Biomedical subjects

J L Keddie

Publications and source records attributed to J L Keddie.

8 recordsLinked to original sources

PubMed2006

Probing particle structure in waterborne pressure-sensitive adhesives with atomic force microscopy.

There is a need to know the nanostructure of pressure-sensitive adhesive (PSA) films obtained from waterborne polymer colloids so that it can be correlated with properties. Intermittent-contact atomic force microscopy (AFM) of an acrylic waterborne PSA film identifies two components, which can be attributed to the polymer and the solids in the serum (mainly surfactant). It is found that when the average AFM tapping force, F(av), is relatively low, the polymer particles appear to be concave. But when F(av) is higher, the particles appear to have a convex shape. This observation is explained by a height artefact caused by differences in the indentation depths into the two components that vary with the tapping amplitude and F(av). To achieve the maximum contrast between the polymer and serum components, F(av) should be set such that the indentation depths are as different as possible. Unlike what is found for the height images, the phase contrast images of the PSA do not show a reversal in contrast over the range of tapping conditions applied. The phase images are thus reliable in distinguishing the two components of the PSA according to their viscoelastic properties. At the surface of films dried at room temperature, the serum component is found in localized regions within permanent depression into the film.

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PubMed2004

Thickness dependence of structural relaxation in spin-cast, glassy polymer thin films.

The isothermal structural relaxation of glassy, spin-cast polymer thin films has been investigated. Specifically, the thickness h of freshly cast poly(methyl methacrylate) thin films was measured over time using spectroscopic ellipsometry. The spin-cast films exhibit a gradual decrease in thickness, which is attributed to structural relaxation of the glass combined with simultaneous solvent loss. In all cases, h was found to be greater than the equilibrium thickness h(infinity) , which is obtained by cooling slowly from the melt. It is observed that both the rate of the volume relaxation and the fractional departure from h(infinity) (referred to as delta(0) ) increase with increasing film thickness. In the limit of very thin films, the initial h is close to h(infinity) , and delta(0) is small, whereas in thick films (>500 nm) , a plateau value of delta(0) of 0.16 is observed, which is close to the volume fraction of the solvent at the vitrification point. This dependence of delta(0) on thickness is observed regardless of the substrate, polymer molecular weight, or angular velocity during spin casting. Enhanced mobility near film surfaces could be leading to greater relaxation in thinner films prior to, and immediately after, the vitrification of the polymer during the deposition process.

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PubMed2004

Structural relaxation of spin-cast glassy polymer thin films as a possible factor in dewetting.

Reiter has recently reported a situation in which the dewetting of quasi-solid films is linked to plastic deformation--rather than viscous flow--resulting from capillary forces. Herein we propose that, in thin films of some glassy polymers--especially poly(methyl methacrylate) (PMMA)--prepared by spin-casting from solvent, structural relaxation might impart sufficient stress to cause plastic deformation. We find that PMMA films decrease in thickness by several percent, which is sufficient to create significant stress in those cases in which the film is attached to a rigid substrate. The floating technique, which can take tens of minutes, might allow most of the structural relaxation to occur prior to dewetting experiments.

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PubMed2003

Influence of the glass transition on solvent loss from spin-cast glassy polymer thin films.

The interdependence of solvent loss and vitrification in spin-cast poly(methyl methacrylate) thin films is explored. Fast measurements of decreases in film thickness, achieved with ellipsometry, indicate that the rate of solvent (toluene) loss decreases sharply when the solvent volume fraction phi(solv) falls below about 0.15 and the film vitrifies. Ellipsometry and microgravimetry show that solvent is lost from a glassy thin film (150 nm thick) over of a period of more than ten hours, which is much longer than would be required if it was limited by diffusion in the glass. These results support the recently-proposed idea that the compression of the glass creates an energy barrier that slows down solvent loss.

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PubMed2002

Vertical water distribution during the drying of polymer films cast from aqueous emulsions.

We present a systematic study of the vertical uniformity of water distribution during the drying of waterborne colloidal films, testing the predictions of a Peclet number Pe defined for this system. Pe indicates the relative contributions of water evaporation and Brownian diffusion in determining the concentration profile in the vertical direction ( i.e. normal to the substrate). When Pe < 1, the water concentration in films cast from an alkyd emulsion is found via magnetic-resonance profiling to be uniform with depth, which is consistent with expectations. When Pe > 1, a gradient in the water concentration develops, with less water near the interface with air. The water profiles reveal that the alkyd particles do not coalesce immediately upon contact in close-packing. At later times, a concentrated surface layer develops, but particles are not coalesced in this layer to form a continuous "skin", but rather the structure is likely to be that of a biliquid foam.

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PubMed1999

The reduced adsorption of lysozyme at the phosphorylcholine incorporated polymer/aqueous solution interface studied by spectroscopic ellipsometry.

Coating hydrogel polymers onto solid substrates can reduce the adsorption of proteins onto these surfaces, but the extent of the reduction in protein adsorption is strongly dependent on how the surface layer is coated. We have examined the effect of coating conditions on the structure of thin polymer films formed from a number of poly(methacrylate)-based hydrogel polymers via the dip-coating method. We show in this work how the polarity of the solvent, the speed of lifting, and the annealing temperature affect the thickness and uniformity of ultrathin phosphorylcholine (PC)-incorporated polymer films coated on the surface of native oxide on silicon and the subsequent interaction of these coated surfaces with lysozyme molecules. Our results show that the uniformity of the polymer film, and thus the smoothness of the outer film surface, influence the extent of reduction in protein adsorption. We suggest that the reduction in lysozyme adsorption is the result of a layer of PC groups on the surface of the polymer film. The improvement of the smoothness of the film results in the formation of a close-packed PC layer on the outer surface of the polymer film, leaving few defects or cavities on which protein molecules can bind.

Adsorption↗

PubMed1999

Bayesian inference analysis of ellipsometry data.

Variable angle spectroscopic ellipsometry is a nondestructive technique for accurately determining the thicknesses and refractive indices of thin films. Experimentally, the ellipsometry parameters psi and Delta are measured, and the sample structure is then determined by one of a variety of approaches, depending on the number of unknown variables. The ellipsometry parameters have been inverted analytically for only a small number of sample types. More general cases require either a model-based numerical technique or a series of approximations combined with a sound knowledge of the test sample structure. In this paper, the combinatorial optimization technique of simulated annealing is used to perform least-squares fits of ellipsometry data (both simulated and experimental) from both a single layer and a bilayer on a semi-infinite substrate using what is effectively a model-free system, in which the thickness and refractive indices of each layer are unknown. The ambiguity inherent in the best-fit solutions is then assessed using Bayesian inference. This is the only way to consistently treat experimental uncertainties along with prior knowledge. The Markov chain Monte Carlo algorithm is used. Mean values of unknown parameters and standard deviations are determined for each and every solution. Rutherford backscattering spectrometry is used to assess the accuracy of the solutions determined by these techniques. With our computer analysis of ellipsometry data, we find all possible models that adequately describe that data. We show that a bilayer consisting of a thin film of poly(styrene) on a thin film of silicon dioxide on a silicon substrate results in data that are ambiguous; there is more than one acceptable description of the sample that will result in the same experimental data.

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PubMedDate not supplied

Magnetic resonance profiling studies of the drying of film-forming aqueous dispersions and glue layers.

We report magnetic resonance profiling experiments to monitor (i) the drying of alkyd emulsion layers, (ii) the cure of wood glue layers and (iii) water transport through glue lines. The alkyd drying is a two stage process. We report new results which support previous evidence that the alkyd drops do not coalesce until the water fraction is below circa 0.02. The profiles recorded from glue layers suggest that MR is a sensitive probe of the curing process and barrier properties of the glue. The measurements were made using GARField (stray field, STRAFI like) magnetic resonance profiling and an improved GARField magnet design characterized by two values of the gradient-to-field strength ratio at two locations offering the same field strength is also reported.

Adhesives↗