Time-resolved resonance fluorescence detection of Br(2Py2) atom disappearance or appearance following 266-nm laser flash photolysis of C F~B~~/ C H~S C H~/ H Z / N Z and C~~C O / C H~S C H~/ H B~/ H~/ N Z mixtures has been employed to study the kinetics of the reactions Br(ZP3/2) + CH3SCH3 -HBr + CH3SCH2 (1, -1) as a function of temperature over the range 386-604 K. Arrhenius expressions in units of cm3 molecule-' s-l which describe the results are kl = (9.0 f 2.9) X 10-l' (exp[(-2386 f 15l)lT'J) and k-1 = (8.6 f 2.5) X {exp[(836 f 140)/ T] ); errors are 2u and represent precision only. To our knowledge, these are the first kinetic data reported for each of the two reactions studied. Second and third law analyses of the equilibrium data for reactions 1 and -1 have been employed to obtain the following enthalpies of reaction in units of kcal mol-': AH298 = 6.1 1 f 1.37 and A H 0 = 5.37 f 1.38. Combining the above enthalpies of reaction with the well-known heats of formation of Br, HBr, and CH3SCH3 gives the following heats of formation of the CH3SCHz radical in units of kcal mol+ m f , 2 9 8 = 32.7 f 1.4 and AHf.0 = 35.3 f 1.4; errors are 2a and represent estimates of absoluteaccuracy. The C-H bond dissociation energy in CH3SCH3 obtained from our data, 93.7 f 1.4 kcal mol-' at 298 K and 92.0 f 1.4 kcal mol-1 at 0 K, agrees well with a recent molecular beam photofragmentation study but is 3 kcal mol-' lower than the value obtained from an iodination kinetics study. IntroductionAccurate thermochemical information for free-radical intermediates is essential toanalysis of reaction mechanisms in complex chemical systems. One experimental approach which can be employed to obtain thermochemical parameters for a radical involves measurement of temperature-dependent rate coefficients for the pair of reactions RH + R' -R'H + R, the ideal reaction pair for such a study is one where the heats of formation and absolute entropies of R', R'H, and RH are well-characterized and where kinetic data for the two reactions can be obtained over the same temperature range.In this paper we report the results of temperature-dependent kinetics studies of the following pair of reactions: Br('P,/,) + CH3SCH3 -CH3SCH2 + HBr CH3SCH2 + HBr -Br('P,/,) + CH3SCH3(1) (-1)The kinetic results have been employed to derive a value for the heat of formation of the CH3SCHz radical, an intermediate in the oxidation of the important atmospheric reduced sulfur compound dimethyl sulfide (CHSSCH,).' Experimental TechniqueThe experimental approach involved coupling reactant radical (i.e., Br or CH3SCHz) production by 266-nm laser flash photolysis of suitable precursors with time-resolved detection of groundstate bromine atom disappearance or appearance by atomic resonance fluorescence spectroscopy. A schematic diagram of the apparatus, as configured for bromine atom detection, can be found elsewhere? as can a detailed description of the experimental Abstract published in Advance ACS Abstracts, July 1, 1994.method~logy.~ Only those aspects of the experimental ap...

This paper demonstrates the results of an experimental study on cross ply carbon/epoxy composite laminates fabricated from high temperature hardener HT972 subjected to impact loading at different velocities and temperatures. The carbon fiber reinforced plastic (CFRP) samples were impacted at velocities 1.5 m/s and 2.5 m/ s, each at a temperature level of 308C, 608C, 908C, and 1208C. The impact response of the material towards various velocities and temperatures was determined using impact parameters like peak force, absorbed energy, maximum deflection, and rebound velocity. Result reveals that the velocity and temperature play a significant role in the impact response of the material. The variation in the trend of Flexural After Impact (FAI) strength of composite laminates at different velocities and temperatures was determined using FAI test and these results were further correlated with impact results. The dominating failure modes affecting the residual strength of the samples were found using acoustic emission (AE) monitoring. POLYM. COMPOS.,

This article investigates the effect of multiphase fiber system based on plain weave woven fabrics and chopped short fibers in glass/epoxy composite laminates on low‐velocity drop weight impact and quasi‐static tensile after impact (TAI) behaviors. In this regard, glass/epoxy composites with two stacking sequences (intercalated and sandwich‐like lay‐ups) were tested and compared with homogeneous standard composites fabricated with either only woven or only chopped fibers. In all the samples with multiphase fiber system, the amount of woven and chopped glass fibers were equal (4 layers and 3 layers, respectively), while stacking sequences were different. This enabled us to investigate the associated influences of fiber architecture and stacking sequence on low‐velocity drop weight impact and TAI behavior of the glass/epoxy composite laminates. All the glass/epoxy samples were impacted at four various impact energy levels, particularly 2, 4, 6, and 8 J. Results depicted that fiber architecture and stacking sequence played a vital role in low‐velocity drop weight impact and TAI behavior (i.e., damage degree, absorbed energy, peak contact force, residual deformation, residual tensile strength, displacement to ultimate failure, and failure profile) of the glass/epoxy samples. The results depicted that the sandwich‐like lay‐up configuration presented superior impact properties and damage tolerance capabilities at low impact energies (up to 4 J). On the contrary, intercalated lay‐up arrangement suited better to withstand widespread damage generated by higher impact energy levels (6 and 8 J), because they enabled a more efficient redistribution of low‐velocity impact damage in the composite. POLYM. COMPOS., 40:1450–1462, 2019. © 2018 Society of Plastics Engineers

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