Browsing by Author "Jan Genzer, Committee Chair"

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Controlling Surface Properties of Materials by Small Molecule and Polymer Grafts.
(2010-11-30) Turgman Cohen, Salomon; Jan Genzer, Committee Chair; Peter Kilpatrick, Committee Chair; Stefan Franzen, Committee Member; Orlin Velev, Committee Member
Design & Synthesis of Silicone Elastomer Networks with Tunable Physico-Chemical Characteristics
(2008-07-25) Willoughby, Julie Ann-Crowe; Orlando Rojas, Committee Member; Joe DeSimone, Committee Member; Saad Khan, Committee Member; Jan Genzer, Committee Chair
Design and Interfacial Activity of Copolymers with Controlled Monomer Sequence Distributions
(2004-01-13) Semler, James Joseph; Jan Genzer, Committee Chair; Carol Hall, Committee Member; Harold Ade, Committee Member; Peter Kilpatrick, Committee Member; Richard Spontak, Committee Member
We study the bulk and interfacial behavior of A-B copolymers. Emphasis is placed upon addressing the role of the monomer sequence distribution in A-B copolymers as it pertains to the copolymer's mobility in confined geometries and its ability to recognize chemical patterns on surfaces. Monte Carlo simulations are used to study the ability of block (A-b-B) and alternating (A-alt-B) copolymers to recognize chemical patterns on flat, impenetrable surfaces comprising two distinct chemical sites, C and D. The copolymer adsorption is driven by the repulsion between A and B segments along the copolymer chain and the attraction between B segments and D sites on the surface. The principle parameters that govern the ability of A-b-B and A-alt-B copolymers to recognize surface patterns are: the strength of the interaction between B segments and D surface sites, the A-B monomer sequence distribution, and the size and spatial distribution of adsorbing D sites. Our simulations reveal that both A-b-B and A-alt-B copolymers are capable of recognizing surface patterns and increasing the B-D attraction enhances the partitioning of A and B segments at the surface. Commensurability between the copolymer's monomer sequence distribution and the size and spatial distribution of the surface heterogeneities is also found to affect the ability of A-b-B and A-alt-B copolymers to recognize surface chemical patterns. When the adsorbing domain size exceeds the size of the copolymer's parallel component to the radius of gyration, A-b-B copolymers are found to transfer the surface pattern into the bulk with high fidelity. A-alt-B copolymers, however, are able to replicate the surface pattern into the bulk material when heterogeneous domain sizes are much smaller. We introduce a novel 'coloring' scheme to synthesize polystyrene-polybromostyrene (PS-co-PBrS) copolymers with statistically random (r-(PS-co-PBrS)) and random-blocky (b-(PS-co-PBrS)) monomer sequence distributions. Our results show that r-(PS-co-PBrS) and b-(PS-co-PBrS) copolymers with equivalent bromine content possess different intrinsic viscosities and radii of gyration. We attribute this behavior to the ability of b-(PS-co-PBrS) coils to form globular structures in toluene where PBrS forms a dense core and PS remains predominantly in a loose corona. This behavior is in contrast to that of r (PS co-PBrS) coils where both the PBrS and PS are homogeneously distributed. The interfacial behavior of the random and blocky copolymers is also found to differ. Specifically, thin films of r-(PS-co-PBrS) deposited on top of flat silica substrates covered with a semifluorinated self-assembled monolayer are found to dewet at a faster rate than b-(PS-co-PBrS) of comparable thickness at the same T−Tg, where Tg is the bulk glass transition temperature of the PS-co-PBrS copolymer. To our knowledge, this is the first experimental evidence that supports claims from computational studies arguing that the sequence distribution of random copolymers affects the chain's mobility on a surface. Molecular insights into the 'coloring' reaction are provided by Monte Carlo simulations of the experimental reaction scheme. The probability of chemically altering expanded homopolymer coils is found to be equal for all units along the length of the chain. In contrast, 'coloring' of collapsed homopolymer coils reveals that the probability of modification is widely distributed. These results further support our claim that copolymers with random and random-blocky monomer sequence distributions can be synthesized by 'coloring' expanded and collapsed homopolymer coils, respectively.
Formation and properties of surface-anchored polymer assemblies with tunable physico-chemical characteristics
(2003-06-16) Wu, Tao; Christine S. Grant, Committee Member; Jan Genzer, Committee Chair; Christopher B. Gorman, Committee Member; Richard J. Spontak, Committee Member; Saad A. Khan, Committee Member
We describe two new methodologies leading to the formation of novel surface-anchored polymer assemblies on solid substrates. While the main goal is to understand the fundamentals pertaining to the preparation and properties of the surface-bound polymer assemblies (including neutral and chargeable polymers), several examples also are mentioned throughout the Thesis that point out to practical applications of such structures. The first method is based on generating assemblies comprising anchored polymers with a gradual variation of grafting densities on solid substrates. These structures are prepared by first covering the substrate with a molecular gradient of the polymerization initiator, followed by polymerization from these substrate-bound initiator centers ("grafting from"). We apply this technique to prepare grafting density gradients of poly(acryl amide) (PAAm) and poly(acrylic acid) (PAA) on silica-covered substrates. We show that using the grafting density gradient geometry, the characteristics of surface-anchored polymers in both the low grafting density ("mushroom") regime as well as the high grafting density ("brush") regime can be accessed conveniently on a single sample. We use a battery of experimental methods, including Fourier transform infrared spectroscopy (FTIR), Near-edge absorption fine structure spectroscopy (NEXAFS), contact angle, ellipsometry, to study the characteristics of the surface-bound polymer layers. We also probe the scaling laws of neutral polymer as a function of grafting density, and for weak polyelectrolyte, in addition to the grafting density, we study the affect of solution ionic strength and pH values. In the second novel method, which we coined as "mechanically assisted polymer assembly" (MAPA), we form surface anchored polymers by "grafting from" polymerization initiators deposited on elastic surfaces that have been previously extended uniaxially by a certain length increment, Λx. Upon releasing the strain in the substrate after completion of polymerization, we show the grafting density of the polymers grafted to flexible substrates can be tuned as a function of Λx.
Formation and Thermodynamics of Heteropolymers with Adjustable Monomer Sequence Distribution
(2008-12-19) Jhon, Young Kuk; Jan Genzer, Committee Chair; Richard J. Spontak, Committee Member; Christopher B. Gorman, Committee Member; Orlando Rojas, Committee Member
Over the past few decades, multiple experimental and theoretical studies have reported on the formation and thermodynamics of copolymers with ordered sequences. Because of challenges associated with synthesizing random copolymers (RCPs) with tunable co-monomer sequences and analyzing their co-monomer sequence distribution, only a few computer simulations and sophisticated theoretical approaches have been employed that provided insight in the thermodynamical behavior of such RCPs. Recently, we developed a methodology facilitating the formation of RCPs with tunable co-monomer sequence distributions by invoking the â€œcoloring schemeâ€ suggested Khokhlov and coworkers in their computer simulations. The RCPs considered in this work are prepared by bromination of polystyrene (PS) with bromine leading to poly(styrene-co-4-bromostyrene) (PBrxS) RCPs, where x is the mole fraction of the 4-bromostyrene (4-BrS) segments in PBrxS. In this Thesis, we demonstrate that the co-monomer sequence distribution in PBrxS can be adjusted by carrying out the bromination reaction in solutions of various solvent quality and reaction geometry. By adjusting the solvent quality during the bromination reaction, either random or random blocky PBrxS, (r-PBrxS or b-PBrxS, respectively), are synthesized. We find that: 1) the bulk bromination of PS follows the second-order kinetic in bromine, 2) the reaction rate increases with increasing the solvent dielectric constant, and 3) decreasing the solvent quality decreases the bromination reaction rate. In addition, we report that the reaction rates for brominating PS brushes are much smaller than those of free PS chains in solution. We attribute this latter behavior to steric hindrance due to PS confinement on the substrate. We also report on the effect of chemical composition, co-monomer distribution, and H/D isotopic substitution in the RCP and the solvent on the phase behavior of PBrxS in cyclohexane (CH). We use turbidity and small angle neutron scattering measurements to assess the temperature-dependence of phase behavior of PBrxS with various x as a function of polymer concentration in CH. Our results reveal that while r-PBrxS chains collapse as individual coils during the coil-to-globule transition, the 1-phase to 2-phase transition in b PBrxS is very complex. Specifically, we report that larger macromolecular aggregates comprising multiple b-PBrxS chains form in solution well above the coil-to-globule transition observed in r-PBrxS having the same x, and that this assembly acts as a precursor to the â€œtrue phase transitionâ€ . This complex phase behavior of b PBrxS is associated with the existence of inter- and intra-chain contacts acting among styrene and 4 BrS units (â€œpattern recognitionâ€ ) leading possibly to â€œflower-likeâ€ micelle formation. Finally, we report on the adsorption kinetics of PBrxS on flat substrates by monitoring the adsorption of PBrxS from various solvents on flat silica surfaces. The PBrxS adsorption is driven by the strong affinity of the 4-BrS units towards silica while the interaction between styrene and the surface is nearly athermal. For a given solvent the amount of PBrxS adsorbed onto the surface increases with increasing the 4-BrS content and the blockiness in the monomer distribution in the RCP. Concurrently, the amount of PBrxS on the substrate also increases with decreasing the quality of the solvent, from which the copolymer is adsorbed. We provide theoretical insight into the various molecular phenomena that govern both the kinetics and the equilibrium amount of the RCPs on the surface as a function of the co monomer distribution in the RCPs.
Investigating Adsorption of Synthetic Nanoparticles and Biological Species using Surface-grafted Molecular and Macromolecular Gradient Assemblies
(2005-08-16) Bhat, Rajendra R; Jan Genzer, Committee Chair; John van Zanten, Committee Member; Daniel Feldheim, Committee Member; Richard Spontak, Committee Member
We utilize novel surface-grafted molecular and macromolecular gradient assemblies to investigate: 1) dispersion of nanoparticles in organic matrices tethered to a substrate, and 2) adsorption of proteins and adhesion of cells to synthetic polymeric surfaces. Application of gradient surfaces facilitates unambiguous identification and analysis of the key parameters governing these complex, multivariate phenomena. First, we demonstrate the formation of and control over the two-dimensional assemblies of nanoparticles bound to a flat substrate via self-assembled monolayer adhesive coating. A molecular gradient template formed by vapor transport of organosilane is used to tune the number of surface-bound particles. Number density of particles is shown to be directly proportional to the surface concentration of organosilane species comprising the monolayer coating. The gradient geometry is further utilized to elucidate the inverse relationship between surface concentration and degree of ionization of organosilane species required to achieve a given particle number density. We also form a new class of nanocomposite materials by dispersing nanoparticles in surface-anchored polymer assemblies. In order to systematically probe the influence of various polymer properties on the structure of resulting nanocomposite, we employ novel architectures of surface-grafted polymers that offer either 1) unidirectional variation of polymer molecular weight (so-called linear gradient) or 2) bidirectional, simultaneous variation of molecular weight and grafting density (called as orthogonal gradient). The number of particles in the polymer brush/particle hybrid is found to increase with increasing polymer molecular weight due to an increase in the number of sites, to which particles can bind. For a given grafting density of polymer brush, larger particles predominantly reside near the brush-air interface. In contrast, smaller nanoparticles penetrate deeper into the polymer brush, thus forming a three-dimensional structure. Upon increasing grafting density of the chains, the number of attached particles exhibits different trends depending on the particle size. For larger particles, a continuous increase in particle loading is detected as a function of increasing grafting density. In contrast, polymer brushes containing smaller particles exhibit a maximum in particle concentration at some intermediate value of grafting density. We rationalize the latter behavior in terms of competition between enthalpic gain upon particle attachment to the polymer chains and entropic penalty induced by the insertion of particles in the dense brush. We also demonstrate that polymer brushes that respond to changes in environmental conditions (temperature in particular) can be harnessed to further tune nanoparticles loading in polymer brush-particle composites. Our experimental results concur very well with theories describing organization of nanoparticles in polymer brushes. Finally, we apply gradients in molecular weight and grafting density of protein-repelling polymer to tailor protein adsorption and cell adhesion to surfaces. Protein adsorption is shown to decrease with increasing surface coverage of polymer, which can be achieved by increasing molecular weight and/or grafting density of tethered polymer. Polymer gradient substrates are utilized to tailor the amount of adsorbed fibronectin (FN), which in turn regulates adhesion of osteoblast precursor cells. Cells are well attached and spread in a polygonal fashion on parts of the gradient with high FN coverage (least polymer coverage) whereas the cells are poorly anchored and elongated in areas of the gradient that are fully decorated by grafted polymer.
An Investigation of the Interfacial Microstructure and Properties in Side-by-Side Bicomponent Polymer Fibers.
(2010-11-03) He, Feng; Melissa Pasquinelli, Committee Chair; Jan Genzer, Committee Chair; Behnam Pourdeyhimi, Committee Member; Nagendra Anantharamaiah, Committee Member
Surface-Grafted Polymer and Copolymer Assemblies with Gradient in Molecular Weight and Composition
(2005-12-29) Tomlinson, Michael Ralph; Sergei Sheiko, Committee Member; Richard Spontak, Committee Member; Jan Genzer, Committee Chair; Chris Gorman, Committee Member; Ken Caster, Committee Member
The chief goal of this Ph.D. dissertation was to develop methodologies facilitating the formation of assemblies comprising grafted polymers on surfaces with gradually varying length (or, alternatively, molecular weight). Our additional goals accomplished include expansion of these methodologies to incorporate multiple monomer systems and block copolymer assemblies. Lastly we demonstrate the utility of these gradient assemblies to study some complex phenomena of scientific interest. Surface-grafted polymer gradients represent important tools in the combinatorial study of tethered polymer layers. This approach can lead to rapid screening of properties and development of new or more efficient technologies involving tethered polymer films. The areas/technologies of interest are cited throughout the work and span organic electronic materials, responsive surfaces, nonfouling coatings, drug delivery applications, and manipulation of matter on 'small scales' leading to developments in nanotechnology. In Chapter 3, I describe procedures, methods, and several evolutions of a gradient chamber designed to create homopolymer gradient assemblies. Chapter 3 also includes studies using polymer gradients to understand polymer surface growth kinetics and an introduction to the concept of orthogonal gradient samples. I include, as supplemental information, my studies involving Atom Transfer Radical Polymerization (ATRP) simulations using a step time based Fortran program. Chapter 4 introduces the concept of block-copolymer gradients and describes my progress and major accomplishments in achieving my goal of formation of tethered copolymer gradient assemblies. I also discuss several studies involving these tethered copolymers. I use a combinatorial approach to create 'step' multiblocks on one sample surface in order to study the efficiency and characteristics of growth of these multiblock layers. I describe how I was able to produce surface-grafted diblock copolymer gradients and how I was able to chemically modify these layers. A major portion of Chapter 4 is devoted to describing a study in which a tethered diblock copolymer gradient was subjected to two selective-solvent exposure procedures designed to collapse the top and bottom blocks, respectively. I was able to study, combinatorially, the formation of and characteristics of micellar and bicontinuous structures formed via these solvent exposure techniques. Clear AFM images, ellipsometric thickness, and wettability measurements made on this sample reveal a possible relationship between surface morphology and the rearrangement of the diblock surface. I also introduce a triangular triblock triple gradient, which I successfully created. Such gradients will yield a wealth of information when applied to the study of tethered triblock copolymers.
Synthesis and Characterization of Surface-confined Responsive Polyelectrolyte Brushes
(2006-05-17) Stone, Jason Lee; Jan Genzer, Committee Chair; John H. van Zanten, Committee Co-Chair; Orlando J. Rojas, Committee Member; Saad A. Khan, Committee Member
Polyelectrolytes contain either acidic or basic monomeric units along their backbones. When the repeat units are weak acids or weak bases, the degree of ionization will depend on the solution pH. In addition to pH, the amount of charge on the macromolecule can be fine-tuned by varying the concentration of salt in the solution. We synthesize surface-grafted weak polyelectrolytes (polyelectrolyte brushes) and explore how their physico-chemical characteristics (most notably the brush height) depend on molecular weight, composition, solution pH, and ionic strength. In addition, we explore how one can utilize pH-dependent chain swelling in the fabrication of stimuli-responsive macromolecular switches and valves.