Search

When explosives detonate in a confined space, repeated boundary reflections result in complex shock interactions and the formation of a uniform quasi-static pressure (QSP). For fuel-rich explosives, mixing of partially oxidized detonation products with an oxygen-rich atmosphere results in a further energy release through rapid secondary combustion or 'afterburn'. While empirical formulae and thermochemical modelling approaches have been developed to predict QSP, a lack of high-fidelity experimental data means questions remain around the deterministic quality of confined explosions, and the magnitude and mechanisms of afterburn reactions. This article presents experimental data for RDX- and PETN-based plastic explosives, demonstrating the high repeatability of the QSP generated in a sealed chamber using pressure transducers and high-speed infrared thermometry. Detonations in air, nitrogen and argon atmospheres are used to identify the contribution of afterburn to total QSP, to estimate the duration of afterburn reactions and to speculate on the flame temperature associated with this mechanism. Computational fluid dynamic modelling of the experiments was also able to accurately predict these effects. Understanding and quantifying explosions in complex environments are critical for the design of effective protective structures: the mechanisms described here provide a significant step towards the development of fast-running engineering models for internal blast events. [ABSTRACT FROM AUTHOR]

The local segmental dynamics of four styrene-b-isoprene-b-styrene-b-isoprene (SISI) tetrablock copolymers with different styrene composition f[sub s] and constant total degree of polymerization Napprox. 120 has been studied in the disordered state in the nano-picosecond time scale, by incoherent quasielastic neutron (QENS), and Brillouin (BS) and depolarized Rayleigh (DRS) light scattering. Far above the glass transition temperature, all three techniques demonstrate the presence of two distinct time scales from which the fast segmental relaxation was quantitatively resolved. This process is associated with the mobility of the polyisoprene (PI) component, and is moderately slower and possesses a broader distribution of relaxation times than in bulk PI. The comparison between the correlation times of DRS and the characteristic times of QENS suggest that segment (hydrogen nucleus) diffusion over a distance of approx. 0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, approx. 0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The critical behavior of six polyisoprene/poly(ethylene–propylene) binary blends has been examined, primarily by dynamic light scattering. Specifically, the decay rate (Γ) of fluctuations in composition has been measured as a function of scattering vector (q) and temperature, above the stability limit. The results are interpreted in the context of mode-coupling corrections to the dynamics. Theoretically predicted crossovers between mean-field and nonclassical regimes, and between mode-coupled and mode-decoupled dynamics, are clearly seen. The mean-field to nonclassical crossover, or Ginzburg criterion, is localized, and the mode-coupled to mode-decoupled transition in the dynamics occurs well into the static mean-field regime. In addition, geometric crossovers between Γ∼q2 (‘‘diffusive’’) and Γ∼q3 or Γ∼q4 (‘‘critical nondiffusive’’) regimes are in quantitative agreement with theory. The applicability of the Kawasaki–Stokes relationship between Γ and the dynamic correlation length is also demonstrated. [ABSTRACT FROM AUTHOR]

Dynamic depolarized light scattering has been utilized to probe the rotational motion of the solvent Aroclor (A1248) in solutions containing either polystyrene (PS) (cPS=0, 0.086 or 0.146 g/cc) or 1,4-polybutadiene (PB) (cPB=0.062, 0.098, or 0.16 g/cc) near the solution glass transition. The orientational time correlation functions cVH(t) for the solvent A1248 were recorded as a function of temperature (-13 to -31 °C) over the time range 10-6 –1 s by photon correlation spectroscopy. A unimodal relaxation time distribution describes cVH(t) at temperatures near Tg and the breadth of the distribution of solvent relaxation time increases upon addition of the polymer. The average orientation time 〈τ> increases in PS/A1248 solutions and decreases in 1,4 PB/A1248 solutions, and the influence of 1,4 PB is temperature dependent. These main experimental results, which provide further support for the notion of polymer-induced modification to the solvent dynamics, are discussed in terms of the Adam–Gibbs description modified according to the coupling relaxation model. [ABSTRACT FROM AUTHOR]

We report evidence of modification of solvent reorientation induced by the presence of polymer in polymer/solvent systems. Depolarized Rayleigh spectroscopy (DRS) has been used to probe the rotational motion of neat Aroclor and polystyrene (PS) solutions with polymer concentration 0.05, 0.086, and 0.146 g/cm3. The depolarized (IVH(ω)) spectra were recorded in the temperature range 40 to 140 °C using Fabry-Perot interferometry. Two Lorentzian lines were found to fit well the experimental IVH(ω) of PS/Aroclor above 60 °C revealing two populations of solvent. The fast relaxation time τf is virtually insensitive to variations of PS content in contrast to a considerable slowing down effect on the slow reorientation time τs obtained more precisely from photon correlation and oscillatory electric birefrigence measurements at low temperatures (<0 °C). The intensity Is of the narrow Lorentzian component, which was found to increase rapidly below a characteristic temperature at given PS concentration, is associated with the slow orientation of the solvent in the PS environment. Surprisingly, the onset of PS induced modification occurs at rather high temperatures (∼Tg+110 K). The values of the intensity ratio Is/If are rationalized in the framework of the restricted rotational diffusion model. Alternatively, the study of the single Lorentzian IVH(ω) spectra of the neat solvent suggests a significant amount of orientational pair correlations and is consistent with clustering in liquid Aroclor. [ABSTRACT FROM AUTHOR]

The rotational relaxation of the solvent Aroclor 1248 (A1248) has been examined in solutions containing either polystyrene (PS), 0≤c≤0.269 g/cc, or polybutadiene (PB), 0≤c≤0.264 g/cc, by oscillatory electric birefringence. Measurements were performed at eight temperatures from -17.31 to 25.00 °C. In all cases the normalized solvent relaxation time τ/τ0, where τ0 was the value in neat solvent, was an exponential function of concentration. For PS/A1248, ∂ ln(τ/τ0)/∂c was independent of temperature and equal to 13±1 cc/g, whereas for PB/A1248, ∂ ln(τ/τ0)/∂c increased steadily with temperature and was negative for measurement temperatures of 2.81 °C and below. These observations were qualitatively consistent with a picture of solvent clustering or ordering, which was enhanced by the addition of PS but disrupted by PB. Although some features of the data were also consistent with changes in solution free volume, as indicated by measurements of the composition-dependent glass transition temperature, such considerations were not sufficient to reconcile all the observed behavior. The ratio τ/τ0 was interpreted as representing an effective, average solvent friction function ζ(c,T), which could also be used to define an effective solvent viscosity, ηe(c,T)=ηs(T)·ζ(c,T). It was shown that ∂ ln ζ/∂c, also equivalent to an intrinsic effective viscosity [ηe], was very close in magnitude and sign to the intrinsic high frequency limiting viscosity [η’∞] for both PS/A1248 and PB/A1248. This implies that the measured values ofη∞ reflect predominantly polymer-induced changes in solvent friction. Therefore, it may not be necessary to invoke additional sources of energy dissipation, such as chain stiffness or internal viscosity, to describe the high frequency viscoelastic or oscillatory... [ABSTRACT FROM AUTHOR]