Suzaku X-ray Spectra and Pulse Profile Variations during the Superorbital Cycle of LMC X-4

We present results from spectral and temporal analyses of Suzaku and RXTE observations of the high mass X-ray binary LMC X-4. Using the full 13 years of available RXTE/ASM data, we apply the ANOVA and Lomb normalized Periodogram methods to obtain an improved superorbital period measurement of 30.32 +/- 0.04 days. The phase-averaged X-ray spectra from Suzaku observations during the high state of the superorbital period can be modeled in the 0.6--50 keV band as the combination of a power-law with Gamma ~ 0.6 and a high-energy cutoff at ~ 25 keV, a blackbody with kT_BB ~ 0.18 keV, and emission lines from Fe K_alpha, O VIII, and Ne IX (X Lyalpha). Assuming a distance of 50 kpc, The source has luminosity L_X ~ 3 x 10^38 ergs s^-1 in the 2--50 keV band, and the luminosity of the soft (blackbody) component is L_BB ~ 1.5 x 10^37 ergs s^-1. The energy resolved pulse profiles show single-peaked soft (0.5-1 keV) and hard (6-10 keV) pulses but a more complex pattern of medium (2-10 keV) pulses; cross-correlation of the hard with the soft pulses shows a phase shift that varies between observations. We interpret these results in terms of a picture in which a precessing disk reprocesses the hard X-rays and produces the observed soft spectral component, as has been suggested for the similar sources Her X-1 and SMC X-1.Comment: 13 emulateapj pages, 11 figures, 4 tables; accepted for publication in Ap

Applications of sensitivity analysis for probit stochastic network equilibrium

Network equilibrium models are widely used by traffic practitioners to aid them in making decisions concerning the operation and management of traffic networks. The common practice is to test a prescribed range of hypothetical changes or policy measures through adjustments to the input data, namely the trip demands, the arc performance (travel time) functions, and policy variables such as tolls or signal timings. Relatively little use is, however, made of the full implicit relationship between model inputs and outputs inherent in these models. By exploiting the representation of such models as an equivalent optimisation problem, classical results on the sensitivity analysis of non-linear programs may be applied, to produce linear relationships between input data perturbations and model outputs. We specifically focus on recent results relating to the probit Stochastic User Equilibrium (PSUE) model, which has the advantage of greater behavioural realism and flexibility relative to the conventional Wardrop user equilibrium and logit SUE models. The paper goes on to explore four applications of these sensitivity expressions in gaining insight into the operation of road traffic networks. These applications are namely: identification of sensitive, ‘critical’ parameters; computation of approximate, re-equilibrated solutions following a change (post-optimisation); robustness analysis of model forecasts to input data errors, in the form of confidence interval estimation; and the solution of problems of the bi-level, optimal network design variety. Finally, numerical experiments applying these methods are reported

Thermogravimetric Studies of PMMA on Silica

The thermal degradation of poly(methyl methacrylate) (PMMA) has been studied extensively. It is well accepted that the degradation process is a radical chain reaction involving initiation, depropagation, transfer, and termination reactions. In the degradation process, the factors having observable effects on the decomposition behavior include molecular mass, polydispersity, tacticity, and sample dimensions. The effects of tacticity on the degradation behavior of PMMA were investigated by Kashiwagi et al., Jellinek et al. And Chiamtore et al. The results indicated that both isotactic and syndiotactic PMMA have similar decomposition pathways and activation energies. Isotactic PMMA will decompose at a slightly lower temperature and over a broader range, compared to syndiotactic PMMA with the similar chain ends and molecular masses. It has also been reported that isotactic PMMA is more sensitive to electron-beam radiation, namely, it degrades more easily, than syndiotactic PMMA. The PMMA-SiO2 system has been investigated by a few groups. The results indicated that the interaction between the carbonyl groups and silica surfaces can increase the decomposition temperature of PMMA. However, how different variables affect the decomposition behavior of PMMA is still far from clear. In the present work, we report studies of the degradation of ultrathin adsorbed PMMA on silica with a specific focus on the effects of adsorbed amount and tacticity on decomposition

Segmental Dynamics of Poly(Isopropyl Acrylate)-d7 on Silica

For a polymer film deposited on a surface, the strength of the surfacesegment interaction affects the mobility of polymer-chain segments. The selfconsistent field lattice model of Scheutjens and Fleer,1 based on mean-field lattice models of polymer at interfaces,2 has been used to describe the distribution of conformations of polymers on surfaces. Adsorbed-polymer segments may be classified as belonging to loops, trains or tails. There are different techniques used to study the molecular motion of the polymer including modulated differential scanning calorimetry (MDSC)3 and nuclear magnetic resonance (NMR).4,5 in this work, solid-state deuterium (2H) NMR was used to characterize the polymer segmental motions. Solid-state 2H NMR is an excellent tool for studying segmental dynamics. The interpretation of solid-state 2H NMR spectra of a deuterium-labeled polymer can provide valuable information on the molecular motion and the physical properties of the polymer.6,7 Relatively narrow polydispersity poly(isopropyl acrylate)-d7 (PIPA-d7) has been studied using deuterium NMR. PIPA has two methyl groups, branched at a methine carbon atom. Substitution of deuterons onto these methyl groups provides a different probe for the segment mobility than that used in previous studies.7-9 the structure of the side chains of PIPA-d7 is different than that of poly(methyl acrylate) previously studied. Bulk and surface-adsorbed PIPA-d7 polymers were investigated using the 2H quadrupole-echo NMR technique as a function of temperature

Segmental Dynamics in Poly(Methyl Acrylate)-d3 on Strongly and Weakly Adsorbing Silica Surfaces

The segmental dynamics of a polymer on a solid surface appears to be significantly different than that in bulk. The strength of the interaction between polymer segments and a solid surface is likely to be an important factor affecting the segmental mobility of the polymers. Studies of polymers on a weakly adsorbing surface would provide additional understanding of their motional characteristics on surfaces. In a recent NMR study by Rivillon et al., chain dynamics was seen to be different for samples, even when there was no specific interaction with the surface. Peanasky et al. have found a restricted segmental dynamics for the polyphenylmethylsiloxane samples at weakly adsorbing surfaces. Zheng et al. Observed a significant difference from bulk dynamics on both strongly and weakly interacting interfaces for poly(styrene) (PS) and poly(2-vinylpyridine). Mechanical properties of weakly interacting poly(methyl acrylate) (PMA)- silica composites were seen to become poorer than the ones with stronger interactions. Contrasting results for PS on silicon surfaces with X-ray reflectivity also highlighted the importance of the surface-polymer interaction for thin films.5,6 Wallace et al. explained that the contrasting results were due to a difference in the surface characteristics. A stronger surface-polymer interaction on a hydrogen-terminated silicon resulted in an increase in glass transition temperature (Tg) for thin films of PS, while Keddie et al.â s work on a silicon native-oxide surface with the same polymer indicated a decrease in Tg for thin films due to a weaker interaction. The segmental dynamics of PMA-d3 on a strongly adsorbing surface was previously studied using the deuterium (2H) quadrupole-echo NMR and modulated differential scanning calorimetry. In this study, the segmental dynamics of PMA on a weakly adsorbing surface was investigated with 2H quadrupole-echo NMR and compared to that in bulk and on a strongly adsorbing surface

Molecular Mass and Dynamics in PMA-d₃ in the Glass Transition Region

The segmental dynamics through the glass transition region, by which the glassy polymer becomes rubbery, are not well understood. Free volume, thermodynamic and kinetic theories have been used to describe the phenomenon behind the glass transition and some of those theories were tested with computer simulations. Different experimental techniques, such as NMR, thermo-mechanical analysis (TMA), differential scanning calorimetry (DSC), dilatometry and dynamic mechanical analysis (DMA) have all been also used to probe this important phenomena. Deuterium (2H) NMR has been a valuable tool for the investigation of the dynamics of macromolecules. Deuteration of macromolecules at specific locations on the chains does not significantly affect the properties of polymers. Spiess et al. Investigated motions in the glass transition region using 1D and 2D exchange NMR experiments on deuterated polystyrene. Rössler et al. studied the molecular dynamics in deuterated binary liquids close to the glass transition temperature (Tg) using 2D Exchange NMR. Blum et al. studied the effect of molecular mass on dynamics through glass transition for poly(methyl acrylate) (PMA). They found a “homogeneity” of the dynamics in the glass transition region for high molecular mass sample, but heterogeneity for the low molecular mass sample. In addition, the polydispersities of the samples were large. In this paper, we report studies of the dynamics of more monodisperse poly(methyl acrylate)-d3 (PMA)-d3 samples around the glass transition region using 2H quadrupole echo NMR and modulated DSC (MDSC)

Thermal Analysis of Adsorbed Poly(Vinyl Acetate) on Silica

The physical properties of polymers at interfaces can be quite different from those in bulk due to the interaction between the absorbed polymer and the surface. This phenomenon can be probed through the dynamic behavior of the polymer chains at the interface which can be experimentally probed by techniques such as nuclear magnetic resonance spectroscopy (NMR),1 viscoelasticity, and calorimetry. At the polymer-air interface, polymer chains have more flexibility than those at the polymer-substrate interface. On the other hand, at the silica-polymer interface, where chains are more restricted on the surface, a higher glass transition temperature (Tg) results as compared with that for the bulk polymer. Absorbed polymers like poly(methyl acrylate) (PMA) with different absorbed amounts behave differently at the silicapolymer- air interface.2 It was found that at low adsorbed amounts, the adsorbed polymer deviates significantly from the behavior of the bulk polymer. This behavior can be observed through measurement of the Tg. In this paper, we report preliminary studies of the interfacial interaction of silica-polymer-air system through the glass transition from the thermal analysis point of view. Modulated differential scanning calorimetry (MDSC) is a widely used technique for identifying the Tg of polymers. Recently, we have reported the thermal behavior of poly(methyl methacrylate) (PMMA) and PMA adsorbed on Cab-O-Sil silica.3,4 in this work, we extend the observation to poly(vinyl acetate) (PVAc), an important polymer used in paints and adhesives applications where surface interactions are crucial to the performance of PVAc-containing syste

NMR and Modulated Differential Scanning Calorimetry of Adsorbed Poly(Methyl Acrylate) on Silica

The interaction between polymers and solid substrates can be studied by a variety of techniques. The results from different experiments can be affected by the nature of the polymer, surface, polymer-surface interactions and, also, the experiment. A great deal has been learned about the behavior of polymers at interfaces, but there is also much to be learned about the interactions and how they affect the physical properties of adsorbed polymers. In the present study, we report both deuterium nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry (MDSC) of poly(methyl acrylate) (PMA) adsorbed on Cab-O-Sil silica. The focus of this study is on the dynamics of polymer segments and the interfacial glass transition temperature (Tg). In our case, very small amounts of polymer were adsorbed, and we can view behavior of polymer segments in close proximity to the silica surface. We found that both NMR and MDSC experiments provide consistent insight into the dynamics of adsorbed polymers. Both experiments show the heterogeneous nature of the adsorbed polymers, with the polymer segments closer to the solid surface being the most motionally restricted resulting in a higher Tg for those segments

Molecular Mass and Dynamics of Poly(Methyl Acrylate) in the Glass Transition Region

The segmental dynamics of bulk poly(methyl acrylate) (PMA) were studied as a function of molecular mass in the glass-transition region using 2H NMR and modulated differential scanning calorimetry (MDSC). Quadrupole-echo 2H NMR spectra were obtained for four samples of methyl-deuterated PMA-d3 with different molecular masses. The resulting spectra were fit using superpositions of simulated spectra generated from the MXQET simulation program, based on a model incorporating nearest-neighbor jumps from positions on the vertices of a truncated icosahedron (soccer-ball shape). The lower molecular-mass samples, influenced by the presence of more chain ends, showed more heterogeneity (broader distribution) and lower glass transitions than the higher molecular-mass samples. The MDSC experiments on both protonated and deuterated samples showed behavior consistent with the NMR results, but temperature shifted due to the different frequency range of the measurements in terms of both the position and breadth of the glass transition as a function of molecular mass

Segmental Dynamics in Poly(Methyl Acrylate) on Silica: Molecular-mass Effects

The effect of molecular mass on the segmental dynamics of poly(methyl acrylate) (PMA) adsorbed on silica was studied using deuterium quadrupole-echo nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry. Samples adsorbed on silica (all about 1.5 mg PMA/m2 silica) were shown to have more restricted segmental mobility, and higher Tg\u27s, than the corresponding bulk PMA samples. Around the glass-transition region, adsorbed samples exhibited segmental mobility, which could be classified as heterogeneous due to a superposition of more-mobile and less-mobile components present in the deuterium NMR spectra. This heterogeneity was consistent with a motional gradient with more-mobile segments near the polymer-air interface and the less-mobile species near the polymer-silica interface. The mobility of the adsorbed 77 kDa PMA sample was the lowest among the four different molecular-mass samples studied. Samples studied with masses both larger and smaller than 77 kDa had larger mobile-component fractions in the adsorbed polymer. The additional mobility was attributed to the presence of either longer tail and loop conformations in the higher molecular-mass samples or the inherent mobility of the tails in the lower molecular-mass samples on the surface