Significance of cross correlations in the stress relaxation of polymer melts

According to linear response theory, all relaxation functions in the linear regime can be obtained using time correlation functions calculated under equilibrium. In this paper, we demonstrate that the cross correlations make a significant contribution to the partial stress relaxation functions in polymer melts. We present two illustrations in the context of polymer rheology using (1) Brownian dynamics simulations of a single chain model for entangled polymers, the slip-spring model, and (2) molecular dynamics simulations of a multichain model. Using the single chain model, we analyze the contribution of the confining potential to the stress relaxation and the plateau modulus. Although the idea is illustrated with a particular model, it applies to any single chain model that uses a potential to confine the motion of the chains. This leads us to question some of the assumptions behind the tube theory, especially the meaning of the entanglement molecular weight obtained from the plateau modulus. To shed some light on this issue, we study the contribution of the nonbonded excluded-volume interactions to the stress relaxation using the multichain model. The proportionality of the bonded/nonbonded contributions to the total stress relaxation (after a density dependent "colloidal" relaxation time) provides some insight into the success of the tube theory in spite of using questionable assumptions. The proportionality indicates that the shape of the relaxation spectrum can indeed be reproduced using the tube theory and the problem is reduced to that of finding the correct prefactor. (c) 2007 American Institute of Physics

Spin spaces and positive decomposition of linear maps on ordered Banach spaces

Spin factors and generalizations are used to revisit positive generation of B(E, F), where E and F are ordered Banach spaces. Interior points of B(E, F)+ are discussed and in many cases it is seen that positive generation of B(E, F) is controlled by spin structure in F when F is a JBW-algebra.

A variational method for the calculation of spin-rovibronic energy levels of any triatomic molecule in an electronic triplet state

The triatomic spin-rovibronic variational code RVIB3 has been extended to include the effect of two uncoupled electrons, for both (3)Sigma(-) and (3)Pi (Renner-Teller) electronic states. The spin-orbital-rotational kinetic energy is included in the usual way, via terms (J+L+S). The phenomenological terms AL.S and lambda 2/3(3S(z)(2)) are introduced to reproduce the 3 spin-orbit and spin-spin splittings, respectively. Calculations are performed to evaluate the spin-rovibronic energy levels of CCO (X) over tilde (3) Sigma(-) and CCO (A) over tilde (3) Pi for which the Born-Oppenheimer potentials are derived from high-accuracy ab initio calculations.

Stress relaxation behaviour of frozen sucrose solutions

The stress relaxation behaviour of two frozen sucrose solutions (7% and 19%) during indentation in the temperature range of -20C to -40C were investigated. The stress relaxation is similar to that of pure polycrystalline ice, which is controlled by steady-state creep. The steady state creep rate exponent, m, of 7% and 19% sucrose solutions lies between 2.3 and 3.6. The steady state creep rate constant, B, of 19% sucrose solution is greater than that of 7% sucrose solution. It is suggested that the steady-state creep rate exponent m depends on contributions from the proportions of favourably oriented grains, unfavourably oriented grains and grain boundaries to creep and that these components depend on the value of internal stress which is related to the hardness of samples at the different testing temperatures. The steady-state creep rate constant B depends on the mobility of dislocations in sucrose solutions which, in turn, depends on the temperature and the concentration of sucrose.

Stress relaxation behavior of wheat dough, gluten, and gluten protein fractions

Relaxation behavior was measured for dough, gluten and gluten protein fractions obtained from the U.K. biscuitmaking flour, Riband, and the U.K. breadmaking flour, Hereward. The relaxation spectrum, in which relaxation times (tau) are related to polymer molecular size, for dough showed a broad molecular size distribution, with two relaxation processes: a major peak at short times and a second peak at times longer than 10 sec, which is thought to correspond to network structure, and which may be attributed to entanglements and physical cross-links of polymers. Relaxation spectra of glutens were similar to those for the corresponding doughs from both flours. Hereward gluten clearly showed a much more pronounced second peak in relaxation spectrum and higher relaxation modulus than Riband gluten at the same water content. In the gluten protein fractions, gliadin and acetic acid soluble glutenin only showed the first relaxation process, but gel protein clearly showed both the first and second relaxation processes. The results show that the relaxation properties of dough depend on its gluten protein and that gel protein is responsible for the network structure for dough and gluten.

Receding and disparity cues aid relaxation of accommodation

Purpose. Accommodation can mask hyperopia and reduce the accuracy of non-cycloplegic refraction. It is, therefore, important to minimize accommodation to obtain a measure of hyperopia as accurate as possible. To characterize the parameters required to measure the maximally hyperopic error using photorefraction, we used different target types and distances to determine which target was most likely to maximally relax accommodation and thus more accurately detect hyperopia in an individual. Methods. A PlusoptiX SO4 infra-red photorefractor was mounted in a remote haploscope which presented the targets. All participants were tested with targets at four fixation distances between 0.3 and 2 m containing all combinations of blur, disparity, and proximity/looming cues. Thirty-eight infants (6 to 44 weeks) were studied longitudinally, and 104 children [4 to 15 years (mean 6.4)] and 85 adults, with a range of refractive errors and binocular vision status, were tested once. Cycloplegic refraction data were available for a sub-set of 59 participants spread across the age range. Results. The maximally hyperopic refraction (MHR) found at any time in the session was most frequently found when fixating the most distant targets and those containing disparity and dynamic proximity/looming cues. Presence or absence of blur was less significant, and targets in which only single cues to depth were present were also less likely to produce MHR. MHR correlated closely with cycloplegic refraction (r = 0.93, mean difference 0.07 D, p = n.s., 95% confidence interval +/-<0.25 D) after correction by a calibration factor. Conclusions. Maximum relaxation of accommodation occurred for binocular targets receding into the distance. Proximal and disparity cues aid relaxation of accommodation to a greater extent than blur, and thus non-cycloplegic refraction targets should incorporate these cues. This is especially important in screening contexts with a brief opportunity to test for significant hyperopia. MHR in our laboratory was found to be a reliable estimation of cycloplegic refraction. (Optom Vis Sci 2009;86:1276-1286)

High signal to noise ratio THz spectroscopy with ASOPS and signal processing schemes for mapping and controlling molecular and bulk relaxation processes

Asynchronous Optical Sampling has the potential to improve signal to noise ratio in THz transient sperctrometry. The design of an inexpensive control scheme for synchronising two femtosecond pulse frequency comb generators at an offset frequency of 20 kHz is discussed. The suitability of a range of signal processing schemes adopted from the Systems Identification and Control Theory community for further processing recorded THz transients in the time and frequency domain are outlined. Finally, possibilities for femtosecond pulse shaping using genetic algorithms are mentioned.

Spin-up and adjustment of the Antarctic Circumpolar Current and global pycnocline

A theory is presented for the adjustment of the Antarctic Circumpolar Current (ACC) and global pycnocline to a sudden and sustained change in wind forcing. The adjustment timescale is controlled by the mesoscale eddy diffusivity across the ACC, the mean width of the ACC, the surface area of the ocean basins to the north, and deep water formation in the North Atlantic. In particular, northern sinking may have the potential to shorten the timescale and reduce its sensitivity to Southern Ocean eddies, but the relative importance of northern sinking and Southern Ocean eddies cannot be determined precisely, largely due to limitations in the parameterization of northern sinking. Although it is clear that the main processes that control the adjustment timescale are those which counteract the deepening of the global pycnocline, the theory also suggests that the timescale can be subtly modified by wind forcing over the ACC and global diapycnal mixing. Results from calculations with a reduced-gravity model compare well with the theory. The multidecadal-centennial adjustment timescale implies that long observational time series will be required to detect dynamic change in the ACC due to anthropogenic forcing. The potential role of Southern Ocean mesoscale eddy activity in determining both the equilibrium state of the ACC and the timescale over which it adjusts suggests that the response to anthropogenic forcing may be different in coupled ocean-atmosphere climate models that parameterize and resolve mesoscale eddies.

Electron spin resonance study of puff-resolved free radical formation in mainstream cigarette smoke

Puff-by-puff resolved gas phase free radicals were measured in mainstream smoke from Kentucky 2R4F reference cigarettes using ESR spectroscopy. Three spin-trapping reagents were evaluated: PBN, DMPO and DEPMPO. Two procedures were used to collect gas phase smoke on a puff-resolved basis: i) the accumulative mode, in which all the gas phase smoke up to a particular puff was bubbled into the trap (i.e., the 5th puff corresponded to the total smoke from the 1st to 5th puffs). In this case, after a specified puff, an aliquot of the spin trap was taken and analysed; or, ii) the individual mode, in which the spin trap was analysed and then replaced after each puff. Spin concentrations were determined by double-integration of the first derivative of the ESR signal. This was compared with the integrals of known standards using the TEMPO free radical. The radicals trapped with PBN were mainly carbon-centred, whilst the oxygen-centred radicals were identified with DMPO and DEPMPO. With each spin trap, the puff-resolved radical concentrations showed a characteristic pattern as a function of the puff number. Based on the spin concentrations, the DMPO and DEPMPO spin traps showed better trapping efficiencies than PBN. The implication for gas phase free radical analysis is that a range of different spin traps should be used to probe complex free radical reactions in cigarette smoke.

Electron paramagnetic resonance investigation of the free radicals in the gas and particulate phases of cigarette smoke using spin-trapping techniques

Free radicals in cigarette smoke have been studied using spin trapping EPR techniques. 2R4F reference cigarettes were smoked using 35 ml puff volumes of 2 seconds duration, once every 60 seconds. The particulate phase of the smoke was separated from the gas phase by passing the smoke through a Cambridge filter pad. For both phases, free radicals were measured and identified. A range of spin-traps was employed: PBN, DMPO, DEPMPO, and DPPH-PBN. In the gas-phase, short-lived carbon- and oxygen- centered radicals were identified; the ratios between them changed during the smoking runs. For the first puffs, C-centered radicals predominated while for the later puffs, O-centered radicals were mainly observed. The particulate phase and the ‘tar’ were studied as well.

Reversible addition of CO to coordinatively unsaturated high-spin iron(II) complexes

Several new coordinatively unsaturated iron(II) complexes of the types [Fe(EN-iPr)X2] (E = P, S, Se; X = Cl, Br) and [Fe(ON-iPr)2X]X containing bidentate EN ligands based on N-(2-pyridinyl)aminophosphines as well as oxo, thio, and seleno derivatives thereof were prepared and characterized by NMR spectroscopy and X-ray crystallography. Mössbauer spectroscopy and magnetization studies confirmed their high-spin nature with magnetic moments very close to 4.9 μB, reflecting the expected four unpaired d-electrons in all these compounds. Stable low-spin carbonyl complexes of the types [Fe(PN-iPr)2(CO)X]X (X = Cl, Br) and cis-CO,cis-Br-[Fe(PN-iPr)(CO)2X2] (X = Br) were obtained by reacting cis-Fe(CO)4X2 with the stronger PN donor ligands, but not with the weaker EN donor ligands (E = O, S, Se). Furthermore, the reactivity of [Fe(PN-iPr)X2] toward CO was investigated by IR spectroscopy. Whereas at room temperature no reaction took place, at −50 °C [Fe(PN-iPr)X2] added readily CO to form, depending on the nature of X, the mono- and dicarbonyl complexes [Fe(PN-iPr)(X)2(CO)] (X = Cl) and [Fe(PN-iPr)(CO)2X2] (X = Cl, Br), respectively. In the case of X = Br, two isomeric dicarbonyl complexes, namely, cis-CO,trans-Br-[Fe(PN-iPr)(CO)2Br2] (major species) and cis-CO,cis-Br-[Fe(PN-iPr)(CO)2Br2] (minor species), are formed. The addition of CO to [Fe(PN-iPr)X2] was investigated in detail by means of DFT/B3LYP calculations. This study strongly supports the experimental findings that at low temperature two isomeric low-spin dicarbonyl complexes are formed. For kinetic reasons cis,trans-[Fe(PN-iPr)(CO)2Br2] releases CO at elevated temperature, re-forming [Fe(PN-iPr)Br2], while the corresponding cis,cis isomer is stable under these conditions.

Dielectric relaxation around the nematic-isotropic transition of liquid crystalline polymers

This study is concerned with a series of acrylate based side-chain liquid crystalline (LC) polymers. Previous studies have shown that these LC polymers have a preference for parallel or perpendicular alignment with respect to the polymer chain which depends on the length of the coupling chain joining the mesogenic unit to the polymer backbone. On the other hand, the dielectric relaxation of these side-chain LC polymers shows a strong relaxation associated to the mesogenic unit dynamics. For samples with parallel alignment, it was found that the dielectric relaxation of the nematic is weaker and broader than the relaxation of the isotropic. By contrast, for samples with perpendicular alignment, the isotropic to nematic transition reduces the broadening the relaxation and increases the relaxation strength. These two features are more evident for samples with short coupling units for which the dielectric relaxation observed appears to be strongly coupled with the backbone dynamics.

A stress relaxation model for the viscoelastic solids based on the steady-state creep equation

Creep and stress relaxation are inherent mechanical behaviors of viscoelastic materials. It is considered that both are different performances of one identical physical phenomenon. The relationship between the decay stress and time during stress relaxation has been derived from the power law equation of the steady-state creep. The model was used to analyse the stress relaxation curves of various different viscoelastic materials (such as pure polycrystalline ice, polymers, foods, bones, metal, animal tissues, etc.). The calculated results using the theoretical model agree with the experimental data very well. Here we show that the new mathematical formula is not only simple but its parameters have the clear physical meanings. It is suitable to materials with a very broad scope and has a strong predictive ability.