Thermal Cross‐Linking of Poly(Vinyl Methyl Ether). I. Effect of Cross‐Linking Process on the Viscoelastic Properties

Abstract

Thermal cross‐linking of poly(vinyl methyl ether) (PVME) in the absence of cross‐linking agent, was detected rheologically. The linear viscoelastic properties of PVME were found to be greatly changed by the onset of the cross‐linking process. The viscoelastic material functions, such as dynamic shear moduli, G′ and G″, complex shear viscosity, η*, and loss tangent, tan δ, were found to be sensitive to the structure changes during the cross‐linking process and the formation of a three‐dimensional polymer network. At the onset temperature of the cross‐linking process, an abrupt increase in G′, G″, and η* (several orders of magnitude) during dynamic temperature ramps (2°C/min heating rate) was observed with some frequency dependence. The temperature dependence of tan δ was found to be frequency independent at the gel‐point, T _gel, that is, the crossover in tan δ regardless of the value of frequency can be taken as an accurate method for determination of T _gel. The coincidence of G′ and G″ at the gel‐point cannot be considered a general method for evaluation of T _gel due to its high frequency dependence, that is, T _gel determined from the crossover of G′ and G″ in the dynamic temperature ramp at 1 rad/sec is about 20°C less than at 100 rad/sec. Furthermore, a dramatic increase in η₀ above the minimum (“v” shape) was observed at T = T _gel in agreement with the value obtained from tan δ vs. T (190°C). The time–temperature‐superposition principle was found to be valid only for temperatures lower than the T _gel (190°C); the principle failed at T ≥ 190°C. This was clearly seen in the low‐frequency region as a deviation from the terminal slope in the G′ curve. Similar behavior was observed in the modified Cole–Cole analyses (G″ vs. G′) that is, the curves start to deviate at 190°C.     

Bonding at Compatible and Incompatible Amorphous Interfaces of Polystyrene and Poly(Methyl Methacrylate) Below the Glass Transition Temperature

Abstract

Films of high‐molecular‐weight amorphous polystyrene (PS, M _w = 225 kg/mol, M _w/M _n = 3, T _g‐bulk = 97°C, where T _g‐bulk is the glass transition temperature of the bulk sample) and poly(methyl methacrylate) (PMMA, M _w = 87 kg/mol, M _w/M _n = 2, T _g‐bulk = 109°C) were brought into contact in a lap‐shear joint geometry at a constant healing temperature T _h, between 44°C and 114°C, for 1 or 24 hr and submitted to tensile loading on an Instron tester at ambient temperature. The development of the lap‐shear strength σ at an incompatible PS–PMMA interface has been followed in regard to those at compatible PS–PS and PMMA–PMMA interfaces. The values of strength for the incompatible PS–PMMA and compatible PMMA–PMMA interfaces were found to be close, both being smaller by a factor of 2 to 3 than the values of σ for the PS–PS interface developed after healing at the same conditions. This observation suggests that the development of the interfacial structure at the PS–PMMA interface is controlled by the slow component, i.e., PMMA. Bonding at the three interfaces investigated was mechanically detected after healing for 24 hr at T _h = 44°C, i.e., well below T _g‐bulks of PS and PMMA, with the observation of very close values of the lap‐shear strength for the three interfaces considered, 0.11–0.13 MPa. This result indicates that the incompatibility between the chain segments of PS and PMMA plays a negligible negative role in the interfacial bonding well below T _g‐bulk.     

Real‐Time Monitoring of Transport of Chlorinated Hydrocarbons from Industrial Areas Using Open‐Path FTIR Spectrometry with COL1SB

Abstract

Open‐path Fourier transform infrared (FTIR) spectrometry was used to continuously monitor 11 chlorinated hydrocarbon species in the region connecting two different industrial complexes for 24 hr. The single‐beam spectra obtained from open‐path FTIR were analyzed by newly developed software named COL1SB, which generates site‐specific background spectra as well as path‐averaged water vapor concentrations in addition to performing regression analyses. The path‐averaged water vapor concentrations calculated by the COL1SB agreed very well with those measured by a thermo‐hygrometer. In addition, site‐specific background spectra were generated by accounting for chemical absorption of 50 chemical species. The accurate estimation of path‐averaged water vapor concentrations, as well as the usage of site‐specific background spectra, enables chlorinated hydrocarbons to be measured by open‐path FTIR spectrometry at the ppb level. Among 11 chlorinated hydrocarbons considered here, trichlorofluoromethane, carbon tetrachloride, trichloroethylene, and tetrachloroethylene were identified as major chlorinated hydrocarbons having concentrations above 1 ppb. Wind affected the temporal variations of these chlorinated hydrocarbons, indicating the importance of local transport. However, the effect of wind differed from one species to another because of different source characteristics.     

Characterization of Mango Juice by High‐Resolution NMR,Hyphenated NMR,and Diffusion‐Ordered Spectroscopy

Abstract

The application of nuclear magnetic resonance (NMR) spectroscopy, hyphenated NMR, and diffusion‐ordered spectroscopy (DOSY) to the characterization of mango juice, as an example of a complex food mixture, is described. The compositional changes taking place as a function of ripening were followed, and selected metabolites were quantified by integration of the corresponding NMR peaks. In this way, an overall view of the metabolite changes is obtained, enabling the study of the biochemical mechanisms involved in the ripening process. More than 50 compounds were identified by 1D‐ and 2D‐NMR, but many ambiguous assignments remain due to spectral overlap or insufficient coupling information. The use of Liquid Chromatography (LC‐NMR) and LC‐NMR/Mass Spectrometry (MS) enables a fuller characterization of the soluble pectin fraction to be made; its dependence on ripening stage is discussed. Finally, DOSY adds information on the M_r of many metabolites, including the pectin fractions of ripe and unripe mango juices, and enables further peak assignments to be made.     

A normalised residence time transport equation for the numerical simulation of combustion with high-temperature air

This study concerns the numerical simulation of turbulent non-premixed combustion in highly preheated air streams. One of the objectives is to settle an efficient computational procedure to proceed with the numerical simulation of large-scale industrial devices. It is also expected that the availability of such a computational framework may facilitate comprehensive sensitivity analyses as well as the development of mathematical models able to represent turbulence-chemistry interactions (TCI) in such conditions. Based on the salient physical ingredients that characterise scalar mixing, propagation, and self-ignition processes, a turbulent combustion modelling framework is thus introduced and applied to the numerical simulation of well-documented laboratory flames. In the corresponding geometries, the bulk flow velocities of the reactants streams can reach rather large values, which lead the flame to lift from the burner rim. Partially premixed flame edges thus stabilise the whole flame structure and the temperature of the oxidising stream can be increased by vitiation with burned gases so as to promote the corresponding flame-stabilisation processes. For sufficiently large values of the vitiated airstream temperature, self-ignition mechanisms may be triggered thus leading to a competition between mixing, propagation, and ignition processes. In this context, the ratio of the residence time to the self-ignition delay is thought to be a relevant variable to delineate the possible influence of ignition phenomena. Therefore, a modelled transport equation for this normalised residence time is considered. The performance of the corresponding modelling proposal is analysed with special emphasis placed on its ability to reproduce ‘memory’ or ‘lagrangian’ effects related to thermal aging processes. In this respect, it is noteworthy that the present set of computations makes use of tabulated quantities associated to (i) steady laminar one-dimensional diffusion flamelets, so as to describe the composition of combustion products, (ii) steady laminar one-dimensional premixed flamelets, to describe the flame brush propagation, and (iii) temporal evolution of zero-dimensional homogeneous mixtures to account for the possible occurrence of self-ignition phenomena. In particular, the tabulated self-ignition time value is used to evaluate the increase in the normalised residence time. Finally, two modelling parameters are put into evidence and studied through a detailed sensitivity analysis.     

Crack growth on the basal plane in single crystal zinc: experiments and computations

Crack propagation on the basal planes in zinc was examined by means of in situ fracture testing of pre-cracked single crystals, with specific attention paid to the fracture mechanism. During quasistatic loading, crack propagation occurred in short bursts of dynamic crack extension followed by periods of arrests, the latter accompanied by plastic deformation and blunting of the crack-tip. In situ observations confirmed nucleation and propagation of microcracks on parallel basal planes and plastic deformation and failure of the linking ligaments. Pre-existing twins in the crack path serve as potent crack arrestors. The crystallographic orientation of the crack growth direction on the basal plane was found to influence both the fracture load as well as the deformation at the crack-tip, producing fracture surfaces of noticeably different appearances. Finite element analysis incorporating crystal plasticity was used to identify dominant slip systems and the stress distribution around the crack-tip in plane stress and plane strain. The computational results are helpful in rationalizing the experimental observations including the mechanism of crack propagation, the orientation dependence of crack-tip plasticity and the fracture surface morphology.     

Thermal conductivity of USb2 and UBi2 single crystals

The thermal conductivity (κ) of single crystals of tetragonal uniaxial antiferromagnets USb₂ (T _N = 202 K) and UBi₂ (T _N = 180.8 K) has been measured along the a-axis (κ_a ) over the temperature range from 0.5 to 300 K and along the c-axis (κ_c ) from 0.5 to 70 K. The as-grown samples have residual resistivity ratio (RRR) values of about 500–600 and 100–150 for UBi₂ and USb₂, respectively. The anisotropy of the thermal conductivity (κ_a (T)/κ_c (T) ～ 5) and the low-T Lorenz ratios are discussed in relation to Fermi surface topology for both compounds.     

Spin-gapped phases in the frustrated Hubbard chain with transverse spin anisotropy

At weak coupling, we investigate the frustrated Hubbard chain including nearest-neighbor and next-nearest-neighbor antiferromagnetic exchange with easy-plane anisotropy $J_1^{xy}$ and $J_2^{xy}$. At half filling, we obtain three different phases in the $J_1^{xy}$–${J_2}^{xy}$ plane: a transverse spin density wave phase, a bond order wave phase and a Luther–Emery metallic phase characterized by the coexistence of singlet superconductivity and charge density wave correlations. The frustrating exchange $J_2^{xy}$ accounts for the spin-gapped phases.     

Mean first passage time of active Brownian particle in one dimension

We investigate the mean first passage time of an active Brownian particle in one dimension using numerical simulations. The activity in one dimension is modelled as a two state model; the particle moves with a constant propulsion strength but its orientation switches from one state to other as in a random telegraphic process. We study the influence of a finite resetting rate r on the mean first passage time to a fixed target of a single free active Brownian particle and map this result using an effective diffusion process. As in the case of a passive Brownian particle, we can find an optimal resetting rate r* for an active Brownian particle for which the target is found with the minimum average time. In the case of the presence of an external potential, we find good agreement between the theory and numerical simulations using an effective potential approach.     

Identifying Particle Correlations in Quantum Hall Regime

We introduce a method that allows the disclosure of correlations between particle positions in an arbitrary many‐body system. The method is based on a well‐known simulated annealing algorithm and the proposed artificial distribution technique. Additionally, we investigate correlations in quantum Hall liquids (we consider many‐body wave functions that have been recently determined via the cyclotron subgroup model) and present three‐dimensional plots of configuration probability distributions that have been established from numerical simulations. We demonstrate that the preferred simultaneous positions of particles (configurations of positions, which correspond to large values of a system's probability distribution, ) tend to form complicated geometric structures, which are equivalent to classical Wigner crystals only for Laughlin states. Furthermore, we claim that quantum Hall liquids attributed to non‐Laughlin fillings are correlated on subdomains rather than on a whole particle domain (due to a quantizing magnetic field, which modifies the topology of a system's dynamics). Finally, we characterize Hall‐like internal orders in terms of statistical correlations (one‐dimensional unitary representations of cyclotron subgroups). Our conclusions concerning the stability of many‐body states agree with transport measurements and various numerical studies.