Mitchell R. Armstrong

Particle size studies to reveal crystallization mechanisms of the metal organic framework HKUST-1 during sonochemical synthesis

Systematic studies of key operating parameters for the sonochemical synthesis of the metal organic framework (MOF) HKUST-1(also called CuBTC) were performed including reaction time, reactor volume, sonication amplitude, sonication tip size, solvent composition, and reactant concentrations analyzed through SEM particle size analysis. Trends in the particle size and size distributions show reproducible control of average particle sizes between 1 and 4μm. These results along with complementary studies in sonofragmentation and temperature control were conducted to compare these results to kinetic crystal growth models found in literature to develop a plausible hypothetical mechanism for ultrasound-assisted growth of metal-organic-frameworks composed of a competitive mechanism including constructive solid-on-solid (SOS) crystal growth and a deconstructive sonofragmentation.

Influence of Particle Size and Loading on Particle Accessibility in Electrospun Poly(ethylene oxide) and ZIF-8 Composite Fibers: Experiments and Theory

Developing electrospun nanofiber/nanoparticle composites (ENNCs) is an emerging strategy for immobilizing functional particles for a variety of applications. The radial location of the particle along the fiber, either at the surface or in the bulk, has implication into the resulting properties. To explore particle location along fibers, ZIF-8 impregnated poly(ethylene oxide) (PEO) nanofibers are formed by electrospinning particle suspensions. Fibers impregnated with two different ZIF-8 particle sizes (200 nm and 12.5 μm) were electrospun and shown by nitrogen porosimetry to be nearly completely wrapped by PEO in each case at loadings near 10 wt %. This was favorably compared to developed theory of polymeric membrane encapsulated particles and extended to other electrospun fiber/particle composites through a brief literature review. ENNCs with varying loadings of nanosized ZIF-8 particles were then fabricated and tested with nitrogen porosimetry to find that the particles became available for adsorption at the surface of the fibers starting from 25 wt % (28 vol %) loading. This suggests that the particles are kinetically trapped at this loading level since, if allowed to exhibit random close-packing, the ZIF-8 would be expected to fully imbedded inside the fibers up to 56 vol % loading.

Modeling Nanoparticle Dispersion in Electrospun Nanofibers

The quality of nanoparticle dispersion in a polymer matrix significantly influences the macroscopic properties of the composite material. Like general polymer-nanoparticle composites, electrospun nanofiber nanoparticle composites do not have an adopted quantitative model for dispersion throughout the polymer matrix, often relying on a qualitative assessment. Being such an influential property, quantifying dispersion is essential for the process of optimization and understanding the factors influencing dispersion. Here, a simulation model was developed to quantify the effects of nanoparticle volume loading (ϕ) and fiber-to-particle diameter ratios (D/d) on the dispersion in an electrospun nanofiber based on the interparticle distance. A dispersion factor is defined to quantify the dispersion along the polymer fiber. In the dilute regime (ϕ < 20%), three distinct regions of the dispersion factor were defined with the highest quality dispersion shown to occur when geometric constraints limit fiber volume accessibility. This model serves as a standard for comparison for future experimental studies and dispersion models through its comparability with microscopy techniques and as a way to quantify and predict dispersion in electrospinning polymer-nanoparticle systems with a single performance metric.