Langevin-elasticity-theory-based description of the tensile properties of double network rubbers

A previously proposed and successfully tested constitutive equation denoted by the ABGIL code (a combination of the Arruda-Boyce equation based on the Langevin elasticity theory and a constraint term based on tube theories; strain-induced increase in the finite extensibility parameter is assumed) has been found to provide a good basis for quantitative interpretation of the stress-strain data recently obtained by Mott and Roland on double networks of natural rubber, prepared by introducing additional crosslinks (second network) into a first network stretched to various extents. Experimental information on properties of the first and second networks has been used to obtain their ABGIL parameters and to calculate, under the common assumption of additivity of contributions, the stress-strain properties and residual stretch of the resulting double networks. The predictive ability of the ABGIL equation has been found to be very good. Effects of the finite extensibility of network chains appear to be significant in double networks while the possible role of orientational crystallization cannot be excluded.     

Phase equilibria of associating fluid mixtures using the perturbed-hard-sphere-chain equation of state combined with the association model

For developing the equation of state which can be applicable to associating fluids, the Perturbed-Hard-Sphere-Chain-Association (PHSC-AS) equation of state is proposed by incorporating the association term of the SAFT model into the PHSC equation of state which has been widely used to describe phase equilibria for the fluid system containing a large molecule such as polymer. In this work, two different types of PHSC models have been examined. One is the original model proposed by Song et al., and the other is the modified model by Kim and Bae whose chain term was replaced with that of the SAFT model. As a result, two types of PHSC-AS models are obtained, and applied to the calculation of phase equilibria for the binary system containing a self-associating compound such as alcohol, amine and carboxylic acid, etc. The calculated results of vapour-liquid equilibria are in good agreement with the experimental data. The proposed models (PHSC-AS) are also compared to the PC-SAFT model.     

Robust estimation for the covariance matrix of multi‐variate time series

In this article, we study the robust estimation for the covariance matrix of stationary multi‐variate time series. As a robust estimator, we propose to use a minimum density power divergence estimator (MDPDE) proposed by Basu et al. (1998) . Particularly, the MDPDE is designed to perform properly when the time series is Gaussian. As a special case, we consider the robust estimator for the autocovariance function of univariate stationary time series. It is shown that the MDPDE is strongly consistent and asymptotically normal under regularity conditions. Simulation results are provided for illustration.     

共聚高分子体系的分子热力学模型

以共聚硬球链流体的分子热力学模型为基础，通过引入链节间方阱位能相互作用的贡献。建立了实际共聚高分子系统的分子热力学模型，对纯共聚高分子ＰＶＴ的关系结果，平均相对误差为０．２２％－０．８０％。     

Effects of magnesium on the structure of aluminoborosilicate glasses: NMR assessment of interatomic potentials models for molecular dynamics

Classical molecular dynamics simulations have been used to investigate the structural role of Mg and its effect when it is incorporated in sodium aluminoborosilicate glasses. The simulations have been performed using three interatomic potentials; one is based on the rigid ionic model parameterized by Wang et al. (2018) and two slightly different parameterization of the core–shell model provided by Stevensson et al. (2018) and Pedone et al. (2020) The accuracies of these models have been assessed by detailed structural analysis and comparing the simulated nuclear magnetic resonance (NMR) spectra for spin active nuclei (²⁹Si, ²⁷Al, ¹¹B, ¹⁷O, ²⁵Mg, and ²³Na) with the experimental counterparts collected in a previous work. Our simulations reveal that the core–shell parameterizations provide better structural models. In fact, they better reproduce the NMR spectra of all the investigated nuclei and give better agreement with known experimental data. Magnesium is found to be five coordinated on average with distances with oxygen in between a network modifier (like Na) and an intermediate network formed (like Al). It prefers to lay closer to three-coordinated B atoms, forming B–NBO bonds, with respect to Si and especially Al. This can explain the formation of AlO₅ and AlO₆ units in the investigated Na-free glass, together with a Si clusterization.     

A stroboscopic X-ray apparatus for the study of the kinetics of strain-induced crystallization in natural rubber

A stroboscopic X-ray diffraction machine is described that allows studying kinetics aspects of strain-induced crystallization in natural rubber. Besides conventional mechanical cycling in extension, samples may be submitted to periodic extension cycling over a 0.05 Hz–30 Hz frequency range and for various amplitudes of elongation. Pertinent parameters as crystalline content, crystallite orientation and segmental order parameter for the amorphous fraction are determined. The existence of a regulation process of the strain experienced by the chains that remain molten due to the phenomenon of strain-induced crystallization is clearly demonstrated during conventional mechanical cycling at low deformation rate. Melting retardation is shown to lead to an equalization of crystalline content in conditions of periodic cycling of moderate amplitude and below some average elongation. On the other hand, crystallization appears to be a rapid process subsisting up to the maximal cycling frequency corresponding to a stretching time of 17 ms. This is confirmed by periodic cycling of higher amplitude that suggests that strain-induced crystallization may appear within millisecond time. An approximate logarithmic dependence of the crystalline content and elongation of the amorphous fraction upon stretching time is further found.     

Drag reduction by DNA-grafting for single microspheres in a dilute λ-DNA solution

The fluid resistance of single micrometre-sized blank and DNA-grafted polystyrene microspheres under shear flow is compared in purified water and dilute λ-DNA solutions by means of optical tweezers experiments with a high spatial (±4 nm) and temporal (±0.2 ms) resolution. The measurement results show that the drag experienced by a colloid in a dilute λ-DNA solution (molecular weight of 48,502 bp per molecule, radius of gyration of 0.5 μm) is significantly decreased if the microsphere bears a grafted DNA brush. This newly discovered drag reduction effect is studied for different parameters, comprising the molecular weight of the grafted DNA molecules (250 bp, 1000 bp and 4000 bp), the concentration of the λ-DNA solution (11, 17 and 23 μg ml^?1, all being significantly smaller than the critical entanglement concentration c¹), the microsphere core diameter (2 μm, 3 μm and 6 μm) as well as the flow speed of the medium (10–50 μm s^?1). The maximum extent of the drag reduction is found to amount to (60 ± 20)% compared to the λ-DNA-induced contribution on the drag acting on blank colloids. We propose a theoretical explanation of this effect based on the combination of the dynamic density functional theory of Rauscher and co-workers [Rauscher M. J. Phys.: Condens. Matter 2010;22:364109] and the stagnation length theory of polymer brushes, as it was established by Kim, Lobaskin et al. [Kim et al. Macromolecules 2008;42(10):3650–3655]. In particular, the solution of the Stokes equation (i.e., the Navier–Stokes equation for creeping flow) for the studied system yields a numerical prediction that is found to be in full accord with our experimental results within measurement uncertainty.     

Constitutive modeling of the behaviour of cermet compacts during reaction sintering

This study deals with the identification of a constitutive equation describing the mechanical behaviour of a nickel ferrite based cermet during sintering. This constitutive equation considers the material as a continuum and may enable one to predict the densification behaviour of a powder under different thermal treatments and the impact of compact geometry, external loading on strain and stress generation. A classical viscous equation of the Newtonian type that includes a term describing free sintering densification has been chosen. The method used for the identification of the parameters of this equation is the one proposed Gillia et al., which is based on dilatometry measurement. It includes a stairway thermal cycle for the determination of the free sintering term and intermittent loading for estimating the viscosity. This approach has been successfully applied to nickel ferrite cermet. The model has been found to be adequate to model the densification behaviour up to 1250 °C, but experimental and theoretical efforts must be accomplished to describe the behaviour above this temperature, when the material exhibits swelling.     

Calculation of the effect of macromolecular architecture on structure and thermodynamic properties of linear-tri-arm polyethylene blends from Monte Carlo simulation

A Monte Carlo simulation formalism proposed recently [Peristeras et al. Macromolecules 2007;40:2904-14] is applied here to linear-tri-arm polyethylene blends using atomistic models. Elementary Monte Carlo moves for long chain and branched molecules are used and shown to result in efficient relaxation of long chains. The effect of chain and arm molecular weight and of temperature on the structure and thermodynamic properties of blends is examined. Chemical potential versus composition diagrams are drawn in order to assess the non-ideality of mixing that may lead to phase separation. All of the blends examined are shown to be fully miscible. The microscopic blend structure is examined by calculating the radial distribution function. Finally, the radii of gyration of linear and branched chains are calculated and scaling exponents are evaluated.     

Measuring nonlinear dependence in time‐series,a distance correlation approach

We extend the concept of distance correlation of Szekely et al. (2007) and propose the auto distance correlation function (ADCF) to measure the temporal dependence structure of time‐series. Unlike the classic measures of correlations such as the autocorrelation function, the proposed measure is zero if and only if the measured time‐series components are independent. In this article, we propose and theoretically verify a subsampling methodology for the inference of sample ADCF for dependent data. Our methodology provides a useful tool for exploring nonlinear dependence structures in time‐series.     

The effect of strain-induced crystallization on the ultimate properties of an elastomeric polymer network

Elastomeric networks made up of chain molecules of sufficient structural regularity generally exhibit strain-induced crystallization. Crystallites thus formed have a pronounced reinforcing effect within the network, and thus increase its ultimate properties (ultimate strength and maximum extensibility), Increase in temperature or addition of diluent (plasticizer) suppresses the strain-induced crystallization and thus diminishes the ultimate properties. These effects are demonstrated using stress-strain isotherms obtained in elongation for crystallizable networks of cis-1, 4-polybutadiene and of polyisobutylene. The magnitude of the effects of strain-induced crystallization are determintud by comparisons of the ultimate properties of these two crystallizable networks with the corresponding ultimate properties of noncrysrallizable networks of polydimethylsiloxane.     

Parameters for the attractive coefficient of the patel-teja-valderrama equation of state

A three-parameter form of the temperature dependence of the attractive term in cubic van der Waals type equations of state (AS; Almeida et al., 1991; Aznar and Silva-Telles, 1992) has been applied to a generalized equation of state known as the Patel-Teja-Valderrama equation (PTV; Valderrama, 1990). The parameters of the AS model in the PTV equation of state were calculated for 457 substances of different polarity, molecular size, and chemical nature. The parameters were calculated using vapor pressure data determined through accurate equations available in the literature (Wagner 1973, Antoine 1888, Frost and Kalkwarf, 1953). The deviations between the equation of state calculations and vapor pressure information are lower than 0.5% for most fluids. The study shows that both the generalized PTV cubic equation and the AS represent good tools for pressure-temperature calculations in engineering applications.     

Relationships between the molecular structure and the flammability of polymers: Study of phosphonate functions using microscale combustion calorimeter

The present work is based on the Van Krevelen’s approach proposed by Lyon et al. [1] to determine the relationships between the molecular structure and the flammability of molecules using Microscale Combustion Calorimeter (MCC). Via this approach, the contributions to Heat Release Capacity (HRC) and Total Heat Release (THR) were successfully calculated for two phosphorus-containing groups: phosphonate and dioxaphosphorinane. We showed that the Van Krevelen’s approach was invalid when the molecules contain both phosphorus and ester or acid groups. We hypothesized that these functions interact between them during the thermal decomposition. In this study we propose to calculate “Interaction Indexes” to estimate the level of interactions according to the chemical structure. The results enable to draw first conclusions about the best chemical architecture for lowering flammability.     

Likelihood inference for discriminating between long‐memory and change‐point models

We develop a likelihood ratio (LR) test procedure for discriminating between a short‐memory time series with a change‐point (CP) and a long‐memory (LM) time series. Under the null hypothesis, the time series consists of two segments of short‐memory time series with different means and possibly different covariance functions. The location of the shift in the mean is unknown. Under the alternative, the time series has no shift in mean but rather is LM. The LR statistic is defined as the normalized log‐ratio of the Whittle likelihood between the CP model and the LM model, which is asymptotically normally distributed under the null. The LR test provides a parametric alternative to the CUSUM test proposed by Berkes et al. (2006) . Moreover, the LR test is more general than the CUSUM test in the sense that it is applicable to changes in other marginal or dependence features other than a change‐in‐mean. We show its good performance in simulations and apply it to two data examples.     

Hydrodynamically induced gel breakdown in pipes

A new model is reported to describe the dynamics of hydrodynamically induced breakdown of oil gel in pipes. Hitherto, the reported models (Davenport and Somper, 1971; Govier and Fogarasi, 1972; Rao et al. 1985a) cannot predict some observed inherent features of the breakdown process relating to its rates. The present model can predict these desiderata as well as delineate the effects of microscopic parameters such as size and shape of wax crystallites on the dynamics of breakdown. The model is based on a concept that energy imparted to the gelled structures is consumed in the breakdown process. Predictions of this model on breakdown time agree reasonably well with measurements. Important considerations relating to the amount of pressure head required for different diameters of pipe as well as for different dimensions of crystallites are analysed using this model.     

含蜡原油的黏弹-触变模型

在胶凝温度以下,因蜡的结晶析出并形成三维空间网络结构,含蜡原油表现出复杂的非牛顿流体特性,如黏弹性、屈服应力和触变性等。目前,含蜡原油的触变性模型均为黏塑性模型,没有考虑屈服点附近及之前物料所具有的黏弹特性。为此,提出了一个包含状态方程和速率方程、共10个未知参数的黏弹-触变模型。模型的剪应力由弹性应力和黏性应力构成,弹性应力为剪切模量与剪应变的乘积,剪切模量正比于结构参数和损耗函数。描述结构参数随时间变化的速率方程,假设结构破坏项与能量耗散率相关,克服了以往速率方程中结构破坏项与剪切率相关这一假设的不足。模型可描述含蜡原油所具有黏弹-触变特性,并且所预测的剪应力能够从以黏弹性为主的阶段平滑过渡到以触变性为主的阶段。利用大庆原油剪切率阶跃和滞回环的实验数据对模型进行验证。结果表明:两种测试的模型拟合值与实测值的平均相对偏差分别在2.0%和5.0%以内;利用由剪切率阶跃测试数据拟合得到的模型参数对滞回环测试进行预测,预测的平均相对偏差约为16%。     

On LM‐type tests for seasonal unit roots in the presence of a break in trend

This article proposes tests for seasonal unit roots allowing for the presence of a break in the trend slope occurring at an unknown date. In particular, new LM‐type tests are derived based on the framework introduced by Hylleberg et al. (1990). Null asymptotic distributions are derived for the no break case as well as when a break is present in the data‐generating process and a Monte Carlo investigation on the finite sample size and power performance of the new procedures is presented.     

“Mechanism of the self-reinforcement of cross-linked NR generated through the strain-induced crystallization”

The author proposed new models and concept for the self-reinforcement of NR. The first model indicates that general rubber vulcanizate consists of the heterogeneous structure, partially continuous cross-linked phase (75%) and continuous uncross-linked phase (25%). In addition, the author proposed other new models and concept for the strain-induced crystallization in vulcanized NR, in which the strain-induced crystallization takes place in the uncross-linked phase in cross-linked rubber. In the uncross-linked phase under large extension, molecular flow and orientation occur due to the very high compressive, shear and tensile stresses generated by the surrounding hard cross-linked phases, which makes the strain-induced crystallization possible in the uncross-linked phase. As macroscopic extension increases, the crystallization spreads over the whole uncross-linked phases, thus the uncross-linked phase changes its character from original soft rubber to the strong super network consisting of a bundle of extended molecules interconnected at the crystals. The characteristic phenomena observed in the stress-strain relation of NR such as the stress-upturn, high tensile strength and large stress-softening (Mullins effect) can be reasonably explained using these models and concepts.