Dewetting Properties of Polystyrene Homopolymer Thin Films on Grafted Polystyrene Brush Surfaces

We have found that the critical molecular weight for auto dewetting of a homopolymer on a polymer brush of the same chemical composition is approximately NH = 0.5 NB, in good agreement with the mean field theory prediction. The measured velocity on the brush surface is at least an order of magnitude faster than that of the polystyrene (PS) on a homopolymer interface, with a greatly decreased dependence on the PS molecular weight. This suggests slippage as a possible mechanism for the dewetting dynamics.

Spatial and Temporal Dependence of Diffusion in Polystyrene Thin Films on Silicon and Carbon Surfaces

The strong inhibition of chain diffusion in thin polystyrene (PS) films has been observed near an attractive silicon surface, and diffusion remains inhibited out to distances of several radii of gyration from the surface. The present study seeks to determine the time dependence of the diffusion coefficient, and to examine the effect of a carbon surface on this diffusion. The sputter-deposited carbon surface may serve as a model for carbon-black particles employed in nanocomposites, which have recently been observed to reduce diffusion throughout a nanocomposite layer. The experiments employed a thin (∼15 nm) deuterated polystyrene (dPS) marker layer sandwiched between two normal PS layers. Deuterium profiles were monitored in the annealed samples by secondary ion mass spectrometry. Strong segregation was observed at the silicon surface, but was inhibited at the carbon surface, allowing the diffusion behaviour to be studied in the latter case over longer annealing times. A finite-element computer program was developed to fit the observed diffusion profiles. The variation of the diffusion coefficient with depth is shown to be consistent with previous results, and diffusion is comparable at both the carbon and silicon surfaces. The diffusion coefficient decreases roughly in proportion to t−1/2, and is discussed in the context of reptation theory.

Evaluation of fibrinogen self-assembly: role of its αC region

BACKGROUND

Exposure of cryptic, functional sites on fibrinogen upon its adsorption to hydrophobic surfaces of biomaterials has been linked to an inflammatory response and fibrosis. Such adsorption also induces ordered fibrinogen aggregation which is poorly understood.

OBJECTIVE

To investigate hydrophobic surface-induced fibrinogen aggregation.

METHODS

Contact and lateral force scanning probe microscopy, yielding topography, image dimensions and fiber elastic modulus measurements were used along with transmission and scanning electron microscopy. Fibrinogen aggregation was induced under non-enzymatic conditions by adsorption on a trioctyl-surface monolayer (trioctylmethylamine) grafted onto silica clay plates.

RESULTS

A more than one molecule thick coating was generated by adsorption on the plate from 100 to 200 μg mL⁻¹ fibrinogen solutions, and three-dimensional networks formed from 4 mg mL⁻¹ fibrinogen incubated with uncoated or fibrinogen-coated plates. Fibrils appeared laterally assembled into branching and overlapping fibers whose heights from the surface ranged from approximately 3 to 740 nm. The elastic modulus of fibrinogen fibers was 1.55 MPa. No fibrils formed when fibrinogen lacking αC-domains was used as a coating or was incubated with intact fibrinogen-coated plates, or when the latter plates were sequentially incubated with anti-Aα529-539 mAb and intact fibrinogen. When an anti-Aα241-476 mAb was used instead, fine, long fibers formed. Similarly, sequential incubations of fibrinogen-coated plates with recombinant αC-domain (Aα392-610 fragment) or αC-connector (Aα221-372 fragment) and fibrinogen resulted in distinctly fine fiber networks.

CONCLUSIONS

Adsorption-induced fibrinogen self-assembly is initiated by a more than one molecule-thick surface layer and eventuates in three-dimensional networks whose formation requires fibrinogen with intact αC-domains.

Evidence for viscoelastic effects in surface capillary waves of molten polymer films

The surface dynamics of supported ultrathin polystyrene films with thickness comparable to the radius of gyration were investigated by surface sensitive x-ray photon correlation spectroscopy. We show for the first time that the conventional model of capillary waves on a viscous liquid has to be modified to include the effects of a shear modulus in order to explain both static and dynamic scattering data from ultrathin molten polymer films.

Supercritical fluid processing of polymer thin films: an X-ray study of molecular-level porosity

This paper reviews our recent experimental results that address the effects of solvent density inhomogeneities in supercritical carbon dioxide (scCO(2)) on polymer thin film processing. The key phenomenon is excess sorption of CO(2) molecules into polymer thin films even when the bulk miscibility with CO(2) is very poor. We have found that the amount of the excess sorption is attributed to the large density fluctuations in scCO(2) near the critical point. Further, taking advantage of the vitrification process of polymer chains through quick evaporation of CO(2), we can preserve the "expanded" structures as they are. The resultant films have large degree of molecular-level porosity that is very useful in producing coatings with low dielectric constants, enhanced adhesion, and metallization properties. These characteristics can be achieved in an environmentally "green" manner, without organic solvents, and are not specific to any class of polymers.

Size-controlled synthesis and characterization of thiol-stabilized gold nanoparticles

Size-controlled synthesis of nanoparticles of less than a few nanometers in size is a challenge due to the spatial resolution limit of most scattering and imaging techniques used for their structural characterization. We present the self-consistent analysis of the extended x-ray absorption fine-structure (EXAFS) spectroscopy data of ligand-stabilized metal nanoclusters. Our method employs the coordination number truncation and the surface-tension models in order to measure the average diameter and analyze the structure of the nanoparticles. EXAFS analysis was performed on the two series of dodecanethiol-stabilized gold nanoparticles prepared by one-phase and two-phase syntheses where the only control parameter was the gold/thiol ratio xi, varied between 6:1 and 1:6. The two-phase synthesis resulted in the smaller particles whose size decreased monotonically and stabilized at 16 A when xi was lowered below 1:1. This behavior is consistent with the theoretically predicted thermodynamic limit obtained previously in the framework of the spherical drop model of Au nanoparticles.

Effect of density fluctuating supercritical carbon dioxide on polymer interfaces

We investigated an effect of CO2 sorption on the compatibility of immiscible polystyrene (PS) and polybutadiene (PB) bilayers by using in situ neutron reflectivity. By labeling either polymer with deuterium, we found that the excess CO2 molecules were adsorbed to both top PS and bottom PB layers when the bilayers were exposed to CO2 at the narrow T and P regime near the critical point of pure CO2. Furthermore, we clarified that this excess sorption of CO2 molecules increased the interfacial width between the layers up to 100 angstroms even near room temperature, while the interfacial width without CO2 exposure has been reported to be at most 40 A even at the highest temperature (T congruent with 175 degrees C).

Dynamics of ultrathin films in the glassy state

We report hole growth experiments in free-standing polystyrene (PS) films at temperatures up to 10 degrees C below the bulk glass transition. The data show an unexpected result: the growth rate of nucleated holes increases with increasing molecular weight, up to a limiting value beyond which the rate is approximately constant. Film thicknesses of 45, 80, and 100 nm were studied, using PS molecular weights ranging from 65K to 11.4 Mg/mol. Hole diameters grew linearly with time, and no growing rims were observed to form around the developing holes. Possible explanations in terms of elasticity, yield, and influence of sample preparation and confinement effects are discussed.

Deviations from liquidlike behavior in molten polymer films at interfaces

We have performed x-ray specular and diffuse scattering on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that films whose molecular weight is close to the entanglement length behave as simple liquids except that the shortest wavelength is determined by the radius of gyration (R(g)) rather than the monomer-monomer distance. When the molecular weight is higher than the entanglement length, the strong deviations from liquidlike behavior are observed. We find that the long wavelength cutoff vector, q(l,c), scales with film thickness, d as d(-1.1+/-0.1) rather than the usual d(-2) expected for simple liquids. If we assume that these deviations are due to surface pinning of the polymer chains, then our results are consistent with the formalism developed by Fredrickson et al. to explain the capillary wave spectrum that can propagate in a polymer brush.

X-ray scattering from freestanding polymer films with geometrically curved surfaces

We show that the x-ray surface scattering from a freestanding polymer film exhibits features that cannot be explained by the usual stochastic formalism for surfaces with random height fluctuations. Instead, a geometric description of the film morphology assuming two curved surfaces characterized by a radius of curvature and a lateral cutoff length successfully accounts for the phase difference between the Kiessig fringes of the nominal "specular" and "off-specular" components of the scattering. The formalism allows one to distinguish unambiguously between conformal and anticonformal curvature morphologies at long length scales.

Mobility of polymer chains confined at a free surface

Dynamic secondary ion mass spectrometry was used to investigate the chain mobility of polystyrene (MW ranging from 4.3 to 957 kg/mol) at the free surface. The data show that the diffusion coefficient was reduced relative to the bulk value within a distance, d < or = 4R(g), from the surface and scaled as 1/N(2.5) at fixed d. These results are in excellent agreement with self-consistent field calculations of the surface segmental distribution and provide the first direct confirmation of various theoretical models that predict asymmetric segmental fluctuation which arises from surface induced orientation of polymer chains.

DNA electrophoresis on a flat surface

We report a new approach for performing DNA electrophoresis. Using experimental studies and molecular dynamics simulations, we show that a perfectly flat silicon wafer, without any surface features, can be used to fractionate DNA in free solution. We determine that the ability of a flat surface to separate DNA molecules results from the local friction between the surface and the adsorbed DNA segments. We control this friction by coating the Si surface with silane monolayer films and show that it is possible to systematically change the size range of DNA that can be separated.

Investigation of Polymer Brushes and Adsorbed Layers under Shear

Neutron reflectometry was used to investigate the density profiles of polymer brushes and an adsorbed polymer layer under shear. The brushes consisted of end-functionalized deuterated polystyrene. The adsorbed polymer was a random copolymer of deuterated styrene and 4-vinylpyridine. The brush and the adsorbed layer were sheared by the flowing of solvents over the polymer, with a maximum shear rate of 104s−1. Density profiles of the brush and the layer were determined before, during and after shear. The shear had no observable effect on the polymer profiles in any of the experiments. We discuss these results in the context of other experiments and a recent theory.