Brian P. Mason

Use of Bifunctional Ureas to Increase the Rate of Proline-Catalyzed α-Aminoxylations

The rate of the proline-catalyzed alpha-aminoxylation of aldehydes is significantly increased in the presence of a bifunctional urea. Structure-activity relationship data indicate that both an amine and a urea are crucial for rate enhancement. The evidence presented herein suggests that this rate enhancement originates from the hydrogen bonding interaction between the bifunctional urea and an oxazolidinone intermediate to increase the rate of enamine formation. Proline derivatives that are incapable of forming oxazolidinones exhibit no rate enhancement in the presence of the bifunctional urea. The rate enhancement is general for a variety of aldehydes, and the faster reactions do not reduce yields or selectivities.

Simplified Mesofluidic Systems for the Formation of Micron to Millimeter Droplets and the Synthesis of Materials

We present and validate simple mesofluidic devices for producing monodisperse droplets and materials. The significance of this work is a demonstration that simple and complex droplet formulations can be prepared uniformly using off-the-shelf small-diameter tubing, barbed tubing adapters, and needles. With these simple tools, multiple droplet-forming devices and a new particle concentrator were produced and validated. We demonstrate that the droplet-forming devices could produce low-dispersity particles from 25 to 1200 μm and that these results are similar to results from more complicated devices. Through a study of the fluid dynamics and a dimensional analysis of the data, we have correlated droplet size with two dimensionless groups, capillary number and viscosity ratio. The flow-focusing device is more sensitive to both parameters than the T-junction geometry. The modular character of our mesofluidic devices allowed us to rapidly assemble compound devices that use flow-focusing and T-junction devices in series to create complex droplet-in-microcapsule materials. This work demonstrates that flow chemistry does not require complicated tools, and an inexpensive tool-kit can allow anyone with interest to enter the field.

Neutron scattering study of internal void structure in RDX

We present the first small and ultrasmall angle neutron scattering (SANS/USANS) measurements of the internal void morphology of the high explosive RDX on length scales from 10 A to 20 μm. Measurements were taken on a range of RDX samples with similar densities and particle size distributions but which have significantly different sensitivities to shock initiation as measured by large-scale gap tests of the samples when formulated in standard polymer blends. Scattering measurements were performed using a contrast match technique to eliminate all features apart from internal void structures. The dominant feature in all samples is a surface fractal scattering that extends from ∼50 nm to above 20 μm, with no observable upper bound for the fractal correlation length. These features are interpreted in terms of scattering from rough surfaces of interior air-filled voids with fractal dimensionality between 2.4 and 2.9. The fractal pattern is proposed to arise from complex growth patterns on void surfaces as inter...

A temperature-mapping molecular sensor for polyurethane-based elastomers

We present a crosslinked polyurethane elastomer featuring a thermochromic molecular sensor for local temperature analysis. The thermochrome is a modified donor-acceptor Stenhouse adduct (DASA) that was dispersed homogeneously into the polymer blend in minuscule amounts. Rapid temperature jump measurements in a pyroprobe and impacts in a Hopkinson bar show that the DASA has suitable kinetics for detecting localized temperature increase following impact or rapid heating. The thermochrome retains a signature of the peak temperature in the elastomer, allowing post-mortem mapping of micron-scale temperature localization in materials such as explosive and propellant composites. We demonstrate the concept by using the kinetics of the DASA activation to determine peak temperatures reached during bullet perforation of the polyurethane.

Equations of state of 2,6-diamino-3,5-dinitropyrazine-1-oxide

2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is an energetic ingredient that has an impact sensitivity close to that of TATB, yet a calculated energy content close to HMX. Reported tests of formulated LLM-105 reveal that it is a good candidate for a new insensitive high-performance explosive. As use of LLM-105 increases, thermodynamic parameters and phase stability will need to be determined for accurate modeling. In order to accomplish this goal, isothermal equations of state of LLM-105 at static high-pressure and temperature were investigated using synchrotron angle-dispersive x-ray diffraction and diamond anvil cells. Data at ambient temperature, 100 °C (373 K), and 180 °C (453 K) were used to obtain isothermal equations of state, and data at ambient pressure were used to obtain the volume thermal expansion coefficient. At ambient temperature, 100 °C (373 K), and 180 °C (453 K) no phase change was evident up to the highest measured pressure; and at ambient pressure, LLM-105 was stable up to 240 °C (513 K) and thermally decomposed by 260 °C (533 K).

Controlling RDX explosive crystallite morphology and inclusion content via simple ultrasonic agitation and solvent evaporation

Uniform crystallite morphology, narrow particle size distribution, and tailored inclusion content have been achieved for cyclotrimethylene trinitramine (RDX) explosive recrystallization by a combination of simple ultrasonic agitation and solvent evaporation, as characterized by optical imaging and confocal microscopy.

Microcapsules with three orthogonal reactive sites

Polymeric microcapsules containing reactive sites on the shell surface and two orthogonally reactive polymers encapsulated within the interior are selectively labeled. The capsules provide three spatially separate and differentially reactive sites. Confocal fluorescence microscopy is used to characterize the distribution of labels. Polymers encapsulated are distributed homogeneously within the core and do not interact with the shell even when oppositely charged.

Optical characterization and confocal fluorescence imaging of mechanochromic acrylate polymers

The development of mechanochromic molecules has opened new pathways for the study of localized stress and failure in polymers. Their application as stress or temperature diagnostics, however, requires suitable measurement techniques capable of detecting the force- and temperature-sensitive chemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial resolution but the energy input during these measurements can itself affect the activation state of the mechanochromic species. Here, we present a systematic study of the effects of laser-based imaging on the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into poly(methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman microspectrometer. Localized stress and temperature activation were studied by means of high-rate compressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excitation wavelength- and power-dependence. Suitable correction functions were established and used to account for the observed laser effects. The presented study demonstrates that confocal imaging using conventional Raman spectrometers is a powerful characterization tool for localized stress analysis in mechanochromic polymers, offering quantifiable information on the activation state with high spatial resolution. However, laser-mechanophore interactions must be well understood and effects of laser excitation and exposure times must be taken into consideration when interpreting the obtained results.

Equations of state of hexanitrostilbene (HNS)

Hexanitrostilbene (HNS) is an energetic ingredient that is widely used in commercial and military explosives for its thermal stability. However, characterization of its thermodynamic parameters and phase stability is lacking. Crystalline properties, such as bulk modulus and thermal expansion, are necessary to accurately predict the behavior of shocked solids using hydrodynamic codes. In order to obtain these values, equations of state of fine-particle (type IV) HNS were investigated using synchrotron angle-dispersive x-ray diffraction experiments at static high-pressure and temperature. The samples were compressed and heated using diamond anvil cells. Pressure - volume data for HNS at ambient temperature were fit to the Birch-Murnaghan and Vinet formalisms to obtain bulk modulus and its first pressure derivative. Temperature - volume data at ambient pressure were fit to obtain the volume thermal expansion coefficient.

Sonocrystallization as a tool for controlling crystalline explosivemorphology and inclusion content

Several research groups have reported preparations of the explosive cyclotrimethylene trinitramine (RDX) that, when formulated into plastic-bonded explosives (PBXs), result in reduced shock sensitivities when compared to the standard formulations. We recently showed a correlation between shock sensitivity of formulated RDX and the void contents of the powders using Small Angle Neutron Scattering (SANS). With this correlation in mind, we present a method for generating RDX crystals with controlled particle size and morphology using ultrasonic agitation and slow evaporation rates.