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Zukoski, Charles F.

Publications and source records attributed to Zukoski, Charles F..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Self-assembly pathways and polymorphism in peptide-based nanostructures

Dipeptide derivative molecules can self-assemble into space-filling nanofiber networks at low volume fractions (<1%), allowing the formation of molecular gels with tunable mechanical properties. The self-assembly of dipeptide-based molecules is reminiscent of pathological amyloid fibril formation by naturally occurring polypeptides. Fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) is the most widely studied such molecule, but the thermodynamic and kinetic phenomena giving rise to Fmoc-FF gel formation remain poorly understood. We have previously presented evidence that the gelation process is a first order phase transition characterized by low energy barriers to nucleation, short induction times, and rapid quasi-one-dimensional crystal growth, stemming from solvent–solute interactions and highly specific molecular packing. Here, we discuss the phase behavior of Fmoc-FF in different solvents. We find that Fmoc-FF gel formation can be induced in apolar solvents, in addition to previously established pathways in aqueous systems. We further show that in certain solvent systems anisotropic crystals (nanofibers) are an initial metastable state, after which macroscopic crystal aggregates with no preferred axis of growth are formed. Furthermore, the molecular conformation is sensitive to solvent composition during assembly, indicating that Fmoc-FF may be a simple model system to study complex thermodynamic and kinetic phenomena involved in peptide self-assembly.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Intrinsic nonlinearities in the mechanics of hard sphere suspensions

The onset of nonlinear responses in near hard sphere suspensions is characterized as a function of oscillatory frequency and strain amplitude. At low frequencies where the viscous behavior dominates, the onset of nonlinearities is driven by increases in rate of strain. At high deformation frequency, where suspension mechanics is dominated by an elastic response, the nonlinear responses occur when deformation exceeds a characteristic strain. This strain is associated with the transient confinement of particles by nearest neighbors and its volume fraction dependence is through cage parameters derived from the high frequency elastic modulus. Furthermore, the onset of nonlinear responses takes on a universal behavior when deformation frequency is normalized by the characteristic time governing the shift from viscous to elastic behavior indicating that this transition is associated with transient particle localization and is expected to be observed for all volume fractions where pair interactions are important.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Gelation of Fmoc-diphenylalanine is a first order phase transition

We explore the gel transition of the aromatic dipeptide derivative molecule fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF). The addition of water to a solution of Fmoc-FF in dimethyl sulfoxide (DMSO) results in increased attractions leading to self-assembly of Fmoc-FF molecules into a space-filling fibrous network. We provide evidence that gel formation is associated with a first order phase transition resulting in nucleation and growth of strongly anisotropic crystals with high aspect ratios. The strength of attraction between Fmoc-FF molecules as a function of water concentration is estimated from long-time self-diffusion measurements using 1H NMR diffusion-ordered spectroscopy (DOSY). The resulting phase behavior follows that observed for a wide range of other crystallizing nanoparticles and small molecules – a result consistent with the short-range nature of the intermolecular attractions. Furthermore, we use NMR to measure the rate of increase in the fraction of bound Fmoc-FF molecules after water is suddenly mixed into the system. We observe a lag time in the formation of the new phase indicative of the existence of a free energy barrier to the formation of a crystal nucleus of critical size. Furthermore, the application of classical nucleation theory for a cylindrical nucleus indicates that one-dimensional crystal growth is driven by an imbalance of the surface energies of the ends and sides of the fiber.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Nanoscale dynamics and aging of fibrous peptide-based gels

Solutions of the aromatic dipeptide derivative molecule fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) in dimethyl sulfoxide produce fibrous gels when mixed with water. We study the evolution of density fluctuations of this three-component system using X-ray photon correlation spectroscopy (XPCS) and compare these results to the macroscopic rheology of the gels and optical observations of the microstructure evolution. At the investigated scattering angles, the intensity autocorrelation functions do not follow behavior expected for simple diffusion of individual Fmoc-FF molecules localized within cages of nearest neighbors. Instead, the dynamics are associated with density fluctuations on length scales of ∼10–100 nm arising from disaggregation and reformation of fibers, leading to an increasingly uniform network. This process is correlated with the growth of the elastic modulus, which saturates at long times. Autocorrelation functions and relaxation times acquired from XPCS measurements are consistent with relaxation rates of structures at dynamic equilibrium. This study provides further support to the concept of exploring peptide-based gelators as valence-limited patchy particles capable of forming equilibrium gels.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Particle Restabilization in Silica/PEG/Ethanol Suspensions: How Strongly do Polymers Need To Adsorb To Stabilize Against Aggregation?

We study the effects of increasing the concentration of a low molecular weight polyethylene glycol on the stability of 44 nm diameter silica nanoparticles suspended in ethanol. Polymer concentration, c{sub p}, is increased from zero to that characterizing the polymer melt. Particle stability is accessed through measurement of the particle second-virial coefficient, B{sub -2}, performed by light scattering and ultrasmall angle X-ray scattering (USAXS). The results show that at low polymer concentration, c{sub p} < 3 wt %, B{sub -2} values are positive, indicating repulsive interactions between particles. B{sub -2} decreases at intermediate concentrations (3 wt % < c{sub p} < 50 wt %), and particles aggregates are formed. At high concentrations (50 wt % < c{sub p}) B{sub -2} increases and stabilizes at a value expected for hard spheres with a diameter near 44 nm, indicating the particles are thermodynamically stable. At intermediate polymer concentrations, rates of aggregation are determined by measuring time-dependent changes in the suspension turbidity, revealing that aggregation is slowed by the necessity of the particles diffusing over a repulsive barrier in the pair potential. The magnitude of the barrier passes through a minimum at c{sub p} {approx} 12 wt % where it has a value of {approx}12kT. These results are understood in terms of a reduction of electrostatic repulsion and van der Waals attractions with increasing c{sub p}. Depletion attractions are found to play a minor role in particle stability. A model is presented suggesting displacement of weakly adsorbed polymer leads to slow aggregation at intermediate concentration, and we conclude that a general model of depletion restabilization may involve increased strength of polymer adsorption with increasing polymer concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Role of Polymer Segment-Particle Surface Interactions in Controlling Nanoparticle Dispersions in Concentrated Polymer Solutions

The microstructure of particles suspended in concentrated polymer solutions is examined with small-angle X-ray scattering and small-angle neutron scattering. Of interest are changes to long wavelength particle density fluctuations in ternary mixtures of silica nanoparticles suspended in concentrated solutions of poly(ethylene glycol). The results are understood in terms of application of the pseudo-two-component polymer reference interaction site model (PRISM) theory modified to account for solvent addition via effective contact strength of interfacial attraction, ε pc , in an implicit manner. The combined experimental-theoretical study emphasizes the complex interactions between solvent, polymer, and particle surface that control particle miscibility but also demonstrate that these factors can all be understood in terms of variations of ε pc .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Dynamic localization and shear-induced hopping of particles: A way to understand the rheology of dense colloidal dispersions

For decades, attempts have been made to understand the formation of colloidal glasses and gels by linking suspension mechanics to particle properties where details of size, shape, and spatial dependencies of pair potentials present a bewildering array of variables that can be manipulated to achieve observed properties. Despite the range of variables that control suspension properties, one consistent observation is the remarkably similarity of flow properties observed as particle properties are varied. Understanding the underlying origins of the commonality in those behaviors (e.g., shear-thinning with increasing stress, diverging zero shear rate viscosity with increasing volume fraction, development of a dynamic yield stress plateau with increases in volume faction or strength of attraction, development of two characteristic relaxation times probed in linear viscoelasticity, the creation of a rubbery plateau modulus at high strain frequencies, and shear-thickening) remains a challenge. Recently, naïve mode coupling and dynamic localization theories have been developed to capture collective behavior giving rise to formation of colloidal glasses and gels. This approach characterizes suspension mechanics of strongly interacting particles in terms of sluggish long-range particle diffusion modulated by varying particle interactions and volume fraction. These theories capture the scaling of the modulus with the volume fraction and strength of interparticle attraction, the frequency dependence of the moduli at the onset of the gel/glass transition, together with the divergence of the zero shear rate viscosity and cessation of diffusivity for hard sphere systems as close packing is approached. In this study, we explore the generality of the predictions of dynamic localization theory for systems of particles composed of bimodal particle size distributions experiencing weak interactions. We find that the mechanical properties of these suspensions are well captured within the framework of dynamic localization theory and that suspension mechanics can be understood in terms of a dynamical potential barrier, the magnitude of which governs the zero shear rate viscosity, and onset of a dynamic yield stress plateau as volume fraction or strength of interaction is raised.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Evidence for equilibrium gels of valence-limited particles

We explore the formation and structure of gels produced from solutions of the aromatic dipeptide derivative molecule fluorenylmethoxycarbonyl–diphenylalanine (Fmoc–FF) in dimethyl sulfoxide (DMSO). Mixing these solutions with water results in the self-assembly of Fmoc–FF molecules into space-filling fibrous networks, exhibiting mechanical properties characteristic of gels. Using confocal fluorescence microscopy, we observe the gel transition in situ and find that, upon the addition of water, the solution undergoes a rapid transition to a non-equilibrium state forming ~2 μm spheres, followed by the formation of fibers 5–10 nm in diameter, nucleating at a sphere surface and expanding into the solution as the remaining spheres dissolve, extending the network. The gel aging process is associated with the network becoming increasingly uniform through apparent redissolution/reaggregation of the Fmoc–FF molecules, corresponding to the observed increase in the elastic modulus to a plateau value. We demonstrate that this increase in uniformity and elastic modulus can be expedited by controlling the temperature of the system, as well as that these gels are thermally reversible, further indicating that the system is in equilibrium in its fibrous network state. X-ray scattering information suggests that the packing of the molecules within a fiber is based on π–π stacking of β-sheets, consistent with models proposed in the literature for similar systems, implying that each particle (molecule) possesses a limited number of interaction sites. Furthermore, these observations provide experimental evidence that these low molecular weight gelator molecules can be considered valence-limited “patchy” particles, which associate at low enough temperature to form equilibrium gels.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Binary Diamondoid Building Blocks for Molecular Gels

Adamantane is a type of diamondoid molecules that has a cage or globular shape with a diameter of 6.34 ± 0.04 Å.8 Anisotropic interactions between these truly nanoscopic particles can be induced through the derivatization of the diamondoid cage. Here we explore the gelation of paired systems of adamantane where attractions are introduced through van der Waals forces and hydrogen bonding. Gels are produced through the mixing of 1-adamantanecarboxylic acid (A1C) and 1-adamantylamine (A1N). Upon mixing dimethyl sulfoxide solutions of these molecules at vanishing concentrations, these diamondoid molecules rapidly precipitate. A space-filling gel of the resulting aggregates is observed at approximately 3% by weight. These resulting gels have elastic moduli of 10 2 –10 4 Pa in the 3–7 wt % concentration range. At a 1:1 mol ratio of 1-adamantanecarboxylic acid (A1C) and 1-adamantylamine (A1N), the gel’s elastic modulus and yield stress increase as volume fractions Φ x and Φ y with x ≈ 4.2 and y ≈ 3.5. Here, the dependencies of moduli and yield stress on the volume fraction display characteristics of colloidal gels. Transmission electron microscope (TEM) images indicate that the gels are formed from a network of interwoven and branched fibers which are composed of ~30 nm crystallites that have undergone oriented aggregation to form fibers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Mechanical Properties of Self-Assembled Fmoc-Diphenylalanine Molecular Gels

We explore the phase diagram and mechanical properties of molecular gels produced from mixing water with a dimethyl sulfoxide (DMSO) solution of the aromatic dipeptide derivative fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF). Highly soluble in DMSO, Fmoc-FF assembles into fibrous networks that form gels upon addition of water. At high water concentrations, rigid gels can be formed at Fmoc-FF concentrations as low as 0.01 wt %. Here, the conditions are established defining the Fmoc-FF and water concentrations at which gels are formed. Below the gel boundary, the solutions are clear and colorless and have long-term stability. Above the gel boundary, gels are formed with increasing rapidity with increasing water or Fmoc-FF concentrations. A systematic characterization of the effect of Fmoc-FF and water concentrations on the mechanical properties of the gels is presented, demonstrating that the elastic behavior of the gels follows a specific, robust scaling with Fmoc-FF volume fraction. Furthermore, we characterize the kinetics of gelation and demonstrate that these gels are reversible in the sense that they can be disrupted mechanically and rebuild strength over time.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

The effect of polymer-induced attraction on dynamical arrests of polymer composites with bimodal particle size distributions

We explore the flow properties of nanocomposite melts where the particles have bimodal size distributions and experience a weak attraction produced by suspending silica particles in polyethylene glycol melts with a molecular weight of 2000 (PEG2000). The polymer is unentangled and adsorbs to the particle surface. Here, the volume fraction ratio of large particles to total particle volume fraction, R, is systematically varied to study the effects of this polymer-induced attraction on suspension rheology. The maximum volume fraction, Φm, of the particles varies in a nonmonotonic manner of R as demonstrated in studies of the same mixtures when suspended in polyethylene glycol with a molecular weight of 400 (PEG400), where the particles experience excluded volume interactions. The dynamical arrest volume fraction Φx, of nanocomposite melts in PEG2000 monotonically increases with R. In frequency sweep experiments, the plateau elastic modulus is dominated by attractive interactions and increases with the total particle volume fraction, Φc, proportionally with 1/(h2< D >), where < D > is the volume average particle diameter and h is the average particle-particle surface separation. As R is varied, this universal yielding behavior occurs at constant surface separation, h, suggesting that the flow properties of the mixtures can be understood as being equivalent to flow properties of homogeneous particle suspensions experiencing short-range attractions with an extent independent of particle size.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Rheology of high density glass of binary colloidal mixtures in unentangled polymer melts

Here, we explore the rheology of high density glasses formed by binary colloidal mixtures in unentangled polyethylene glycol melts with a molecular weight of 400. A particle size ratio δ = d l /d s = 4.8 is chosen to ensure that mixing results in a reduction in composite rheology without phase separation for a fixed high total volume fraction, Φ c . We explore frequency dependent linear viscoelastic behaviors, yielding and shear-thickening behaviors, and study properties as a function of the volume fraction ratio of the large particles, R . We characterize the cross-over volume fractions which denote the onset of transient particle localization, Φ x , and the maximum packing fractions, Φ m , and find that both are non-monotonic functions of R , with Φ m /Φ x being a constant as R is varied from 0 to 1. Hard sphere scaling fails to capture the effects of particle size when R = 0 and R = 1, demonstrating that, even for these low molecular weight oligomers, polymer absorption and alterations of polymer configurations modify pair potentials away from being solely volume exclusion in nature. Changes in linear rheology, yielding behavior and shear-thickening are discussed in terms of a pseudo one-component model that depends on the dimensionless volume fraction Φ* = ( Φ m - Φ x )/( Φ m -Φ c ).

36 MATERIALS SCIENCE↗
DOE OSTI2012

Role of Particle Size and Polymer Length in Rheology of Colloid–Polymer Composites

The effect of particle size on the flow properties of composite melts is explored. In this study we investigate a system composed of silica particles with diameter D c = 127–730 nm in unentangled poly(ethylene glycol) of two molecular weights (400 with degree of polymerization ~9 and 2000 with degree of polymerization of 45). At low concentration, silica particles are stabilized by the adsorbed polymer layer as indicated by intrinsic viscosities slightly larger than the Einstein value of 2.5. Huggins coefficients indicate that at low concentration in PEG400 the particles behave very much like hard spheres, while in PEG2000 the particles experience very weak attractions. At high volume fraction, the absorbed polymer layers begin to interact such that hard-sphere scaling no longer applies to linear and nonlinear rheological responses. In PEG2000, at elevated volume fractions, the particles with stronger attractions experience kinetic arrest with average particle surface separations much larger than the polymer radius of gyration, R g . We show that the linear rheology, yielding behavior, and shear thickening response of dense composites are varied with R g / D c , with volume exclusion glass formation observed at low R g / D c and gelation at high R g /D c , and with yielding and shear thickening properties changed by R g and D c independently. The polymer layers are found to alter the approach to the jamming transition even for R g / D c as small as 6 × 10 –3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Rheology of dense suspensions of shape anisotropic particles designed to show pH-sensitive anisotropic pair potentials

Here we investigate the flow properties of suspensions of dicolloidal particles composed of interpenetrating spheres where one sphere is rich in polystyrene and the second is rich in poly 2-vinyl pyridine. The synthesis method is designed to create both anisotropic shape and anisotropic interaction potentials that should lead to head to tail clustering. Furthermore, these particles are referred to as copolymer dicolloids (CDCs). The viscoelastic properties of stable and gelled suspensions of CDC particles are compared with analogs composed of homopolymer dicolloids (HDCs), having the same shape but not displaying the anisotropic attractions. After coating the particles with a nonionic surfactant to minimize van der Waals attractions, the flow properties of glassy and gelled suspensions of CDCs and HDCs are studied as a function of volume fraction, ionic strength and pH. Suspensions of HDC particles display a high kinetic arrest volume fraction (Φ g > 0.5) over a wide range of pH and ionic strength up to [I]=0.5 M, demonstrating that the particles experience repulsive or weakly attractive pair potentials. Suspensions of CDC particles behave in a similar manner at high or low pH when [I]=0.001 M, but gel at a volume fraction of Φ g < 0.3 and display anomalously large elastic moduli at and above the gel transition point for intermediate pH or for pH=9 when [I]=0.5 M. The gelation processes for the CDC particles are reversible by adjusting the solution pH. Interaction potential anisotropy is evident in the processes, during which the CDC particles yield on increasing oscillatory strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗