Biomedical subjects

John C Berg

Publications and source records attributed to John C Berg.

13 recordsLinked to original sources

PubMed2005

Transient aggregation during dry-down of solvent-borne dispersions.

The present work investigates the role of changing solvency conditions on the stability of sterically stabilized colloids in evaporating solvent-borne dispersions and the recovery time for the redispersion of aggregates if destabilization occurs. Process conditions during the conversion of the coated fluid dispersion to a solid film must be carefully controlled to ensure that aggregation, leading to uneven pigment distribution, does not occur. Although a polymeric binder serves as a retention aid in a solvent-borne coating, it also act as a steric stabilizer during dry-down or curing of the coating. Because the dispersion medium in the coatings often is a mixture of solvents and nonsolvents, the binder's interactions with the dispersion medium govern whether aggregation is likely to occur. Tracking such an occurrence may be difficult due to rapidly changing solvent compositions. Light scattering is utilized herein to indicate if the conditions of a high speed coating and drying process (in this case, the production of magnetic data storage media) may lead to aggregation and, if aggregation is detected, the ability of aggregates to fully redisperse. Conformational changes of free polymeric binder chains in solution are followed to gauge the polymer-solvent interactions and, therefore, the binder's ability to prevent aggregation. A "danger zone" (conditions where aggregation is probable) is constructed from these measurements, and further dynamic light scattering measurements on binder-grafted-oxide particles detect aggregation in this zone. The aggregates are redispersed in a series of "good" solvents, and redispersion is found not to be instantaneous, suggesting that high-speed drying processes may not allow the flocculated colloids enough recovery time.

Journal Article↗

PubMed2005

Relating clay yield stress to colloidal parameters.

The ability to predict rheological behavior of clay dispersions would be useful in formulating systems rheologically modified by clays as well as designing equipment to handle clay based materials. Deriving predictive equations for these materials has been hindered due to the various mechanisms by which clay systems develop microstructure. This work seeks to observe the relationship between the yield stress, tauy, and the zeta potential, zeta, and determine the nature of the particle interactions for dispersions of laponite (a synthetic clay) and dispersions of a mixture of naturally occurring kaolinites and bentonites. It is found that the relationship of tauy to zeta2 for clay suspensions is opposite to that found for homogeneously charged spheroidal colloids. This result can be traced to the type of particle interactions occurring, which for the systems studied appear to be edge-to-face attractions.

Journal Article↗

PubMed2005

High-salt stabilization of Laponite clay particles.

Colloidal radioactive transuranic wastes are currently buried in large tanks in the form of dense colloids in high salt, high pH aqueous media. These facilities are beginning to fail, so it is necessary to transport and "package" them for more permanent disposal, processes requiring understanding of the microstructure that develops under such conditions. Laponite RD clay is believed to be a good simulant for the colloids in the waste tanks, and the present study addresses their behavior under high salt conditions, where previous studies have frequently observed the phenomenon of "restabilization," i.e., the attainment of aggregation stability at high electrolyte conditions. Specifically, the aggregation kinetics and the resulting cluster structure (fractal dimension) of Laponite RD clay colloids at high concentrations of BaCl2 (an electrolyte previously shown to lead to restabilization) are investigated. At low-to-intermediate electrolyte concentrations, the clay is found to behave in accord with DLVO theory, i.e., low salt conditions yield slow aggregation into densely-packed aggregates, whereas intermediate salt concentrations, sufficient to cause double layer collapse, produce rapid aggregation into open aggregates. High salt concentrations, however, show slow rates of aggregation. The aggregate structure under these conditions is found to mimic that found for very low electrolyte concentrations, i.e., high fractal dimension. Further experiments show that a sudden increase in salt concentration in a system containing young open aggregates produced under intermediate salt concentrations causes them to reform into more compact structures.

Journal Article↗

PubMed2005

Gel trapping of dense colloids.

Phase density differences in sols, foams, or emulsions often lead to sedimentation or creaming, causing problems for materials where spatial uniformity over extended periods of time is essential. The problem may be addressed through the use of rheology modifiers in the continuous phase. Weak polymer gels have found use for this purpose in the food industry where they appear to be capable of trapping dispersoid particles in a three-dimensional matrix while displaying water-like viscosities at low shear. Attempts to predict sedimentation stability in terms of particle properties (size, shape, density difference) and gel yield stress have led to qualitative success for suspensions of large particles. The effect of particle size, however, in particular the case in which colloidal dimensions are approached, has not been investigated. The present work seeks to determine useful stability criteria for colloidal dispersions in terms of readily accessible viscoelastic descriptors. Results are reported for systems consisting of 12 microm poly(methyl methacrylate) (PMMA) spheres dispersed in aqueous gellan gum. Monovalent salt concentration is varied to control rheological properties, and sedimentation/centrifugation experiments are performed to determine dispersion stability. Necessary conditions for stability consist of a minimum yield stress together with a value of tan delta less than unity.

Journal Article↗

PubMed2004

Aggregate restructuring by polymer solvency effects.

This study investigates the aggregation in cyclohexane of silica particles initially stabilized by grafted polystyrene and destabilized by temperature reduction. It complements an earlier study by Zhu and Napper (P.W. Zhu, D.H. Napper, Phys. Rev. E 50 (1994) 1360) in which the aggregation of polystyrene latex particles with tethered poly(N-isopropyl acrylamide) (PNIPAM) in water was investigated. Their dynamic light scattering results showed that both the rate of aggregation and the aggregate fractal dimension increased with a sufficient decrease in the PNIPAM adlayer solvency, achieved by means of either salt (NaNO3) addition or temperature rise. This result stands in contrast to those obtained when an electrostatically stabilized colloid is destabilized, i.e., that the more rapidly aggregates are formed, the lower the resulting fractal dimension. The authors explained their results in terms of the effects of both salt effects and increased temperature on the extent of the hydrophobic interactions between the adlayer-covered surfaces in the water. The present study examines a sterically-stabilized colloid in a nonaqueous solvent, where neither salt effects nor hydrophobic effects play a role. Temperature is decreased to bring the system from better-than-theta-conditions to worse-than-theta-conditions. Power-law aggregation kinetics are observed at 15.7 degrees C by dynamic light scattering. The particles first undergo reduced rate aggregation, producing low-fractal-dimension aggregates, which after some time, restructure into more compact aged clusters. The fractal dimension of these aged clusters increases with increasing initial aggregation rate, consistent with results seen by Zhu and Napper, but without the presence of hydrophobic effects. The ability of the polymer-grafted particles to rearrange suggests aggregation into a secondary minimum, with the ability to slide over one another to achieve a more energetically favorable, denser configuration. The reversible nature of the aggregation is verified by additional experiments gradually bringing the system from worse-than-theta-conditions back to better-than-theta-conditions, with an attendant decrease in aggregate fractal dimension, and ultimately full redispersion.

Cyclohexanes↗

PubMed2003

A review of the different techniques for solid surface acid-base characterization.

In this work, various techniques for solid surface acid-base (AB) characterization are reviewed. Different techniques employ different scales to rank acid-base properties. Based on the results from literature and the authors' own investigations for mineral oxides, these scales are compared. The comparison shows that Isoelectric Point (IEP), the most commonly used AB scale, is not a description of the absolute basicity or acidity of a surface, but a description of their relative strength. That is, a high IEP surface shows more basic functionality comparing with its acidic functionality, whereas a low IEP surface shows less basic functionality comparing with its acidic functionality. The choice of technique and scale for AB characterization depends on the specific application. For the cases in which the overall AB property is of interest, IEP (by electrokinetic titration) and H(0,max) (by indicator dye adsorption) are appropriate. For the cases in which the absolute AB property is of interest such as in the study of adhesion, it is more pertinent to use chemical shift (by XPS) and the heat of adsorption of probe gases (by calorimetry or IGC).

Acids↗

PubMed2003

The effective surface energy of heterogeneous solids measured by inverse gas chromatography at infinite dilution.

Inverse gas chromatography (IGC) at infinite dilution has been widely used to access the nonspecific surface free energy of solid materials. Since most practical surfaces are heterogeneous, the effective surface energy given by IGC at infinite dilution is somehow averaged over the whole sample surface, but the rule of averaging has thus far not been established. To address this problem, infinite dilution IGC analysis was carried out on mixtures of known heterogeneity. These materials are obtained by mixing two types of solid particles with significantly different surface energies as characterized individually with IGC, and results are obtained for binary combinations in varying proportions. It is found that when all surface components have the same accessibility by probe molecules, the effective surface energy of such a heterogeneous surface is related to the surface energy distribution by a square root linear relationship, square root sigma(eff)(LW)= summation operator (i)phi(i) square root sigma(i)(LW), where sigma(i)(LW) refers to the nonspecific (Lifshitz-van der Waals) surface energy of patches i, and phi(i) to their area fraction.

Journal Article↗

PubMed2003

A simplified method for predicting the dynamic surface tension of concentrated surfactant solutions.

A simplified method for predicting the dynamic surface tension of concentrated surfactant solutions is proposed. It is implemented using the framework of the Henry's Law analytical solution to the Ward and Tordai equation for diffusion-controlled adsorption, with the necessary parameters being deduced from the measured equilibrium surface tension equation and a value for the surfactant monomer diffusivity. The method is tested by calculating the dynamic surface tension relaxations of aqueous C10E6 and C10E8 solutions over concentration ranges from well below to well above their critical micelle concentrations (cmc). Results are compared with measured relaxations over 0.001-50 s, and semiquantitative agreement is found, with the best results obtained for concentrations near the cmc. The predictive method may prove useful in such applications as the screening of candidate surfactants for inks used in inkjet printing.

Journal Article↗

PubMed2003

Linear chains and chain-like fractals from electrostatic heteroaggregation.

The internal structure of materials prepared by aggregation of oppositely charged polystyrene spheres (electrostatic heteroaggregation) is investigated by static light scattering, optical microscopy, and Brownian dynamics simulation. Light scattering indicates ultralow mass fractal dimensions, as low as 1.2. Such low fractal dimensions, approaching the theoretical limit of a linear object, imply a chaining mechanism. Optical micrographs reveal linear chains with the particle charge alternating down the chains. Brownian dynamics simulation gives additional support for a chaining mechanism. For the polystyrene system (120-nm primary particle diameters), the fractal dimension is found to increase from 1.2 to 1.7 as the background electrolyte is increased. In terms of electrostatic screening, the results match those reported recently for larger polystyrene spheres. The low fractal dimensions appear to represent a crossover from linear chains to a structure of diffusion-limited aggregates; however, experiments under density-neutral conditions imply that sedimentation plays an important role in the formation of ultralow fractal dimensions. The practical implication is that microcomposites with a locally uniform distribution of starting materials and almost any degree of branching can be prepared from oppositely charged particles.

Journal Article↗

PubMed2003

Temperature effects on the rheological properties of current polyether and polysiloxane impression materials during setting.

STATEMENT OF PROBLEM: Rheological tests of elastomeric impression materials during setting have been most often conducted at room temperature rather than at intraoral temperature. Because temperature may affect properties and the setting kinetics, clinically relevant inferences may not be accurate with studies conducted at room temperature. PURPOSE: The purpose of this study was to determine the viscoelastic properties of new low- and medium-viscosity elastomeric impression materials during setting at 33 degrees C and to evaluate the medium-viscosity materials at 3 additional temperatures. MATERIAL AND METHODS: The impression materials investigated at 33 degrees C were 2 polyvinylsiloxanes (PVS) (Aquasil Deca and Aquasil LV) and 5 polyethers (PE) (Impregum Penta, Impregum Penta Soft H, Impregum Penta Soft L, Impregum Garant Soft L, and Permadyne Garant L). Three impression materials (Aquasil Deca, Impregum Penta, and Impregum Penta Soft H) were also investigated at 25 degrees, 29 degrees, and 37 degrees C. Time-dependent oscillatory rheometry was carried out on these materials (n=3) with a rheometer with a 25-mm diameter parallel plate cell. The storage modulus (G') and the loss tangent (tandelta) were determined as functions of time over a period from 0 seconds to 900 seconds, commencing 40 seconds after mixing. Induction time (t(ind)) or initial setting time and tandelta, the relative liquidlike behavior, were also computed. A single-factor analysis of variance (ANOVA) was used for the properties determined at 33 degrees C and a 2-factor ANOVA was used for the temperature studies, with hypothesis testing at alpha=.05. RESULTS: The G'(t) curves for all materials displayed the expected sigmoidal shape with time, with the solid-like behavior rising slowly, then more rapidly, and again slowly to final set. The initial setting time (t(ind)) was found to be approximately 2.8 minutes for the PVS materials and for Impregum Penta and Impregum Penta Soft H, but was significantly longer for the remaining 3 PE low viscosity materials. The solid-like behavior (G') at final set or shear modulus differed among all materials, ranging from 1.0 MPa for Aquasil Decca, 1.69 for Impregum Penta Soft H, and 1.8 MPa for Impregum Penta. G' for low-viscosity materials ranged from 0.66 MPa for Aquasil LV and 0.79 MPa for Permadyne Garant L to 1.2 MPa for Impregum Penta Soft L. The loss tangent at 40 seconds tandelta (t(0)) varied among medium and low viscosity materials, ranging from liquid-like behavior of 4.3 for Permadyne Garant and less than unity or significant solid-like behavior for Impregum Penta. All materials showed tandelta values less than unity at their setting times. The temperature studies revealed significant changes in the kinetics of setting, with the setting time decreasing more than 3-fold between 25 degrees and 37 degrees C for the Aquasil Deca and the Impregum Penta Soft H and more than 2-fold for the Impregum Penta. At room temperature of 25 degrees C, the storage modulus for Impregum Penta Soft H and Aquasil Deca was 1.1% and 37.5% lower than Impregum Penta, respectively. CONCLUSION: The development of the viscoelastic rheological properties with time for 3 medium- and 4 low-viscosity impression materials at 33 degrees C showed significant differences in the setting time and the magnitude of the storage modulus. Compared with Impregum Penta, the new Impregum Penta Soft H was 6% less stiff when set, compared with 44% lower stiffness for the medium viscosity addition silicone. Temperature studies between 25 degrees C and 37 degrees C revealed strong temperature sensitivity of the kinetics of setting and evolution with time of their rheological properties.

Analysis of Variance↗

PubMed2002

Effect of moisture on the surface free energy and acid-base properties of mineral oxides.

Surface energetic properties of mineral oxides are important in many applications. Since oxide surfaces in practice have generally come in contact with water molecules, it is important to know how water coverage affects the surface properties. In this work, five oxide samples, namely MgO, Al2O3, TiO2, SnO2 and SiO2 are heat-treated to various extents, to produce different degrees of hydration, and characterized thereafter by inverse gas chromatography. Water contents of the treated samples are determined independently by Karl Fischer titration, and specific surface areas are measured by the BET method. The results show that in general as water coverage decreases, the Lifshitz-van der Waals component of the specific surface free energy (sigma(S)LW) increases, but the acid-base interaction potential (-deltaG(AB)) decreases. These attributes are more sensitive to changes in water coverage at lower coverages, where the surface is presumed to consist of patches of molecular water and unhydrated hydroxyl groups.

Chromatography, Gas↗

PubMed2002

Structural evolution of octyltriethoxysilane films on glass surfaces during annealing at elevated temperature.

Glass samples treated by octyltriethoxysilane were annealed for times ranging from 0 to 16 h, and their topography, adhesion, and stiffness properties were examined using pulsed-force mode (PFM) atomic force microscopy. While the surfaces of samples dried at room temperature were structurally featureless, those annealed at 145 degrees C showed formation of islands, increasing in size and number with heating time. PFM adhesion and stiffness maps suggested that the glass substrate remained at least partially covered by silanes even after island formation.

Glass↗

PubMed2002

Wettability, imbibition, and mass change of disinfected low-viscosity impression materials.

STATEMENT OF PROBLEM: There is an ongoing effort by dental manufacturers to create impression materials with improved wetting properties. Disinfection solutions may alter the surface characteristics of these newer materials. PURPOSE: This study compared wettability, imbibition, and mass change of various recently introduced automixed low-viscosity addition silicone and polyether materials before and after immersion disinfection. MATERIAL AND METHODS: The Wilhelmy technique was used for deriving wetting properties of 5 addition silicone materials (Clinician's Choice Affinity, Clinician's Choice Superhydrophilic [experimental], Kerr's Take One, 3M's Imprint II, and Dentsply's Aquasil LV) and 2 polyether materials (ESPE's Permadyne Garant and Impregum Garant). Conditions included a control with no disinfection (0 hours), as well as (1/2) hour of immersion disinfection in a full-strength solution of 2% acid glutaraldehyde disinfectant (Banicide). Weight changes before and after disinfection and weight loss in air were measured over an 18-hour period to detect imbibition and mass change over time. The data were analyzed with a 1-way analysis of variance at alpha=0.05, with n = 3 for advancing (ACA) and receding (RCA) contact angles and n = 2 for imbibition and mass change. RESULTS: Statistical significant differences in wettability (P<.001) were found among nondisinfection control groups, as well as among (1/2)-hour disinfection groups. Polyethers were the most wettable materials overall. Impregum Garant polyether demonstrated significantly lower RCA for the control (48.4 degrees) and at (1/2) hour of disinfection (51.8 degrees). The 2 polyethers and Take One lost mass, whereas Aquasil LV gained mass in air; however, all materials exhibited some degree of imbibition during disinfection. CONCLUSION: Within the limitations of this study, the 2 polyether materials tested exhibited significantly lower ACA's and RCA's compared with the 5 addition silicones tested. Imbibition for the 2 polyether materials was significantly higher (P<.001). Polyether materials lost significantly more (0.6% to 0.8%) and Aquasil LV gained significantly more (0.6%) mass in air.

Absorption↗