controlled by judicious choice of the compression rate. Defects in the brush layer depended on chain length; we showed that quick compression for short chains allowed the layer no time to relax into coil form. Quick compression for long chains increased entanglement and allowed the chains no time to form a fully-relaxed brush. After compressing the systems to high surface densities, the brushes were allowed to relax to a lower surface density. It was shown that higher compression/relaxation rates led to an increase in disparity between the brush thicknesses found during the compression and relaxation stages; this disparity was largely due to inadequate equilibration time. Last, compressing non-uniformly in the x- and y-directions showed negligible effects on monolayer height and structure. We also investigated the effect of poor-solvent conditions on MAM formation. Square-well chains composed of 20 to 100 units end-grafted to a hard surface at low density were compressed laterally at varying rates. Brush thickness depended on the interplay between solvent quality and the substrate compression rate. Brushes formed in poor solvents at fast compression rates were thinner and exhibited more heterogeneity in coverage than brushes formed in good solvents at slow compression rates. End-monomer trapping increased with increasing compression rate and/or decreasing solvent quality. By varying relaxation/compression rate, we could modify the effects of brush thickness hysteresis. Finally, we suggested a general blueprint for efficient formation of defect-free monolayers. We investigated the formation of heteropolymers with adjustable monomer sequences (HAMs) by simulating a “coloring†reaction performed on A-type homopolymers of length ranging from 100 to 300 units. The transformation of selected A-type monomers to B-type monomers along the macromolecule led to A1-x-co-Bx random copolymers, where x is the mole fraction of B. We showed that for a fixed A-B interaction, the distribution of A and B units in A1-x-co-Bx could be tuned by adjusting both the degree of “coloring†and the solubility of the A and B segments with respect to the implicit solvent. In general, increasing the solubility of the A-type homopolymer or the degree of coloring led to a decrease in blockiness in the co-monomer distribution. Decreasing the solubility of the B species increased the blockiness of the final A1 xBx copolymer. Last, we investigated the effect of tethering the polymers to an impenetrable, surface during the coloring process. Polymers of length 50 to 300 were tethered at various surface densities and then “colored†over a range of temperature and monomer solubilities, resulting in A1-x-co-Bx HAMs. We showed that while blockiness can be adjusted over a small range by varying temperature and solubility, we could significantly affect blockiness by varying chain length and surface density. We showed that for systems of long chains in poor solvents, blocky copolymers formed due to chain-chain overlap; in good solvents, blocky copolymers formed due to chain extension. We observed that for a given change in surface density, blockiness increased more for systems of long chains than for shorter chains.
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    Computer Simulation Studies of Self-Assembly of Dipolar and Quadrupolar Colloid Particles
    (2009-03-24) Goyal, Amit; Carol K. Hall, Committee Chair; Keith E. Gubbins, Committee Member; Orlin D. Velev, Committee Co-Chair; Donald W. Brenner, Committee Member
    Colloidal particles with directional interactions that self-assemble into pre-defined microstructures have the potential to serve as the foundation for a new generation of micro- and nano-structures of remarkable complexity and precision. Dipolar colloid particles tend to align end-to-end and self-assemble into variety of micro- and nano-structures ranging from co-crystals of novel symmetry, to open networks (gels) of cross-linked chains of particles. Quadrupolar colloid particles also tend to self-assemble in a wide variety of structural motifs including sheets, tubes and shells depending upon external conditions. We use molecular dynamics computer simulation to explore the self-assembly, structure, crystallization and/or gelation of systems of colloid particles with permanent dipole moments or quadrupole moments immersed in a high-dielectric solvent. Particle-particle interactions are modeled with discontinuous potentials in order to take advantage of discontinuous molecular dynamics (DMD), a fast simulation technique that is very computationally efficient. We first calculate the phase diagram of monodisperse system of dipolar colloid particles using DMD in the temperature-packing fraction plane. Several types of phases are found in our simulations: ordered phases including face centered cubic (FCC), hexagonal close packing (HCP) and body centered tetragonal (BCT) at high packing fractions, and fluid, string-fluid and gel phases at low packing fractions. The very low volume fraction gel phases and the well ordered crystal phases are promising for advanced materials applications. We then examine how the phase diagram changes upon varying the sizes of the two species as well as their dipole moments. The phase diagrams of an equimolar binary mixture of dipolar colloid particles with different diameter ratios and different dipole moment ratios are calculated in the temperature-volume fraction plane. These systems exhibit six distinct phases: isotropic fluid, string-fluid, gel, FCC, HCP, BCT, and ten coexisting phases: Fluida + String-fluidb, Fluida + Gelb, String- Fluida + Gelb, Gela + BCTb, FCCa + FCCb, FCCa + HCPb, FCCa + FCCb+ Fluid, HCPa + HCPb, BCTa + BCTb, BCTa + BCTb + large voids depending upon size ratio and dipole moment ratio. An interesting aspect of these phase diagrams is the appearance of co-crystals containing large and small dipolar colloid particles at size ratio equal to 0.414. Even more interesting is the appearance of two unique bicontinuous gel structures - the first gel consists of two independent, but interpenetrating, networks of cross-linked chains formed by particles with high dipole moment and chains formed by particles with low dipole moment. The second type of gel consists of network of cross-linked chains formed by particles with high dipole moment, while particles with low dipole moment form a sheath around the chains. Such bicontinuous gels may have unusual rheological and transport properties such as multiple yield stresses and multiple percolation temperatures and could form the basis of new classes of soft-solid materials with unique properties and multiple applications. We also explore the structure formation of systems of colloid particles with permanent quadrupole moment. We introduce simple quadrupole-quadrupole discontinuous potential model that give rise to the self-organization of random surface (membrane), tubular (nanotubes) structures. We find that the discrete rotational symmetry of the quadrupolar particles give rise to extended two-dimensional random sheet or surface structures that preserve the local symmetry within the organized structure. A new type of anisotropic colloid particle is introduced having displaced quadrupole moment of a unique symmetry that leads to the formation of tubular structure. The precise diameter and length of the tubes can be controlled by tuning the interactions and temperature. Our simulations predict the optimal conditions for making the tubes of precise diameter and length using quadrupolar colloidal particles which may be the route to the formation of high quality nanotubes.
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    A Fundamental Study of the Molecular Structure, Interactions and Self-Organization of 1,3:2,4-Dibenzylidene-D-Sorbitol
    (2003-04-16) Wilder, Elizabeth A; Keith E. Gubbins, Committee Member; Saad A. Khan, Committee Member; Richard J. Spontak, Committee Chair; Carol K. Hall, Committee Co-Chair
    1,3:2,4-Dibenzylidene-D-sorbitol (DBS) is a relatively low-molecular-weight amphiphile that is capable of self-organizing into nanoscopic fibrils. At sufficiently high DBS concentrations, these fibrils assemble into a nanofibrillar network in a wide variety of organic solvents and polymer melts to produce "organogels." DBS has been shown to induce physical gelation at surprisingly low concentrations (< 1 wt%), making it ideal for applications requiring uncompromised physical or chemical properties of the matrix medium. Contemporary applications of DBS include personal cosmetics, biomedical materials, and (opto)electronic devices. Despite the many and diverse uses of DBS in existing, as well as emerging, technologies, a comprehensive study addressing the molecular structure, intermolecular interactions, nanofibrillar morphology and macroscopic properties of DBS-containing systems remains lacking. In this work, we seek to elucidate the molecular interactions governing DBS self-assembly, the impact of molecular structure on resultant nanofibrillar morphology, and the effect of this nanostructure on macroscopic mechanical properties. Molecular mechanics calculations performed with Cerius2 and InsightII software reveal two important features of the DBS molecule: (i) the pendant hydroxyl group tends to form intramolecular hydrogen bonds, and (ii) the phenyl rings prefer to lie in an equatorial position. The terminal hydroxyl group, however, possesses tremendous flexibility, indicating that it may be able to participate in intermolecular interactions. Molecular self-organization of DBS molecules, as discerned from both molecular mechanics calculations and molecular dynamics simulations of dimers, is sensitive to hydrogen bonding of the hydroxyl groups and pi interactions between phenyl rings, suggesting that the mechanism of network formation is complex, involving more than one type of local interaction. Transmission electron microscopy of organogels composed of poly(ethylene glycol) (PEG) and DBS reveals that DBS nanofibrils measure from about 10 to 70 nm in diameter, with a primary nanofibrillar diameter closer to 10 nm. Dynamic rheological measurements of DBS-containing PEG and PEG derivatives differing in endgroup substitution and, hence, polarity exhibit several interesting features. The rate of gelation, the gel dissolution/formation temperatures, and the magnitude of the dynamic elastic modulus are all sensitive to both DBS concentration and matrix polarity. Hydroxy-endcapped PEG/DBS systems require more time to gel and dissolve faster than their methoxy-endcapped analogs at constant DBS concentration. The elastic modulus, however, is less dependent on matrix polarity. Time-temperature superposition analyses provide evidence that the activation energy of gelation increases linearly with: (i) decreasing DBS concentration at constant matrix polarity and (ii) increasing matrix polarity at constant DBS concentration. Addition of DBS to a series of amphiphilic polypropylene glycol-b-polyethylene glycol-b-polypropylene glycol (PPG-b-PEG-b-PPG) triblock copolymers yields organogels with properties intermediate between those observed in PEG/DBS and PPG/DBS systems. Dynamic rheology reveals a maximum in the elastic modulus at temperatures near the gel dissolution and formation temperatures, both of which increase with increasing DBS concentration and PPG content. The magnitude of the elastic modulus is sensitive to copolymer composition and block length at low DBS concentration, but becomes matrix-independent as the DBS network saturates at a DBS concentration in excess of about 1 wt%. Transmission electron microscopy and microtomography of DBS networks in a nonpolar thermoplastic such as poly(ethyl methacrylate) reveal the existence of DBS nanofibrils measuring ca. 10 nm in diameter and ranging up to several hundred nanometers in length. At sufficiently high DBS concentration, these nanofibrils form a highly interconnected 3D network that can be altered through the further addition of a siliceous nanoparticle, such as colloidal silica. Dynamic mechanical property analysis reveals that, while DBS has little effect on glassy PEMA, it serves to increase, in systematic fashion, the elastic modulus of molten PEMA above the glass transition temperature.
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    Modeling and Computer Simulation of Block Copolymer/Nanoparticle Composites
    (2004-02-04) Schultz, Andrew Jeremy; Keith E. Gubbins, Committee Member; Richard J. Spontak, Committee Member; Jan Genzer, Committee Co-Chair; Carol K. Hall, Committee Chair
    Molecular dynamics computer simulation is used to explore the phase behavior and structural properties of block copolymers and block copolymer nanocomposites. Block copolymers microphase separate into ordered structures with domains on a nanometer length scale, which can then be used as a template for nanoparticles. This research provides insight into the fundamental physics that govern phase behavior and properties of these materials. We first focus on the case of neat diblock copolymers. We performed discontinuous molecular dynamics simulation to study the phase behavior of diblock copolymers modeled as chains of tangent hard spheres with square shoulder repulsions between unlike species as a function of chain length, volume fraction (f) and interaction strength (χ). The location of the order-disorder transition for a symmetric copolymer is close to the predictions of Fredrickson and Helfand. Our simulation results for packing fractions of 0.35, 0.40 and 0.45 and chain lengths 10 and 20 are summarized in phase diagrams which display disordered, lamellae, perforated lamellae, cylindrical and BCC spherical phases in the χN vs. f plane. These phase diagrams are consistent with phase diagrams from other simulation studies. Contrary to theoretical predictions we observe the perforated lamellar phase near regions of predicted gyroid stability, and the spherical phase only in the systems with high packing fraction and long chain length. These discrepancies may be due to the short chain lengths considered, as they are less evident in the 20-bead chains than the 10-bead chains. We examine the structural spacing of the microphases and the variation of that spacing with χN. We also examine the internal energy and entropy and their variation with χN. Our results are consistent with self-consistent field theory results for the strong segregation limit. We then extend our simulations to study the phase behavior and properties of diblock copolymer/nanoparticle composites. The nanoparticles are modeled as hard spheres with a square shoulder repulsion with one of the copolymer blocks. The resulting phase diagrams are presented for composites containing nanoparticles of various sizes and interaction strengths, and include lamellae, perforated lamellae, cylinders and disordered phases. Composites containing large nanoparticles also exhibit two-phase coexistence between different copolymer phases, or between a copolymer phase and a nanoparticle phase, depending upon the nanoparticle interaction strength. We also present concentration profiles perpendicular to the lamellar interface for nanoparticles of different sizes and interaction strengths. Neutral nanoparticles concentrate at the interface between copolymer domains while interacting nanoparticles concentrate within the favorable domain. The larger nanoparticles are more easily localized, but have less impact on the copolymer concentration profiles. The lamellar spacing increases with nanoparticle volume fraction for interacting nanoparticles, but decreases with nanoparticle size. The locations of the phase transitions are in qualitative agreement with theoretical predictions, but the concentration profiles are inconsistent with theoretical predictions. The variation of the spacing with nanoparticle volume fraction is consistent with experimental data.
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    Phase Equilibria of Diatomic Lennard-Jones Molecules Using Monte Carlo Simulation
    (2006-08-06) Galbraith, Aysa Lamia; Orlin Velev, Committee Member; Keith E. Gubbins, Committee Member; Carol K. Hall, Committee Chair; Peter K. Kilpatrick, Committee Member
    The overall aim of this research is to use computer simulations to study vapor-liquid and solid-liquid phase behavior of simple nonspherical molecules with a special focus on determining how the differences in the components' molecular size and intermolecular interactions affect the type of phase diagrams observed. We first calculate vapor-liquid phase diagrams for binary mixtures of diatomic Lennard-Jones molecules using Monte Carlo simulations and the Gibbs-Duhem integration method. We plot pressure versus composition vapor-liquid phase diagrams for the binary mixtures O₂-N₂, CO₂-C₂H₆ and N₂-C₂H₆ at different temperatures. We then add a quadrupole term to the two-center Lennard-Jones potential model and we observe that this further improves the agreement with experimental data. We also investigate the dependence of Henry's constant on the temperature, pressure and binary interaction parameter. We explore the effect of varying the molecular size ratio from σ₁₁/σ₂₂=1.0 to 1.40, intermolecular attraction ratio from ε₁₁/ε₂₂=0.70 to 1.20 and binary interaction parameter from δ₁₂=0.70 to 1.10 on the dumbbell mixture's phase behavior. We then examine the solid-liquid phase equilibria for systems containing pure Lennard-Jones dumbbell molecules and their mixtures. We begin by calculating the equations of state for systems containing C₂H₆, CO₂ and F₂, all of which are modeled by two-center Lennard-Jones potential for linear diatomic molecules. We then use the Frenkel-Ladd thermodynamic integration method to calculate the free energies. The equations of state and the free energies are used to obtain solid-liquid coexistence points which are needed to start the Gibbs-Duhem integration. The solid-liquid phase equilibria for pure and binary mixtures of Lennard-Jones dumbbells are predicted using the Gibbs-Duhem integration method. We use three model Lennard-Jones binary mixtures with varying bondlengths, σ₁₁/σ₂₂, ε₁₁/ε₂₂ ratios to calculate the solid-liquid phase diagram. Mixtures I, II and III show solid solution, azeotrope and eutectic phase diagrams, respectively. We then investigate the effects of molecular size and intermolecular attractions on the solid-liquid phase diagrams of binary Lennard-Jones mixtures using our three model mixtures.