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Results for “Polystyrenes”

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At least 991 records · Page 55Linked to original sources

PubMed2001

X-ray reflectivity study on the surface and bulk glass transition of polystyrene.

The surfaces of polystyrene (PS) films decorated with gold nanoclusters were investigated by x-ray reflectivity measurements. The thicknesses of the films are much larger than the radii of gyration of the different PS samples. By annealing the films above the glass transition temperature T(g) an embedding process of the clusters into the polymer is detected which is accompanied by a substantial increase in the cluster layer thickness due to Brownian motion. These processes start at a sufficiently low viscosity and may be regarded as a probe of the glass transition in the near surface region of the PS films. Simultaneously the thermal expansion of the entire film and hence its approximate bulk behavior were monitored. Two samples of different molecular weight do not show a significant difference between the surface and bulk T(g) values.

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PubMed2002

Time evolution of dynamic shear moduli in a physical gelation process of 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol in polystyrene melt: critical exponent and gel strength.

We investigated time evolution of shear moduli in the physical gelation process of 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol (PDTS) in the polystyrene melt system containing 2.5 wt % of PDTS. At the gel point, storage and loss shear moduli, G' and G", were described by the power law of frequency omega, G' approximately G" approximately omega(n), with the critical exponent n equal to 2 / 3, in agreement with the value predicted by the percolation theory. The exponent n and the gel strength S at the gel point measured as a function of quench depth DeltaT indicated that the fractal network structure of PDTS does not change with DeltaT but that the mechanical strength of the network increases with DeltaT. Before reaching the gel point, G'(omega) and G"(omega) obtained at various times can be well superposed onto the master curves, indicating that the mechanical as well as structural self-similarity hold in this gelation process.

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PubMed2003

Relaxation of a rubbed polystyrene surface.

The relaxation dynamics of a rubbed polystyrene (PS) surface have been characterized using infrared-visible sum frequency generation spectroscopy (SFG). The SFG results were compared with previous relaxation of retardation measurements, and the results show that the rubbed PS surface has the same T(g) as the bulk where T(g) is defined as tau(T(g))=5 s, however, the surface has a lower activation energy (deltaE) and a larger stretching exponent (beta(KWW)) than bulk PS. This indicates that the surface region relaxes more quickly than the bulk. The thickness of this region of lower deltaE and larger beta(KWW) is estimated to be roughly 12 nm.

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PubMed2004

Polystyrene thin films in CO(2).

In air, or vacuum environments, liquid polystyrene (PS) thin films (thickness, h<100 nm ) supported by SiOx /Si substrates are structurally metastable or unstable, depending on film thickness. They rupture and eventually form droplets on the SiOx /Si substrates (dewet) due to the influence of destabilizing long-ranged van der Waals dispersion forces. We used scanning force microscopy to examine the structural stability of liquid PS films in the thickness range 5 nm<h<100 nm in liquid and in supercritical carbon dioxide ( CO2 ) environments. All films in this thickness range were metastable; holes developed throughout the films and over time these holes grew, impinged, and eventually formed droplets. The rate of destabilization is controlled by three factors: film thickness, temperature, and CO2 pressure (which dictates CO2 volume fraction in the films). Calculations of the effective interface potentials suggest that the energy barrier for nucleation and growth of holes in CO2 is larger than that in air, and in the limit of vanishingly low PS volume fraction the films should be stable.

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PubMed2005

Petawatt-laser direct heating of uniformly imploded deuterated-polystyrene shell target.

A uniformly imploded deuterated polystyrene (CD) shell target is fast-heated by a Petawatt (PW) laser without cone guide. The best illumination timing is found to be in a narrow region around 80+/-20 picoseconds from the onset of the stagnation phase, where thermal neutrons are enhanced four to five times by the PW laser of energy less than 10% of the implosion laser. The timing agrees with the timings of enhancement of the x-ray emission from the core and reduction of the bremsstrahlung radiation from scattered hot electrons. The PW laser, focused to the critical density point, generates the energetic electrons within as narrow an angle as 30 degrees , which then heats the imploded CD shell to enhance thermal neutrons. These results first demonstrate that the PW laser directly heats the imploded core without any conelike laser guide.

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PubMed2005

Pattern formation in thin polystyrene films induced by an enhanced mobility in ambient air.

Thin polystyrene films (prepared in a sandwich geometry between two aluminum electrodes) develop a characteristic pattern in ambient air when kept above the glass-transition temperature, whereas they remain unchanged in vacuum or in a pure nitrogen atmosphere. Measurements by broadband dielectric spectroscopy, capacitive dilatometry, and AC calorimetry reveal that the pattern formation is initiated in ambient air by an increase in the average relaxation rate of the dynamic glass transition, which results in a corresponding reduction of the glass-transition temperature. Infrared measurements evidence that this is caused by a shift of the molecular weight distribution to lower values as a consequence of oxygen-induced chain scissions.

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PubMed2005

Evidence of convective constraint release during hole growth in freely standing polystyrene films at low temperatures.

Hole growth measurements were performed using optical microscopy on freely standing polystyrene films at temperatures that were slightly larger than the bulk value of the glass transition temperature T(bulk)g. For the measured range of temperatures, we have observed a transition from linear growth of the hole radius R during the early stages to exponential growth of R at later times. We have characterized this transition as a function of molecular weight 120 x 10(3) < Mw <2240 x 10(3) , film thickness 61 nm<h<125 nm , and temperature 101 degrees C<T<117 degrees C . The viscosity at the edge of the hole inferred from the long time exponential growth regime exhibits shear thinning due to the large shear strain rates present at the edge of the hole. The R (t) data for all times can be fit very well using an expression that describes exponential hole growth with a time-dependent viscosity that allows for an initial, transient response due to the decay of elastic entanglements. The time scale for the decay of the transient behavior is interpreted in terms of the decay of entanglements by the convective constraint release mechanism of the tube theory of entangled polymer dynamics.

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PubMed2005

Hole growth in freely standing polystyrene films probed using a differential pressure experiment.

We have probed the whole chain mobility of polymer molecules confined to freely standing films by measuring the flow of gas through holes growing in the films at elevated temperatures using a differential pressure experiment. Freely standing polystyrene films were measured for the temperature range 92 degrees C<T<105 degrees C for films with two different molecular weights Mw=717 x 10(3) and 2240 x 10(3) , with thicknesses 51 nm<h<97 nm . This range of film thicknesses is of particular interest because large reductions in the glass transition temperature Tg have been measured previously for freely standing PS films in this thickness range. We find that hole formation and growth, and therefore substantial chain mobility, does not occur until temperatures close to the bulk value of the glass transition temperature T(bulk)g. The characteristic growth times tau for the thinnest films, which have reduced values of Tg, are not substantially less than those for thicker films, and we find that these small differences in tau can be understood in terms of the bulk phenomenon of shear thinning. We also show that the viscosity at the edge of the hole inferred from the characteristic growth times obtained in this and previous studies exhibit shear thinning with reduced shear strain rates beta that span twelve orders of magnitude.

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PubMed2006

Measured long-ranged attractive interaction between charged polystyrene latex spheres at a water-air interface.

We report results of a systematic experimental study of interactions between charged polystyrene (PS) latex spheres at a water-air interface. Optical observations of stable bonded particle clusters and formation of circular chainlike structures at the interface demonstrate that the interaction potential is of dipole origin. Atomic force microscopy (AFM) is used to examine the distribution of charge groups on the colloidal surface. AFM phase images show patchy domains of size approximately 100 nm on the particle surface, indicating that the surface charge distribution of the PS spheres is not uniform, as is commonly believed. Such patchy charges can introduce fluctuating in-plane dipoles, leading to an attraction at short interparticle separations. A theoretical analysis is given to explain the mechanism for attractions between like-charged particles at the interface.

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PubMed2006

Low-energy excitations and the fast process of polystyrene thin supported films studied by inelastic and quasielastic neutron scattering.

We studied the low-energy excitations in the meV region as well as the picosecond fast process in polystyrene thin supported films using inelastic and quasielastic neutron scattering in a temperature range from 11 to 430 K , covering the bulk glass transition temperature Tg. It was found that the mean square displacement u2 decreased with the film thickness below and above the glass transition temperature Tg, suggesting that the hardening occurs with decreasing the film thickness. Corresponding to the decrease in u2, it was also found that the density of phonon states G(omega) decreased with film thickness. This decrease occurs mainly in the Debye mode while the contribution of the boson peak mode also decreases with the film thickness without changing the boson peak energy. As for the fast process, which appeared above at about 150 K as a quasielastic scattering, the fraction A(fast)(Q) decreased with the film thickness in a similar manner with the boson peak without changing the relaxation rate, suggesting a common origin for the boson peak and the fast process. These observations showing the hardening were well explained by assuming a hard dense layer at the interface, where the numbers of the boson peak mode as well as the fast process are less than the bulk state.

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PubMed2000

Absence of scaling for the intermediate scattering function of a hard-sphere suspension: static and dynamic x-ray scattering from concentrated polystyrene latex spheres.

X-ray photon correlation spectroscopy and small-angle scattering measurements are presented of the dynamics and structure of concentrated suspensions of charge-stabilized polystyrene latex spheres dispersed in glycerol, for volume fractions from 3% to 52%. The static structures of the suspensions show essentially hard-sphere behavior, and the short-time dynamics shows good agreement with predictions for the wave-vector-dependent collective diffusion coefficient. However, the intermediate scattering function is found to violate a scaling behavior found previously for a sterically stabilized hard-sphere suspension.

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PubMed2000

Molecular structure of polystyrene at Air/Polymer and Solid/Polymer interfaces

IR-visible sum-frequency generation (SFG) spectroscopy has been used in a total internal reflection geometry to study the molecular structure of polystyrene (PS) at PS/sapphire and PS/air interfaces, simultaneously. The symmetric vibrational modes of the phenyl rings dominate the SFG spectra at the PS/air interface as compared to the antisymmetric vibrational modes at the PS/sapphire interface. This indicates approximately parallel orientation of the phenyl rings at the PS/air interface while nearly perpendicular orientation at the PS/sapphire interface, with respect to the surface normal.

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PubMed2003

Free surfaces cause reductions in the glass transition temperature of thin polystyrene films.

The effect of free surfaces on the glass transition temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm<h<290 nm) with one free surface and on films capped with a 5 nm thick metal layer (no free surfaces). Potential problems with evaporative deposition were eliminated by studying samples made of two supported films placed with their free surfaces in contact and annealed. Uncapped films displayed reduced T(g) values for h less, similar 40 nm while all "properly" capped films exhibited a T(g) value the same as that of the bulk polymer (370+/-1 K). When the free surface was restored, the measured T(g) values the same as those of uncapped films of the same thickness. These results show that free surfaces are crucial for observing T(g) reductions in thin polymer films and address the role of the sample preparation history.

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PubMed2005

Experimental benchmark for an improved simulation of absolute soft-x-ray emission from polystyrene targets irradiated with the Nike laser.

Absolutely calibrated, time-resolved spectral intensity measurements of soft-x-ray emission (hnu approximately 0.1-1.0 keV) from laser-irradiated polystyrene targets are compared to radiation-hydrodynamic simulations that include our new postprocessor, Virtual Spectro. This new capability allows a unified, detailed treatment of atomic physics and radiative transfer in nonlocal thermodynamic equilibrium conditions for simple spectra from low-Z materials as well as complex spectra from high-Z materials. The excellent agreement (within a factor of approximately 1.5) demonstrates the powerful predictive capability of the codes for the complex conditions in the ablating plasma. A comparison to data with high spectral resolution (E/deltaE approximately 1000) emphasizes the importance of including radiation coupling in the quantitative simulation of emission spectra.

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PubMed1975

Chromatography of Cytokinins on a Neutral Polystyrene Resin: A Simple Procedure for the Separation of the cis and trans Isomers of Zeatin or Ribosylzeatin.

A simple procedure for the separation of the cis and trans isomers of zeatin and ribosylzeatin by column chromatography on a neutral polystyrene resin, Porapak Q, in aqueous ethanol solutions is reported. The method has been used to examine the stereoisomer composition of ribosylzeatin isolated from wheat germ transfer RNA. Chromatographic data for several other cytokinins are also presented.

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PubMed1992

An enzyme-linked antiglobulin test to quantify nanogram quantities of IgG on polystyrene microspheres.

A method is described in which polystyrene latex beads are used for constructing standard curves to estimate the number of protein molecules on cell surfaces by an enzyme-liked immunoassay test (ELAT). A series of immune globulin (IgG) dilutions in pH 9.8 carbonate buffer were coated on 3-microns microbeads by incubation overnight at 4 degrees C and subjected to ELAT. The r value of the curve derived from four assays was 0.9991. This standard curve applied to previously recorded ELAT data resulted in estimating that normal RBC have 63 +/- 19 (SD) IgG molecules and that the lower level of sensitivity of the antiglobulin test is 155 IgG molecules per RBC. The method should be useful for more precise standardization of procedures for measuring proteins on cell surfaces.

Enzyme-Linked Immunosorbent Assay↗

PubMed1981

Positive identification of human leukaemic cells with scanning immuno-electron microscopy, using antibody coated polystyrene (latex) beads as markers.

Scanning immuno-electron microscopic labelling techniques were successfully used to identify a variety of leukaemia-lymphoma cells. Anti-T, -B and -common type acute lymphoblastic leukaemia (ALL) antisera were covalently conjugated to polystyrene latex spheres by means of glutaraldehyde. Peripheral blood mononuclear cells obtained from healthy individuals and from patients with T-type ALL, B-type chronic lymphocytic leukaemia (CLL), acute and chronic non-lymphoblastic leukaemias (ANLL) and cultured leukaemia-lymphoma cells were incubated with the above antisera-latex conjugates. Positively labelled cells were easily identified by the presence of latex spheres on their surfaces. Cross reactivity of the antisera used and non-specific attachment of the markers were infrequently encountered. The results of this study confirm earlier scanning electron microscopic reports indicating that different leukaemic cells have characteristic surface features and show that scanning immuno-electron microscopy (SIEM) can readily be applied to the study of human leukaemic cells.

Humans↗

PubMed1990

Effect of cetylpyridinium chloride on microbial adhesion to hexadecane and polystyrene.

Microbial adhesion at the oil-water interface is a subject of both basic interest (e.g., as a technique for the measurement of hydrophobicity) and applied interest (e.g., for use in two-phase oil-water mouthwashes for the desorption of oral microorganisms). In general, surfactants inhibit microbial adhesion to oils and other hydrophobic surfaces. In the present study, we demonstrated that the cationic surfactant cetylpyridinium chloride (CPC) significantly enhanced microbial adhesion to hexadecane and various oils, as well as to the solid hydrophobic surface polystyrene. CPC increased adhesion to hexadecane of Escherichia coli, Candida albicans and Acinetobacter calcoaceticus MR-481 and of expectorated oral bacteria from near 0% to over 90%. The CPC concentration required for optimal enhancement of adhesion was a function of the initial cell density. This phenomenon was inhibited by high salt concentrations and, in the case of E. coli, by a low pH. CPC-pretreated cells were able to bind to hexadecane, but CPC-pretreated hexadecane was unable to bind untreated cells. Another cationic, surface-active antimicrobial agent, chlorhexidine gluconate, was similarly able to promote microbial adhesion to hexadecane. The results suggest that (i) CPC enhances microbial adhesion to hexadecane by binding via electrostatic interactions at the cell surface, thus diminishing surface charge and increasing cell surface hydrophobicity, and (ii) this phenomenon may have applications in oral formulations and in the use of hydrocarbon droplets as a support for cell immobilization.

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