bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterizations of Nanostructured Films as Responsive Electrode Materials
Authors: Kariuki Nancy , Luo Jin , Moussa Laura , Israel Lisa B. , Zhong Chuan-Jian , Hepel Maria
Published: April 2011

Nanostructured thin films were assembled as metal-responsive electrode materials from monolayer-capped gold nanoparticles (2 nm) and carboxylic acid functionalized alkyl thiol linkers via an exchange-crosslinking-precipitation reaction pathway. The network assemblies have open frameworks in which void space forms channels or chambers with the nanometer sized cores defining its size and the shell structures defining its chemical specificity. Such nanostructures were investigated as responsive materials for the detection of metal ion fluxes. Cyclic voltammetry, in-situ electrochemical quartz-crystal nanobalance, and surface infrared reflection spectroscopy techniques were used to characterize the interfacial redox reactivity and mass fluxes at the nanostructured electrode materials. The system showed remarkable reversible mass loading arising from incorporation of ionic species into the film. The diagnostic stretching bands of the carboxylic and carboxylate groups at the shell allowed the identification and assessment of the interfacial carboxylate-metal ion reactivity.
Microstructure and Magnetic Properties of Fe(C) and Fe(O) Nanoparticles
Authors: Sun Xiang-Cheng , Nava N. , Reyes-Gasga J.
Published: April 2011

Two types of iron (Fe) nanoparticles, carbon-coated Fe nanoparticles (Fe(C)) and pure α-Fe nanoparticles that coated with oxide layers (Fe(O)), have been successfully synthesized using modified graphite arc-discharge method. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HREM) and electron diffraction (SAED) analysis have been used to characterize these distinct structural morphologies. It is indicated that those two Fe nanoparticles have an average grain size of 15-20nm. The presence of carbon encapsulated α-Fe, γ-Fe and Fe3C phases are clearly identified by X-ray diffraction and SAED patterns in those Fe(C) particles. However, the evidence of pure α-Fe nanocrystal coated with oxide layer is also revealed by HR-TEM images and SAED patterns in these Fe(O) particles.

Mössbauer spectra and hyperfine magnetic fields at room temperature for the assemblies of Fe(C) and Fe(O) nanoparticles further confirm their distinct nanophases that detected by XRD analysis and HRTEM observation. Specially, the assemblies of Fe(O) nanoparticles exhibit ferromagnetic properties at room temperature due to the stronger interparticle interaction and bigger magnetocrystalline anisotropy effects among these Fe(O) nanoparticles. Moreover, modified superparamagnetic relaxation is observed in the assemblies of Fe(C) nanoparticles, which is attributed to the nanocrystalline nature of the carbon-coated nanoparticles.
Carbon-Nanofibre-Filled Thermoplastic Composites
Authors: Sandler Jan , Shaffer MiloS.P. , Lam Yeng-Ming , Windle Alan H. , Werner Philipp , Altstädt Volker , Nastalczyk Jacek , Broza Georg , Schulte Karl , Keun Christian-André
Published: April 2011

Macroscopic poly(ether ether ketone) (PEEK) and polypropylene (PP) nanocomposites containing vapour-grown carbon nanofibres (CNF) were produced using standard polymer processing. Tensile tests revealed a linear increase in composite stiffness with nanofibre content. A detailed DSC study verified that under standard processing conditions the degree of crystallinity and the crystalline structure of these semicrystalline thermoplastics were not affected by the nanofibres. Nevertheless, we provide evidence that the nanoscale filler can alter the polymer morphology under certain conditions, an effect which needs to be considered when evaluating nanocomposite properties. Given the absence of morphological changes in the standard nanocomposites we were able to calculate the intrinsic nanofibre modulus using short fibre theory; both distinctively different matrix systems show a similar effective nanofibre modulus.
Fabrication and Characterization of Carbon Nanotube Field Emitters
Authors: Jiao Jun , Dong Lifeng , Tuggle David W. , Mosher Catherine L. , Foxley Sean , Tawdekar Jaswant
Published: April 2011

We report an effective procedure for fabricating carbon nanotube emitters by directly synthesizing carbon nanotubes on an electrochemically sharpened tungsten tip. The nanotubes adhere very well to the tip of tungsten without any painting materials. Thermal cleaning of the tungsten tip under applied electric field reduced the number of nanotubes formed on the tip resulting in a single nanotube emitter. Electron field emission properties were investigated by employing a field emission microscope with a base pressure ~ 1 × 10-9 Torr. The emission images with respect to the applied field and time were obtained. Different emission images consisting of one to four lobes at different applied fields were observed. The characteristic of the emission current vs. applied voltage was analyzed. Applied potentials up to 3000 V were tested. The estimated field on the emitter was on the order of several tens of volts per nanometer. Our investigation suggests that at lower fields, the I-V characteristic of the nanotube emitter follows Fowler-Nordheim (F-N) emission behavior. At higher applied field, current saturation was observed.
Pneumatic Actuator Response from Carbon Nanotube Sheets
Authors: Spinks Geoffrey M. , Wallace Gordon G. , Fifield Leonard S. , Dalton Larry R. , Mazzoldi Alberto , Rossi Danilo De , Khayrullin Ilyas I. , Baughman Ray H.
Published: April 2011

Reversible actuation strains in excess of 2% in the sheet direction and over 300% in the thickness direction have been produced by single wall carbon nanotube mats when electrochemically charged to +1.5V (vs. SCE) in aqueous sodium chloride solution. The observed strains represent a ten-fold increase over that previously reported for carbon nanotube actuators, and is considerably larger than that achievable with polymer ferroelectric actuators. The enhanced actuator strains result from a new mechanism of electrochemically induced “pnuematic actuation” where high pressure gas forms within the porous structure of the nanotube mat causing partial delamination and swelling. An erasable “memory” effect was also observed for pneumatic actuation driven by hydrogen gas evolution/storage in the nanotube electrodes.
Focused Electron Beam Induced Deposition of High Resolution Magnetic Scanning Probe Tips
Authors: Utke I. , Cicoira F. , Jaenchen G. , Hoffmann P. , Scandella L. , Dwir B. , Kapon E. , Laub D. , Buffat Ph. , Xanthopoulos N. , Mathieu H.J.
Published: April 2011

Apexes of commercial pyramidal silicon scanning microscopy tips were magnetically functionalized by means of local focused electron beam induced deposition. High aspect ratio supertips and local tip coatings with varying apex diameters can be produced by varying exposure time, beam current, and scan mode. The carbonyl precursor Co2(CO)8 was used as source of magnetic metal. Tip performance was tested with magnetic force microscopy (tapping / lift-retrace mode) and magnetically actuated cantilever atomic force microscopy. The deposit contains 34±2 at.% Co, dispersed as 2-5 nm metal nanocrystals in a carbonaceous matrix. Specific surface reactions and Boudouard reactions are proposed to explain the resulting deposit composition measured by Auger spectroscopy. The electrical resistivity is 104 higher than bulk Co resistivity.
Atomistic Study of Mechanical Properties of Carbon Nanotubes
Authors: Narita T. , Shintani K.
Published: April 2011

The mechanical properties of single-walled carbon nanotubes are investigated by means of molecular dynamics simulations. The Tersoff-Brenner potential is used for the calculation of the interatomic forces. Two kinds of simulation cells are considered; one adopts the periodic boundary condition along the tube axis and the other corresponds to tube clusters. The atoms at the ends of a simulation cell are translated along its axis, and the Young's modulus and Poisson's ratio are estimated. How these mechanical properties of carbon nanotubes depend on their chiralities is discussed.
Effect of Solvent Quality on the Friction Forces Between Polymer Brushes
Authors: Forster Aaron M. , Kilbey S. Michael
Published: April 2011

We have used the surface forces apparatus to measure the structural and frictional force profiles between opposing, solvated brush layers as a function of temperature. Two different polyvinylpyridine-polystyrene [PVP-PS] diblock copolymers were used to make PS brushes. The molecular weights (in thousands) of these PVP-PS materials were [114/103]k, [30/70]k, respectively. Structural and frictional force profiles in toluene and cyclohexane were measured, and the cyclohexane experiments were conducted at temperatures ranging from the theta-point to 50 C. In toluene the PS brushes needed to be compressed to ∼1/5th of their equilibrium height before frictional forces were measured, but this onset of frictional forces was detected at a much lower level of compression in near-theta cyclohexane. In cyclohexane the structural force profiles were basically insensitive to the temperature change, but the frictional forces depended strongly on the solvent temperature. When the cyclohexane temperature was raised, the onset of frictional forces decreased toward the good-solvent onset. We also discuss the dependence of frictional force on shearing parameters.
Photonic Crystals at Near-Infrared and Optical Wavelengths
Authors: Moroz Alexander
Published: April 2011

As demonstrated for the example of a diamond and zinc blende structure of dielectric spheres, small inclusions of a low absorbing metal with the volume fraction can have a fm dramatic effect on a complete photonic band gap (CPBG) between the 2nd-3rd bands. For example, in the case of silica coated silver spheres, the CPBG opens for 1:1% and fm exceeds 5% for 2:5%. Consequently, any dielectric material can be used to fabricate fm a photonic crystal with a sizeable and robust CPBG in three dimensions. Absorption in the CPBG of 5% remains very small ( 2:6% for 750 nm). The structure enjoys almost perfect scaling, enabling one to scale the CPBG from microwaves down to ultraviolet wavelengths.
A Theoretical Study of Structural Disorder and Photoluminescence Linewidth in InGaAs/GaAs Self Assembled Quantum Dots
Authors: Lin Yih-Yin , Jiang Hongtao , Singh Jasprit
Published: April 2011

The past few years have seen considerable efforts in growth and device application of self-assembled quantum dots. However, the photoluminescence (PL) linewidth, which represents structural fluctuations in dot sizes, is still in the range of 30-50 meV. This large linewidth has deleterious effects on devices such as lasers based on self-assembled dots. In this paper we will examine the configuration-energy diagram of self-assembled dots. Our formalism is based on: (1) an atomistic Monte Carlo method which allows us to find the minimum energy configuration and strain tensors as well as intermediate configurations of dots; (2) an 8-band k p method to calculate the electronic spectra. We present results on the strain energy per unit cell for various distributions of InAs/GaAs quantum dots and relate them to published experimental results. In particular we examine uncovered InAs/GaAs dots and show that in the uncovered state a well-defined minimum exists in the configuration energy plot. The minimum corresponds to the size that agrees well with experiments.
InAs Quantum Dots in AlAs/GaAs Short Period Superlattices: Structure, Optical Characteristics and Laser Diodes
Authors: Tokranov Vadim , Yakimov M. , Katsnelson A. , Dovidenko K. , Todt R. , Oktyabrsky S.
Published: April 2011

The influence of two monolayer - thick AlAs under- and overlayers on the formation and properties of self-assembled InAs quantum dots (QDs) has been studied using transmission electron microscopy (TEM) and photoluminescence (PL). Single sheets of InAs QDs were grown inside a 2ML/8ML AlAs/GaAs short-period superlattice with various combinations of under- and overlayers. It was found that 2.4ML InAs QDs with GaAs underlayer and 2ML AlAs overlayer exhibited the lowest QD surface density of 4.2x1010 cm-2 and the largest QD lateral size of about 19 nm as compared to the other combinations of cladding layers. This InAs QD ensemble has also shown the highest room temperature PL intensity with a peak at 1210 nm and the narrowest linewidth, 34 meV. Fabricated edge-emitting lasers using triple layers of InAs QDs with AlAs overlayer demonstrated 120 A/cm2 threshold current density and 1230 nm emission wavelength at room temperature. Excited state QD lasers have shown high thermal stability of threshold current up to 130°C.
Fabrication and Properties of Self-Assembled Nanosized Magnetic Particles
Authors: Salazar-Alvarez G. , Mikhailova M. , Toprak M. , Zhang Y. , Muhammed M.
Published: April 2011

The synthesis and characterisation of gold-coated cobalt nanoparticles, as well as their chemically- and magnetically-induced self-organisation have been studied. Metallic core-shell nanoparticles were prepared using two different experimental techniques: bulk reductive precipitation, with average particles size ∼15 nm, and microemulsion confining method, with average particle size of ∼6 nm. The self-assembly of prepared nanoparticles on flat substrates was achieved by derivatising the substrate and particle surfaces with bifunctional organic molecules that attaches to both particles and substrates. Examination of the self-assembled systems was carried out by a number of characterisation techniques including transmission electron microscopy (TEM), UV-visible spectrophotometry (UV-VIS), and atomic force microscopy (AFM).
Thermoassisted Photorecording in Chiral Smectic Azodye Copolymers
Authors: Kozlovsky Mikhail V. , Haase Wolfgang , Rehahn Matthias
Published: April 2011

Illumination of photochromic liquid-crystalline (PCLC) copolymer films with polarized light during the mesophase formation has been studied. Both thick pressed and thin spin-coated, the films show prominent increase in photoinduced orientation, as compared with conventional isothermal photorecording. Two methods of the thermoassisted recording have been suggested, one of those with simultaneous cooling of the film and another one after a shock heating and quenching. The photoinduced birefringence, Δnind, and the order parameter of photochromic groups, Sazo, have been measured. The thermally assisted recording produces holographic gratings with high diffraction efficiency, η, up to 20-30%.
Frequency Agile Optical Filters using Nanoscopic Polymer Dispersed Liquid Crystals
Authors: Lewis Keith , Smith Gilbert , Mason Ian , Rochester Kate
Published: April 2011

The ability to realise frequency agile filters has always been a major goal of materials and device scientists. Polymer dispersed liquid crystal materials (PDLCs), in which the droplets are deliberately produced with dimensions smaller than the Rayleigh limit for optical scatter, provide a means of realising this goal. At visible wavelengths the material has the appearance of a phase-homogeneous medium both with and without applied electric field. Refractive indices are an average of those of the liquid crystal and those of its host and can be described on the basis of effective medium approximations using orientationally averaged values for the randomly aligned LC material for the ambient state. On application of electric field, the individual liquid crystal molecules align with a resulting reduction in the effective index of the medium. Extensive studies of the variation of process parameters and their effect on the morphology of the composite have enabled field induced changes in refractive index in excess of 0.05 to be achieved without recourse to the use of polarisers and with little or no hysteresis effects. At wavelengths associated with WDM devices for telecommunication applications, the degree of scatter loss is negligible. Design trade-offs have been assessed for candidate tunable WDM filters based on resonant cavity devices.
Thermal and Liquid Crystalline Properties of a Polymer with Amphiphilic-Mesogenic Side-Chains
Authors: Ujiie Seiji , Yano Yumi , Mori Akira
Published: April 2011

A low-molecular-weight liquid crystalline amphiphilic compound (Az6Bu) and a liquid crystalline polymethacrylate [P(Az6Bu)] with amphiphilic-mesogenic side-chains are reported in this present paper. A polyethyleneimine chain with 2-hydroxyethyl side-groups was used as the water-soluble spacer. Az6Bu exhibited a lyotropic columnar phase and no thermotropic mesophase. P(Az6Bu) showed thermotropic and lyotropic liquid crystalline properties. P(Az6Bu) formed thermotropic smectic A and C mesophases on the heating and cooling processes. The lyotropic systems consisting of P(Az6Bu) and water exhibited a smectic C lyomesophase with oilystreak and schlieren textures. The layered structures formed in the thermotropic and lyotropic mesophases are composed of a sublayer produced by the anisotropic alignment of aromatic-mesogenic groups and a sublayer of hydrophilic groups.
Numerical Simulation Of A Voltage-Dependent Surface-Induced Molecular Reorientation In A Confined Nematic Liquid Crystal
Authors: Castro J.J. , Calles A. , Valladares R. M.
Published: April 2011

We present a computer simulation of the voltage-dependent orientational ordering of a nematic liquid crystal within the lattice approximation, confined between two substrates with different anchoring for each surface. The system is studied by analyzing the response of the molecular reorientation near the surfaces as a function of the applied voltage. The model considers a molecular interaction responsible for the nematic phase that includes a superposition of the anisotropic induced-dipole-induced-dipole and isotropic Maier-Saupe interactions, whereas for the nematic-surface intermolecular interaction we assume the Rapini-Papoular form. The response to the external electric field is studied by calculating the dielectric constant tensor, based on a model through the molecular polarizability. We particularly discussed the case for negative dielectric anisotropy that could have relevance for the voltage controlled twist effect. The simulation is carried through a numerical relaxation method for the total energy of the system.
Real-time Study of the Evolution of Anisotropic Phase Separation in H-PDLC’s
Authors: Natarajan Lalgudi V. , Tondiglia V.P. , Sutherland R.L. , Tomlin D.W. , Bunning Timothy J.
Published: April 2011

We have investigated the dynamics of formation of a reflection hologram in a photosensitive formulation containing pre-polymer and liquid crystal. Kogelnik’s two beam coupling theory of an isotropic material has previously been expanded to account for variations of refractive index Δn in the x, y, and z directions. This theory predicts a non-zero p-polarized coupling coefficient, κp at 45o internal angle, only when a macroscopic anisotropy in the grating is present. A broadband source was used as a probe to monitor the diffraction efficiencies (DE) during exposure for both s- and p-polarized light. The onset of a macroscopic ordering of the liquid crystal is observed at the same time as the onset of scattering. We report here the effects of laser writing power on the temporal evolution of s- and p-polarized diffraction efficiency and ppolarized scattered intensity.
Liquid Crystalline Nano-Segregated Structures in Hydrogen-Bonded Complexes of Fluoroalkyl Substituted Benzoic Acids
Authors: Nishikawa Etsushi , Yamamoto Jun , Yokoyama Hiroshi
Published: April 2011

Several new benzoic acid derivatives having perfluorinated substituents were synthesized and their hydrogen-bonded complexes with 4, 4’-dipyridyl were prepared. In these acid/base complexes the incompatibility between perfluoroalkyl moieties and hydrocarbon parts is large, which can lead to organize nano-segregation structures. We have found in one of such complexes, that has a long flexible fluorinated moiety, a thermotropic cubic phase with Ia3d symmetry formed by double gyroid of two interpenetrating jointed rod networks with an estimated cell parameter of 10.9 nm. Furthermore another complex with a branched long perfluoroalkyl terminal chain exhibits a first order smectic A to smectic C phase transition, which is confirmed with thermal analysis detecting a large enthalpy change of 5.3 kJxmol-1, X-ray scattering experiments revealing a tilt angle jump, and polarized optical microscopy observing a remarkable texture change at the phase transition temperature.
Adhesion and Interfaces Involving Polymers, Studied by Electrical Resistance Measurement
Authors: Mei Zhen , Chung D.D.L.
Published: April 2011

The adhesion and interfaces involving polymers were studied by electrical resistance measurement. Adhesive bonding and bond degradation were monitored by measuring the resistance in real time during temperature or stress variation. The resistance measured was either the contact resistance of the joint interface or the apparent volume resistance of one of the two adjoining components in a direction in the plane of the joint interface. The polymers were in the form continuous carbon fiber thermoplastic-matrix composites, due to the importance of these composites for lightweight structures and due to the conductivity of carbon fibers and the necessity of conductivity for electrical resistance measurement. The technique was used for the case of both of the adjoining components being such composites, and for the case of one component being such a composite and the other component being concrete. The concrete case is relevant to the use of the composite to retrofit concrete structures.
Effect of Solvent Quality on the Friction Forces Between Polymer Brushes
Authors: Forster Aaron M. , Kilbey S. Michael
Published: April 2011

We have used the surface forces apparatus to measure the structural and frictional force profiles between opposing, solvated brush layers as a function of temperature. Two different polyvinylpyridine-polystyrene [PVP-PS] diblock copolymers were used to make PS brushes. The molecular weights (in thousands) of these PVP-PS materials were [114/103]k, [30/70]k, respectively. Structural and frictional force profiles in toluene and cyclohexane were measured, and the cyclohexane experiments were conducted at temperatures ranging from the theta-point to 50°C. In toluene the PS brushes needed to be compressed to ∼1/5th of their equilibrium height before frictional forces were measured, but this onset of frictional forces was detected at a much lower level of compression in near-theta cyclohexane. In cyclohexane the structural force profiles were basically insensitive to the temperature change, but the frictional forces depended strongly on the solvent temperature. When the cyclohexane temperature was raised, the onset of frictional forces decreased toward the good-solvent onset. We also discuss the dependence of frictional force on shearing parameters.
In Situ Measurement of the Second Harmonic Signal of Adsorbing Nonlinear Optical Ionically Self-assembled Monolayers
Authors: Brands C. , Neyman P.J. , Guzy M.T. , Shah S. , Van Cott K.E , Davis R.M. , Wang H. , Gibson H.W. , Heflin J.R.
Published: April 2011

Ionically self-assembled monolayers (ISAMs) have recently been shown to spontaneously exhibit a polar ordering that gives rise to substantial second order nonlinear optical response. The deposition of ISAMs has been studied in situ via second harmonic generation. This is a particularly sensitive probe of the growth of nanometer-thick films since the centrosymmetry of the immersion solutions, the substrate, and the container yields no SHG contribution from these bulk components. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute. When a film is immersed into salt water, the SHG decreases significantly only to be restored when the salt solution is replaced with deionized water.
Nanoscratch Behavior of Sputtered Metal Films on PAMAM Dendrimer Monolayers
Authors: Xu Feng-Ting , Curry Mike , Huang Feng , Ye Ping-Ping , Rar Andrei , Street Shane C. , Barnard John A.
Published: April 2011

Striking differences have been observed in the deformation modes associated with 2mN constant load nanoscratch events on ultrathin Cr layers deposited on SiOx with and without a dendrimer interlayer. In the absence of the dendrimer distinct ridge formation along the wear track indicative of plowing and wedge formation are observed. By contrast, dendrimer monolayer-mediated films exhibit ridgeless scratches apparently formed in a nearly pure cutting mode. In addition, the width of the scratch trench in the sample without dendrimer is larger than the one with dendrimer.
Molecular Weight Dependence of the Surface Glass Transition of Polystyrene Films Investigated by the Embedding of Gold Nanoclusters
Authors: Erichsen J. , Günther-Schade K. , Dolgner K. , Strunskus T. , Zaporojtchenko V. , Faupel F.
Published: April 2011

We probed the glass transition temperature Tg at the surface of polystyrene (PS) samples with different molecular weights by embedding of gold nanoclusters. The embedding of the nano-clusters was monitored by x-ray photoelectron spectroscopy (XPS). The onset embedding temperature determined by XPS was taken as a measure for the Tg at the surface. We observed a small decrease of Tg at the surface relative to the bulk for all molecular weights (ΔTg= 2.5- 8.5 K). This decrease is small for short chains (MW = 3.6 kg/mol) and grows to a saturation for MW ≥ 44 kg/mol (ΔTg ≍ 8 K). This is in accord with the smaller relative increase in the number of degrees of freedom between bulk and surface for shorter chains.
X-ray Reflectivity Study of Gold Nanoparticles in a Polymer Matrix
Authors: Shin Kwanwoo , Wang Howard , Satija Sushil K. , Han Charles C. , Josell Daniel
Published: April 2011

We have examined the structural evolution of a thin gold layer sandwiched between two polymer films using specular X-ray reflectivity. We used a depth-profiling model consisting of three independent layers, upper and bottom PS layers and an intermediate gold layer. The results show that a well-defined as-deposited Au layer of about 62 ± 3 Å breaks within 10 min of annealing at 120 °C. During the breakage of the Au film, the bottom low-Mw PS flows to fill into the void between the Au particles, causing the reduction of the bottom PS film, while the top polymer layer remains relatively constant.
Diffusion Limited Aggregation of PMMA Stereocomplex in Thin Films
Authors: Grohens Yves , Castelein Gilles , Carriere Pascal , Spevacek Jiri
Published: April 2011

The nanoscale patterns formed by poly(methyl methacrylate) (PMMA) stereocomplexes at the surface of silicon wafers, glass and mica, were investigated by tapping mode atomic force microscopy (TM-AFM). The effects of the solvent nature, PMMA concentration, i/s-ratio (stoechimetry) and surface nature on the morphology of the stereocomplex thin layer at a surface were addressed. The aggregation phenomena are well described by the diffusion limited cluster-cluster aggregation model (DLA) and the fractal exponent D calculated. The i/s-ratio strongly influences the fractal exponent D which is equal to 1.35 for the 1:2 ratio is lower than for the other i:s ratios which are 1.46, 1.61, 1.82 for 1:1, 2:1 and 4:1 ratios, respectively. The low values of the fractal dimension D are indicative of a fast aggregation process and higher values of D correspond to a slow aggregation process.
Near-Interface Structure and Mobility of Polymeric Chains in Polymer-Based Nanocomposites
Authors: Picu Catalin R. , Ozmusul Murat S.
Published: April 2011

The effect of a curved solid wall on the conformations of long, flexible polymer chains is studied in a dense polymer system and in the athermal limit by means of lattice Monte Carlo simulations. It is found that the wall does not perturb the atom number density, which is constant throughout the volume of the polymer, however the chain end density is higher at the wall compared to the bulk. The segregation effect decreases with increasing wall curvature. The bonds are preferentially oriented in the direction tangential to the wall over a two-bond thick layer next to the wall. When probing the preferential orientation on the chain segment length scale, the perturbed region has a thickness on the order of the probing metric considered. The chain mobility is evaluated by computing the bead diffusion coefficient in the direction perpendicular and tangential to the curved wall, and as a function of the distance from the interface. The bead mobility is reduced in a two-bond-thick layer in the vicinity of the wall in both the normal and tangential directions, the reduction being more pronounced in the normal direction. The mobility is seen to decrease with increasing wall curvature. The reduction is most pronounced when the particle size becomes smaller than the chain radius of gyration in the bulk, situation in which the average chain surrounds the particle, which hence becomes an effective entanglement site for the chains. This explains in part the increased strength of polymer-based nanocomposites with small fillers.
Frequency Specific Characterization of Very Soft Polymeric Materials Using Nanoindentation Testing
Authors: Conte Nicolas , Jardret Vincent
Published: April 2011

Polymer thin films and surfaces play a major role in the functionality of many components in the microelectronic and automotive industries. The characterization of their mechanical properties at a nanometer scale remains a technological challenge. Instrumented indentation testing with frequency-specific dynamic analysis provides access to viscoelastic measurement of near surface properties of polymeric materials. Development of biomaterials for tissue replacement raises the interest for testing polymers with storage modulus of less than 1 MPa. The development of specific testing methodologies with improved accuracy and precision for evaluating very soft polymeric materials are presented in this work. Results at various frequencies for polyurethane and polydimethylsilane using both Berkovich and Flat punch geometries are compared.
Microprobing of Interfacial Behavior of Solid Polymers by Means of Normal Mechanical Contacts
Authors: Garif Y. S. , Gerberich W. W. , Macosko C. W. , Pocius A.
Published: April 2011

Adhesion properties of acrylic pressure sensitive adhesives were examined by means of normal (JKR) contact at room temperature. Cylindrically shaped samples were synthesized in capillary tubes in presence of a cross-linking agent in order to obtain an elastic response. The results reveal power law increase of adhesion at higher rates of interfacial separation. Transition from low- to high-power is revealed and explained as a result of bulk shielding of interfacial fracture mechanism. Further results will be available soon. Bulk viscoelastic dissipation and molecular interactions at the interface are thought to jointly govern this behavior.
Measurement of Polymer-Polymer Adhesion in Thin Multi-Layer Systems by Means of Micro-Scratch Probing
Authors: Garif Y. S. , Gerberich W. W. , Macosko C. W. , Pocius A. V.
Published: April 2011

A novel technique called Cross-Sectional Scratch (CSS) has been established. The technique employs a sliding contact of an indenter tip across a polymer stack cross-section. The main advantage of the technique is that it allows to measure/quantify interfacial adhesion in multilayer polymer films that are only a few microns thick. CSS has successfully tested both glassy/glassy and glassy/soft polymer interfaces. Thus, PS/PE, PET/PE, PS/PMMA and PVdF/PMMA co-extruded films were chosen as sample systems and tested at room temperature. It was found that indenter speed has a strong effect on the magnitude of the adhesion energy measured. The dependence has a power law form with indices ∼ 0.1-0.2. More viscoelastic systems have higher power law index, a measure of rate sensitivity.
Bulk Modulus and Density Measurements of Small Compressible and Incompressible Samples
Authors: Hay Jack C. , Lucas Barry N.
Published: April 2011

A novel system is described for determining physical properties, including bulk modulus and density, of small solid samples. The system consists of a chamber in which the sample weight is measured by a weighing device immersed in a gas of controllable density. Thus, the method of measuring density is based on Archimedes' principle where the weight of an object is reduced by the weight of the displaced fluid. This particular device has been designed for examining the density of disk-shaped samples 3 mm in diameter and 0.4 mm thick. The weighing device has a repeatability of 4 nN and sample densities can be determined to 0.5%.

A significant feature of this device is the ability to measure buoyancy forces at a plurality of gas densities, which allows one to capture nonlinear behaviors associated with closed-cell compressible media. Results are presented for a quasi-closed cell foam that experiences volume reduction as the gas pressure is increased. Volumetric strains are determined as the difference between the observed behavior and the linear behavior of incompressible media. Plots of hydrostatic stress versus volumetric strain are initially linear, as described by the bulk modulus, and exhibit a “kink” at high pressures, presumably due to the complete compression of internal cells.
Investigation of the Melting Behavior of the Alternating PEO/PCL Multiblock Copolymer by SAXS and WAXS
Authors: Kim Tae Chul , Kim Jong Ho , Lee Hyun Hwi , Bae You Han , Noh Do Young
Published: April 2011

The melting of the semicrystalline poly(ethylene oxide)(PEO)/ poly(ε -caprolactone) (PCL) multiblock copolymer was investigated by small and wide angle x-ray scattering. The two-stage melting was found in which the PEO block and the PCL block melt independently. The crystalline PEO Bragg peaks disappear at 30.5 °C while the block spacing increases due to the chain relaxation. The PEO crystalline domain size decreases continuously in proportion to the amount of the crystalline part. Above 40 °C, the PCL lamellae domains started to melt and the integrated intensity of the PCL Bragg peak decreases with increasing temperature. In the case of the PCL, however, the crystal domain size decreases only slightly. This indicates that the number of the PCL crystalline domains decreases during the melting.
Assessing Structure-Property Relations of Diseased Tissues Using Nanoindentation and FTIR
Authors: Ebenstein Donna M. , Chapman Joan M. , Li Cheng , Saloner David , Rapp Joseph , Pruitt Lisa A.
Published: April 2011

Disease processes are often associated with changes in tissue composition. For example, in atherosclerosis lipid and calcification are often found in the artery wall, whereas in healthy arteries the tissue microstructure is dominated by highly organized collagen. Such variations in composition likely result in changes in the material properties of the tissue. However, this relationship has not been fully investigated in atherosclerotic vessels. Using a combination of nanoindentation and spectroscopic techniques, our goal was to assess how changes in tissue composition affect the tissue's mechanical properties. Fourier Transform Infrared Spectroscopy (FTIR) was used to assess the biochemical composition of the tissue samples, such as the lipid and calcium content of fibrous tissues in diseased arteries. Nanoindentation was used to measure the local mechanical properties of the same tissue samples. This information was then correlated by position in the sample to assess the contributions of different constituents to the overall structure-property relations of these tissues.
Nematic liquid crystalline formation of F-actin displays features of a continuous transition
Authors: Viamontes Jorge , Tang Jay X.
Published: April 2011

The phase transition of solutions of the protein filaments F-actin from the isotropic (I) to the nematic (N) liquid crystalline state was studied by quantitative measurements of optical birefringence and fluorescence. The threshold protein concentration for the transition varies inversely with the average filament length, consistent with the prediction of statistical mechanics of rodlike suspensions based on the excluded volume effect. By measurements of local optical birefringence, a range of actin concentration is identified as the transition region between the isotropic and nematic phases. However, local measurements of the protein concentrations detect no discontinuity within a large number of samples in the transition region, suggesting that the I-N transition for F-actin occurs continuously over a defined concentration range. Thus the I-N transition appears to be of a higher order than the 1st, for F-actin of average filament length 3 μm or longer. Additionally, by mixing a tiny number of labeled actin filaments with the unlabeled ones, we observed thermal motions of individual filaments, thus ruling out an alternative picture, viewing an F-actin solution as an amorphous gel in which long filaments are too entangled or even cross-linked to allow partitioning of filaments into domains of discontinuous concentrations. We propose based on these experimental findings that either the extreme polydispersity of F-actin, or the large average filament length itself, renders the I-N transition to be a continuous one, in terms of both the average alignment of filaments and the protein concentration.
A Quartz Crystal Microbalance Cell Biosensor: Detecting Nocodazole Dependent Microtubule Disruption Dynamics In Living Cells
Authors: Marx Kenneth A. , Zhou Tiean , Montrone Anne , Braunhut Susan J.
Published: April 2011

A Quartz Crystal Microbalance (QCM) was used to create a biosensor utilizing living adherent endothelial cells (ECs) as the biological sensing element. This EC QCM biosensor detected the effect of varying concentrations of nocodazole, a microtubule binding and disrupting drug, on the adherent cells as they altered the underlying QCM device state frequency, Δf, and motional Resistance, ΔR, shift values. Over the dose range 0.11-15 μM nocodazole, the Δf shift values decreased significantly in magnitude in a dose dependent fashion over a 5-6 hr incubation period following drug addition to a limiting value, with a 900 nM midpoint. This effect is consistent with nocodazole's known dose dependent effect on the disruption of microtubules. At all drug concentrations, the relative Δf decrease with time was found to be very similar and well fit by a single exponential decay equation. For all nocodazole doses, t 0.5 was invariant, averaging t 0.5 = 0.83 ± 0.14 hr. These data demonstrate that a single dynamic sensing system within the cell, the microtubules, responds to the addition of nocodazole and its response can be quantified by the biosensor. These results indicate that the EC QCM biosensor can be used to detect EC cytoskeletal alterations and dynamics and may be a valuable screening method for all classes of biologically active drugs or biological macromolecules that affect cytoskeleton perturbations or cellular attachment.
Low Voltage Table-Top Electron Microscopy of Polymer and Organic Molecular Thin Films
Authors: Martin David C. , Drummy Lawrence F. , Yang Junyan , Coufalova Eva
Published: April 2011

There is intense current interest in the use of polymer and organic molecular materials for high technology devices including transistors, biosensors, and actuators [1]. In order for these materials to achieve their ultimate potential it is imperative to obtain detailed information about their microstructure, especially in the thin film forms central to technological applications. Our research group has been actively developing techniques for high-resolution electron optical examinations of polymers and organic materials, with particular emphasis on low dose high-resolution electron microscopy (HREM) [2]. While this method of examining organic materials' structure has proven to be particularly powerful, current generations of electron microscopes suffer from several problems. They are expensive, require considerable amounts of space, and are time-consuming and fairly difficult to operate. Furthermore, the high voltages normally used in conventional electron microscopes (200-400 kV) provide little contrast when imaging thin films composed of low atomic number components.
Characterisation of Biomedical Materials, Cells & Interfaces using Environmental SEM (ESEM)
Authors: Stokes D.J. , Rea S.M. , Porter A. E. , Best S. M. , Bonfield W.
Published: April 2011

The ability of Environmental Scanning Electron Microscopy (ESEM) to image insulating and/or moist specimens without the need for the removal of volatile components or the application of a conductive coating has significantly increased the potential range of experiments and observations that can be performed at the high resolution of electron microscopy. Such a technological advance has particularly important implications for the study of soft matter, complex fluids and biological specimens [1]. Thus an important field of research to which ESEM can be applied is the study of materials for biomedical applications such as tissue engineering. The bioactivity of these materials is dependent upon such factors as phase composition, chemical composition, surface activity, crystallinity and microstructure. Using ESEM it is possible to obtain surface-sensitive, specimen-dependent secondary electron images (in the absence of specimen coating), yielding potentially new perspectives on microstructure to complement information derived from other techniques. We have used ESEM to study the apposition of bone on hydroxyapatite-based biomedical materials, from both in vitro and in vivo investigations.
Optical Characterization of Biological and Other Systems
Authors: Bezrukova Alexandra G.
Published: April 2011

Multiparametric optical analysis (MOA) can provide further progress in characterization and on-line control of biological and other disperse systems with nanoparticles as the disperse phase in various dispersive media. MOA includes the nondestructive simultaneous testing of disperse systems by different compatible optical methods such as refractometry, absorption, fluorescence, light scattering (integral and differential, static and dynamic, unpolarized and polarized). For the solution of inverse optical problems various techniques can be applied. By optical methods it is possible to determine parameters of disperse system's state (mean equivalent diameter and number of particles, mean refractive index and mass of disperse phase, number and mass distributions) and parameters of particle's structure. This paper is devoted to characterization of biological particles that can be approximated as bilayered spheres (blood-substitutes, liposomes, etc.) and to polarization measurements of disperse systems (viruses, bacterial cells, etc.) which can help to develop on-line control of environment.
Characterization of Chemically and Topographically Modified Siloxane Elastomer for Controlled Cell Growth
Authors: Gibson Amy L. , Wilson Leslie H. , Wilkerson Wade R. , Feinberg Adam W. , Seegert Charles A. , Baney Ronald H. , Brennan Anthony B.
Published: April 2011

A main limitation of biomedical devices is the inability to start, stop, and control cell growth making it crucial to develop biomaterial surfaces that induce a desired cellular response. Micropatterns of ridges and pillars were created in a siloxane elastomer (Dow Corning) by casting against epoxy replicates of a micromachined silicon wafer. Silicone oils were incorporated to determine the change in modulus and surface energy caused by these additives. SEM and white light interference profilometry verified that the micropatterning process produced high fidelity, low defect micropatterns. Mechanical analysis indicated that varying the viscosity, weight percent and functionality of the added silicone oil could change the elastic modulus by over an order of magnitude (0.1-2.3 MPa). As a self-wetting resin, silicone oils migrate to the surface, hence changing the surface properties from the bulk. Both topographical and chemical features define the surface energy, which in combination with elastic modulus, dictate biological activity. The results imply that the morphology, mechanical properties and surface energy of the siloxane elastomer can be modified to elicit a specific cell response as a function of engineered topographical and chemical functionalization.
Polymeric Tissue Culture Substrates patterned by UV Irradiation
Authors: Welle Alexander , Gottwald Eric , Weibezahn Karl-Friedrich , Dertinger Herman
Published: April 2011

We studied the physico/chemical effects of deep UV irradiation of polystyrene, PMMA and polycarbonate with respect to cell adhesion and protein immobilization. Photochemical modifications of the polymer surfaces yielded unstable peroxides and carboxylic acid groups. Patterned protein adsorbates were realized by coupling via carbodiimid activation of the COOH moieties. We have immobilized enzymes and antibodies. It was observed that hepatoma cells (HepG2) and fibroblasts (L929) adhered in the presence of serum proteins in the culture medium on the irradiated regions of the substrate without any further treatment.
Controlled Growth Factor Delivery By Mechanical Stimulation
Authors: Lee Kuen Yong , Peters Martin C. , Anderson Kenneth W. , Mooney David J.
Published: April 2011

Growth factor delivery using polymer matrices is one exciting approach to replace or regenerate tissues. Most growth factor delivery systems, however, have been designed to operate under static conditions, regardless of dynamic environments in the body. Considering the dynamic environment of our body (e.g., bone, muscle, and blood vessel), mechanical stimulation is an important signal that could be readily exploited. We hypothesize that polymer matrices, which release growth factors in response to mechanical stimulation, could provide a novel approach to engineer tissues in mechanically stressed environments. We report here a model system, comprised of alginate hydrogel and vascular endothelial growth factor (VEGF), which upregulates the release of the growth factor in response to mechanical stimulation and subsequently promotes granulation tissue formation in animals. This approach may find a number of potential applications in tissue engineering, as well as in drug delivery.
An In Vitro Study of Nano-fiber Polymers for Guided Vascular Regeneration
Authors: Miller Derick C. , Thapa Anil , Haberstroh Karen M. , Webster Thomas J.
Published: April 2011

Biomaterials that successfully integrate into surrounding tissue should match not only the tissue's mechanical properties, but also the dimensions of the associated nano-structured extra-cellular matrix (ECM) components. The goal of this research was to use these ideals to develop a synthetic, nano-structured, polymeric biomaterial that has cytocompatible and mechanical behaviors similar to that of natural vascular tissue. In a novel manner, poly-lactic acid/polyglycolic acid (PLGA) (50/50 wt.% mix) and polyurethane were separately synthesized to possess a range of fiber dimensions in the micron and nanometer regime. Preliminary results indicated that decreasing fiber diameter on both PLGA and PU enhanced arterial smooth muscle cell adhesion; specifically, arterial smooth muscle cell adhesion increased 23% when PLGA fiber dimensions decreased from 500 to 50 nm and increased 76% on nano-structured, compared to conventional structured, polyurethane. However, nano-structured PLGA decreased endothelial cell adhesion by 52%, whereas adhesion of these same cells was increased by 50% on polyurethane. For these reasons, the present in vitro study provides the first evidence that polymer fiber dimensions can be used to selectively control cell functions for vascular prosthesis.