Benjamin J. Ash

Glass transition behavior of alumina/polymethylmethacrylate nanocomposites

Abstract Alumina/polymethylmethacrylate (PMMA) nanocomposites were synthesized using 39-nm nanoparticles and in situ free-radical polymerization. At filler concentrations greater than 0.5 wt.%, the glass transition temperature, Tg, was observed to decrease precipitously by 25 °C compared to the neat polymer. At smaller weight fractions, there were no changes in the composite Tg. The abrupt changes seem to indicate a threshold at which a significant volume fraction of polymer has higher mobility that brings about the decrease in Tg. Consistent with this behavior, the Tg depression was suppressed by coating the nanoparticles to make them compatible with the matrix.

High-precision thermal conductivity measurements as a probe of polymer/nanoparticle interfaces

We use the 3ω method to study the thermal conductivity of composites of nanoscale alumina particles in polymethylmethacrylate (PMMA) matrices. Effective medium theory and data for the changes in conductivity produced by low volume fractions of particle fillers are used to estimate the thermal conductance G of PMMA/alumina interfaces in the temperature range of 40<T<280 K. Near room temperature, G≈30±10 MW m−2 K−1 and the critical particle radius, r0=Λ0/G, is extremely small, r0≈7.5±2.5 nm. (Λ0=0.205 W m−1 K−1 is the conductivity of the PMMA matrix.) Therefore, high volume fractions of typical ceramic nanoparticles with r≫r0 can be used in thermal interface materials such as adhesives, pastes, and pads.

Investigation into the Thermal and Mechanical Behavior of PMMA/Alumina Nanocomposites

Polymethylmethacrylate (PMMA) nanocomposites were synthesized by free radical polymerization in the presence of various weight percentages of alumina (Al 2 O 3 ) nanoparticles. The resulting nanocomposites show an average increase of 600% in strain-to-failure and the appearance of a well-defined yield point. Concurrently, the glass transition temperature (Tg) of the composites decreased 20°C, while the ultimate strength and the Youngs modulus decreased by 20% and 15%, respectively.

Elucidating the mysteries of wetting.

Nearly every manufacturing and many technologies central to Sandias business involve physical processes controlled by interfacial wetting. Interfacial forces, e.g. conjoining/disjoining pressure, electrostatics, and capillary condensation, are ubiquitous and can surpass and even dominate bulk inertial or viscous effects on a continuum level. Moreover, the statics and dynamics of three-phase contact lines exhibit a wide range of complex behavior, such as contact angle hysteresis due to surface roughness, surface reaction, or compositional heterogeneities. These thermodynamically and kinetically driven interactions are essential to the development of new materials and processes. A detailed understanding was developed for the factors controlling wettability in multicomponent systems from computational modeling tools, and experimental diagnostics for systems, and processes dominated by interfacial effects. Wettability probed by dynamic advancing and receding contact angle measurements, ellipsometry, and direct determination of the capillary and disjoining forces. Molecular scale experiments determined the relationships between the fundamental interactions between molecular species and with the substrate. Atomistic simulations studied the equilibrium concentration profiles near the solid and vapor interfaces and tested the basic assumptions used in the continuum approaches. These simulations provide guidance in developing constitutive equations, which more accurately take into account the effects of surface induced phase separation and concentration gradients near the three-phase contact line. The development of these accurate models for dynamic multicomponent wetting allows improvement in science based engineering of manufacturing processes previously developed through costly trial and error by varying material formulation and geometry modification.

A multiscale approach to multi-component wetting.

Laser scanning confocal microscopy has been applied to study segregation in multi-component wetting. By labeling the two components of a blend with contrasting fluorescent dyes, the approximate local concentration can be determined from the relative fluorescence intensities. As a proof of concept, a coarsely blended mixture was imaged and parameters were adjusted to achieve good spectral separation of the two components. The technique was then applied to a well-blended drop of the two components and one component was observed to segregate to the air interface.Copyright