Method for determining resin cure kinetics with low-frequency Raman spectroscopy.

Characterizing resin extent of cure kinetics is critical to understanding the structure-property-processing relationships of polymers. The disorder band present in the low-frequency region of the Raman spectrum is directly related to conformational entropy and the modulus of amorphous materials, both of which change as the resin polymerizes. Normalizing the disorder band to its shoulder (∼85 cm-1) provides structural conversion kinetics, which we can directly correlate to chemical conversion kinetics for methacrylate and epoxy-amine based resin systems. In addition to fitting both the structural and chemical conversion data to a phenomenological kinetic rate equation, we also demonstrate a relationship between the chemical and structural kinetics which appears to relate to the softness of the material. Lastly, we use the method to investigate a methacrylate/epoxy interpenetrating polymer network resin system. We find that the structural and chemical conversions occur simultaneously during the formation of the primary (methacrylate) network, but there is a lag between the two during the formation of the secondary (epoxy-amine) network.

Reconversion of Parahydrogen Gas in Surfactant-Coated Glass NMR Tubes.

The application of parahydrogen gas to enhance the magnetic resonance signals of a diversity of chemical species has increased substantially in the last decade. Parahydrogen is prepared by lowering the temperature of hydrogen gas in the presence of a catalyst; this enriches the para spin isomer beyond its normal abundance of 25% at thermal equilibrium. Indeed, parahydrogen fractions that approach unity can be attained at sufficiently low temperatures. Once enriched, the gas will revert to its normal isomeric ratio over the course of hours or days, depending on the surface chemistry of the storage container. Although parahydrogen enjoys long lifetimes when stored in aluminum cylinders, the reconversion rate is significantly faster in glass containers due to the prevalence of paramagnetic impurities that are present within the glass. This accelerated reconversion is especially relevant for nuclear magnetic resonance (NMR) applications due to the use of glass sample tubes. The work presented here investigates how the parahydrogen reconversion rate is affected by surfactant coatings on the inside surface of valved borosilicate glass NMR sample tubes. Raman spectroscopy was used to monitor changes to the ratio of the (J: 0 → 2) vs. (J: 1 → 3) transitions that are indicative of the para and ortho spin isomers, respectively. Nine different silane and siloxane-based surfactants of varying size and branching structures were examined, and most increased the parahydrogen reconversion time by 1.5×-2× compared with equivalent sample tubes that were not treated with surfactant. This includes expanding the pH2 reconversion time from 280 min in a control sample to 625 min when the same tube is coated with (3-Glycidoxypropyl)trimethoxysilane.

Experimental verification of sparse frequency linearly frequency modulated ladar signals modeling.

We present the results of an experiment designed to verify the results of a previously published theoretical model that predicts the range resolution and peak-to-side lobe ratio of sparse frequency linearly frequency modulated (SF-LFM) ladar signals. We use two ultra stable diode lasers which are frequency locked and can be current tuned in order to adjust the difference frequency between the two lasers. The results of the experiment verify the previously developed model proving that SF-LFM ladar signals have the ability to increase the range resolution of a ladar system without the need for larger bandwidth modulators. Finally we simulate a target at a range of approximately 150 meters through the use of a fiber optic delay line, and demonstrate the ability of SF-LFM ladar signals to detect a target at range.

Sparse frequency LFM ladar signals.

Through modeling we explored the possibility of utilizing a sparse frequency linear frequency modulation (LFM) signal for laser radar (ladar) applications. We propose a potential transmit and receive experiment utilizing the superposition of two LFM laser sources with a known difference frequency to provide the necessary segmented bandwidth. Finally we analyzed the signal performance of the proposed system showing that the range resolution of the signal can be improved by two to three times while utilizing the same modulator bandwidth as that of a continuous LFM signal.