Browsing by Author "Jan Genzer, Committee Co-Chair"

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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 Pattern Recognition in Biomimetic Polymers
    (2006-04-06) Jayaraman, Arthi; Keith Gubbins, Committee Member; Saad Khan, Committee Member; Carol Hall, Committee Chair; Jan Genzer, Committee Co-Chair
    The overall aim of this research has been to understand the molecular phenomena governing recognition in biological processes such as antibody-antigen binding, transmembrane signaling, viral-inhibition, etc. Specifically, we use computer simulations to study the thermodynamics of recognition of patterns in a copolymer bulk adsorbed on a heterogeneous surfaces, recognition of monomer sequences in copolymers adsorbed on heterogeneous surfaces, and recognition of nucleotide sequences in DNA microarrays. We first focus on the recognition of a bulk pattern in an AB diblock copolymer film adsorbed on a heterogeneous surface. We investigate how a pattern imposed in a copolymer film at a certain distance from the surface propagates through the film onto an adsorbing heterogeneous surface. We bias the copolymer film to adopt a specified target pattern and then use simulation to design a surface pattern that helps the adsorbed film to maintain that target pattern. We examine the effect of varying the copolymer chain length, the size of the target pattern, and the distance, $z'$, from the surface where the target pattern is applied on the extent of pattern transfer. At constant chain length, pattern transfer is best when the pattern size brings the energy of the system close to the energy when no pattern is applied. At constant pattern size, pattern transfer is best in the systems with longer chains because longer chains are more likely to adsorb as brushes and loops, which then helps transfer the pattern through the adsorbed film down to the surface. We extend our work to designing heterogeneous surfaces that can recognize and selectively adsorb copolymer sequences. In most of the theoretical work in the area of the pattern recognition by biomimetic polymers the question of how to design optimal surface patterns for recognizing specific monomer sequences in copolymers, has not been answered yet. We have developed a novel simulation method to design surfaces for recognizing specific monomer sequences in copolymers. We fix the sequence statistics of the AB copolymers and adsorb them on a surface containing two types of sites. We allow the simulation to iterate towards an optimal surface pattern that can "recognize" and selectively adsorb the copolymer sequence. For copolymers with less blocky sequences the designed surfaces recognize the correct sequence well when the segment-surface interactions dominate over the intersegment interactions. For copolymers with more blocky sequences recognition is good when the segment-surface interactions are only slightly stronger than the intersegment interactions. We further extend our study to the recognition of nucleotide sequences in DNA microarrays. DNA microarrays has been widely adopted by the scientific community for a variety of applications. In order to improve performance and to design next generation microarrays there is a need for a fundamental understanding of the interplay between the various factors that affect microarray performance. To gain such an understanding we study the thermodynamics and the kinetics of hybridization of single stranded "target" genes in solution with complementary "probe" DNA molecules immobilized on a microarray surface. Using Monte Carlo simulations and a coarse-grained lattice model we examine how various parameters affect the hybridization process. We observe that as the probe length decreases the specificity increases. Probes with segments that are complementary to the segments at either ends of the target have higher specificity than the probes complementary to the center portion of the target. Analysis of the hybridization kinetics reveals that the segments at the ends of the probe have a high probability of starting the hybridization and the segments towards the center of the probe remain bound to the target for a longer time.
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    Lateral Structuring and Stability Phenomena Induced by Block Copolymers and Core-Shell Nanogel Particles at Immiscible Polymer/Polymer Interfaces
    (2010-03-08) Gozen, Arif Omer; Jan Genzer, Committee Co-Chair; Richard J. Spontak, Committee Co-Chair; Orlin Velev, Committee Member; Peter Kilpatrick, Committee Member
    We have investigated the parameters such as copolymer/nanoparticle concentration, architecture and molecular weight combined with film thickness, time and temperature in order to develop a molecular-level insight on how lateral interfacial structuring occurs at immiscible polymer/polymer interfaces. In order to develop a molecular-level understanding of how these ‘smart’ self-assembling materials and core-shell nanogel particles interact both intra- and inter-molecularly and form ordered structures in bulk, as well as at immiscible interfaces, we first focused on the response of core-shell polymer nanoparticles, designated CSNGs, composed of a cross-linked divinylbenzene core and poly(methyl methacrylate) (PMMA) arms as they segregate from PMMA homopolymer. We have demonstrated that these nanogel particles exhibit autophobic character when dispersed in high molecular weight homopolymer matrices and segregate to the interface with another fluid. We have further explored the migration of these new-generation nanogel particles (CSNG-Rs) segregating from PS homopolymer to PS/PMMA interfaces. Unlike the instability patterns observed with the CSNGs, which exhibit classical nucleation and growth mechanism with circular hole formation, we have observed an intriguing dewetting pattern and CSNG-Rs forming lateral aggregates and tentacle-like structures at the interface. In parallel with our core-shell particle studies, we have also explored the structuring of copolymer molecules that are far from equilibrium in bulk and complex laminate of polymer thin films. Our early triblock copolymer studies have proven that molecular asymmetry has a profound effect on order-disorder transition temperature. We focused primarily on the effect of the copolymer chemical composition (i.e., block sizes) on the dewetting behavior of PS/SM thin films on PMMA. We elucidate the interfacial segregation and concurrent micellization of diblock copolymers in a dynamically evolving environment with changing boundary conditions as spherical holes develop. These studies reveal that in-plane interfacial nanostructures produced by block copolymers may not always provide stabilization of the bilayer; this behavior has been attributed to the interplay between copolymer micellization and copolymer segregation at the immiscible polymer interface. Lastly, we have investigated the dewetting behavior of PS/PMMA assemblies containing compositionally varied mixtures of mirrored copolymers, such as PS50-b-PMMA10 / PS10-b-PMMA50 and PS50-b-PMMA20 / PS20-b-PMMA50. The dewetting rates of systems composed of copolymer mixtures lie between those of systems modified with the neat copolymers. This observation suggests that the dewetting behavior of the double layer with a copolymer mixture may be approximated satisfactorily by a linear rule of mixtures.
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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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    Modification of Polymer/Polymer Interfaces using Block Copolymers and Microgels
    (2006-03-13) Wei, Bin; Jan Genzer, Committee Co-Chair; John van Zanten, Committee Member; Alan E. Tonelli, Committee Member; Richard J. Spontak, Committee Chair
    A guest macromolecular material, either a block copolymer (BCP) or core-shell microgel (MG) particles, has been used to stabilize a polystyrene (PS) film positioned atop an immiscible homopolymer substrate of poly(methyl methacrylate) (PMMA). Modification of the PS/PMMA interface due to interfacial partitioning of the guest macromolecule significantly increases the stability of the PS film by either slowing down or completely eliminating dewetting of the top PS layer. Our work has revealed that the dewetting mechanism of the top layer may change between nucleation and growth of holes and spinodal-like surface fluctuations, depending on the extent of interfacial heterogeneities induced by the BCP. Due to the shape retention of the MG, autophobicity is observed between the MG particles with a PS-like core and PMMA arms and a chemically identical long-chain PMMA homopolymer. This behavior is attributed to entropic exclusion of the PMMA matrix polymer from the PMMA arms of high graft density. We have demonstrated that autophobicity is strong enough to overcome the resistance of interfacial tension γAB so that the MG could be pushed from the PMMA matrix to the PS/PMMA interface, whereas the PMMA/MG surface remained free of MG, because the PMMA surface energy suppresses the surface roughening that accompanies autophobic segregation. Such MG-induced interfacial patterning in areas in contact with PS is completely reversible. Further annealing the PMMA/MG after the PS is removed permits the surface energy of PMMA to force the MG back into the PMMA substrate. Based on this reversible autophobic segregation, we have developed a simple stamping strategy of controlled and reversible patterning of MG particles on the film surface. A patterned poly(dimethyl siloxane) (PDMS) film with periodic ridges and valleys can be utilized as a stamp to control the in-plane distribution of the segregating MG particles. The MG blended in the PMMA/MG film migrates to the surface only in areas in contact with the PDMS. Further annealing with the stamp removed causes the film surface to smoothen due solely to surface tension.
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    Polymerization in Confined Geometries
    (2006-08-09) Petrie, Randall James; Jan Genzer, Committee Co-Chair; Chris Gorman, Committee Co-Chair; Kenneth Hanck, Committee Member; Edmond Bowden, Committee Member
    The work presented in this PhD thesis is centered on nanometer-sized pores. Two detailed objectives include: 1) investigating the confinement effect on "grafting from" polymerization carried out directly inside the pore, and 2) using porous silicon as a novel platform for controlled motion of liquid drops moving along wettability gradients created on the pores. In chapter 2 we investigate the confinement effect of the pore (< 50 nm) on the polymerization of poly methyl methacryrlate (PMMA) in porous silicon. Porous silicon has the unique quality of acting as a replacement for the conventional organic matrix used in matrix assited laser desorption ionization (MALDI). To this end, porous silicon not only acts as the substrate in which the polymerization takes place, but also serves as an in situ platform for the direct molecular weight analysis of the pore-grown polymer. We also report on the efficiency of porous silicon to produce MALDI spectra of PMMA as compared to MALDI spectra obtained using a conventional organic matrix. Chapter 3 focuses upon the use of an alternative substrate, anodic aluminum oxide (AAO), for the in-pore polymerization of PMMA. AAO is attractive for its homogeneous pore distribution and commercial availability. Although AAO does not serve as an organic matrix replacement for MALDI like porous silicon, a procedure for the ex situ characterization of the pore grown PMMA via MALDI is discussed. In chapter 4 we report on the motion of water droplets on porous and flat silicon surfaces decorated with molecular gradients comprising semifluorinated (SF) organosilanes. SF molecular gradients deposited on flat silica substrates facilitate faster motion of water droplets relative to the specimens covered with an analogous hydrocarbon gradient. Further increase in the drop speed is achieved by advancing it along porous substrates coated with the SF wettability gradients. The results of our experiments are in quantitative agreement with a simple scaling theory that describes the faster liquid motion in terms of reduced friction at the liquid/substrate interface.
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    Self-Assembled Thin Films: Peptides in Hybrid Bilayers and Mixed Organosilanes on Silica
    (2008-01-19) Smith, Matthew Brian; Robert M. Kelly, Committee Member; Jan Genzer, Committee Co-Chair; Peter K. Kilpatrick, Committee Chair; Peter S. Fedkiw, Committee Member; John H. van Zanten, Committee Member