Recent Developments in Solid-Propellant Binders

Abstract

The complex and important role of the propellant binder in solid-propellant rockets is described. The severe weight problems of space exploratrion cause high propellant performance to be of major concern in a highly competitive field. Binders contribute to performance, not only in terms of fuel value, but by being compatible with energetic components and by providing good mechanical properties. Both sterilization required for planetary landings and compatibility problems of new energetic oxidizers generate new requirements too stringent for existing binders; only binders composed essentially of saturated aliphatic hydrocarbon are predicted to be suitable. Several saturated-binder developments are in progress. The achievements of one of these programs, based on free-radical synthesis, are evaluated against the goals of the new binder.     

Rapid-scan EPR with triangular scans and fourier deconvolution to recover the slow-scan spectrum

Direct-detected rapid-scan EPR signals were recorded using triangular field scan rates between 1.7 and 150 kG/s for deoxygenated samples of lithium phthalocyanine (LiPc) and Nycomed trityl-CD3. These scan rates are rapid relative to the reciprocals of the electron spin relaxation times and cause characteristic oscillations in the signals. Fourier deconvolution with an analytical function permitted recovery of lineshapes that are in good agreement with experimental slow-scan spectra. Unlike slow-scan EPR, direct detection rapid-scan EPR does not use phase sensitive detection and records the absorption signal directly instead of the first derivative of the absorption signal. The amplitude of the signal decreases approximately linearly with applied magnetic field gradient. Images of phantoms constructed from samples of LiPc and trityl-CD3 were reconstructed by filtered back-projection from data sets with a missing angle. The lineshapes in spectral slices from the image are in good agreement with slow-scan spectra and the spacing between sample tubes matches well with the known sample geometry.     

Formal hydride transfer mechanism for photoreduction of 3-phenylquinoxalin-2-ones by amines. Association Of 3-phenylquinoxalin-2-one with aliphatic amines

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin-2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a bathochromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ and [HNQ-amine], both MeNQ and HNQ are photoreduced efficiently on irradiation in the presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(-) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxalin-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XNQ in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mechanism for the reversible photoreduction.     

ABSORPTION AND EMISSION STUDIES OF ELECTRONIC STATES OF N-ARYLBENZAMIDES

Abstract— Absorption and emission spectra of several N-arylbenzamides have been measured. The quantum yields for their fluorescence were found to be dependent on matrix viscosity and temperature. Singlet-triplet splittings for these compounds were determined from their emission spectra and found to be abnormally small for π. π* states (˜ 1500 cm^-1). Indeed, the phosphorescence maxima of N-arylbenzamides occur slightly to the blue relative to their fluorescence maxima. Intersystem crossing efficiencies were determined for several of these compounds and are consistent with S₁→ S ₀ radiationless decay.     

EPR Free Induction Decay Coherence Observed after a Single Pulse in Saturation Recovery Experiments for Samples with Resolved Multiline CW Spectra

An electron paramagnetic resonance (EPR) spin-coherence signal has been observed following a single pulse for rapidly tumbling radicals with well-resolved nuclear hyperfine splitting in fluid solution when B ₁ is large enough to excite multiple hyperfine lines. This signal, which has the shape of a spin echo, arises from constructive interference of overlapping free induction decays (FIDs) from the hyperfine lines. It has been observed for 2,6-di-t-butyl-1,4-benzosemiquinone, 2,5-di-t-butyl-1,4-benzosemiquinone, 2,3,5,6-tetramethoxy-1,4-benzosemiquinone, 2,4,6-tri-t-butylphenoxyl radical, and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy. It occurs at a time after the pulse that is equal to the inverse of the nuclear hyperfine splitting, independent of EPR resonance frequency from 250 MHz to 9.1 GHz. As the length of the pulse is increased, separate coherence signals can be observed that correspond to the beginning and end of the pulse. This coherence is distinct from the "single-pulse echo" signals discussed in the literature. For 2,6-di-t-butyl-1,4-benzosemiquinone, which has two resolved couplings (1.24 and 0.052 G), FID oscillations with a period that corresponds to the larger hyperfine coupling are observed on the coherence signal that arises from the smaller hyperfine coupling. If phase cycling is not perfect, the coherence signal can interfere with measurements of T ₁ by saturation recovery. Authors' address: Gareth R. Eaton, Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, USA     

Multifrequency electron paramagnetic resonance of ultramarine blue

Electron paramagnetic resonance spectra of the S^3/? radical center in ultramarine blue over a factor of about 2500 in frequency (258 MHz to 670 GHz) reveal a substantially Lorentzian shape, without resolution of g anisotropy. Variable temperature measurements found that the line width is independent of temperature, within experimental uncertainty, up to about 90 K at 9.5 GHz and between ca. 5 K and room temperature at 95 and 217 GHz, as expected for an exchange-narrowed signal. Analysis of the increase in the low-temperature line width as a function of frequency above 9 GHz is consistent with an exchange interaction of about 2· 10^?2 K. The line width increases as frequency is decreased from 2.7 GHz to 258 MHz which is attributed to the contribution from nonsecular terms that has been denoted the “10/3” effect.