Analysis of the surface diffusion of alkylbenzenes and p-alkylphenols in reversed-phase liquid chromatography using the surface-restricted molecular diffusion model

Surface diffusion of alkylbenzene and p-alkylphenol derivatives was measured in reversed-phase liquid chromatography on a C18-silica column with a methanol/water mixture (70/30, v/v) as the mobile phase. They were analyzed on the basis of the restricted molecular diffusion model for surface diffusion. The temperature dependence of the surface diffusion coefficient (Ds) arises probably from that of molecular diffusivity, suggesting a correlation between surface and molecular diffusion. Other correlations were also observed: (1) an enthalpy-entropy compensation of the retention equilibrium constant, (2) a linear free energy relationship between the retention and surface diffusion, and (3) a linear correlation between the restriction energy of surface diffusion and the isosteric heat of adsorption. The physical meaning of parameters involved in the surface-restricted molecular diffusion model is discussed on the basis of these results. A practical and convenient procedure for the estimation of Ds is also suggested. This study demonstrates how surface diffusion phenomena can be accounted for on the basis of the correlation between surface and molecular diffusion.     

Theoretical modelling of carrier transports in molecular semiconductors: molecular design of triphenylamine dimer systems

Charge transport in molecular systems and biosystems can be different from that in inorganic, rigid semiconductors. The electron-nuclear motion couplings play an important role in the former case. We have developed a theoretical scheme to employ the Marcus electron transfer theory coupled with a direct diabatic dimer model and the Brownian diffusion assumption to predict the carrier mobility for molecular materials. For triphenylamine, a typical molecular transport material, the design strategies regarding the formation a cyclic or a linear dimer are evaluated from theoretical calculations for the carrier mobility. We made a comparison between the mobility and the electrical polarizability. It is found that in the case of triphenylamine dimer, these two quantities have different trends. The fact that the macrocycle possesses higher mobility but lower polarizability than the linear chain is due to the difference in the reorganization energy. The theoretical predicted temperature dependences are analysed within the hopping mechanism. The calculated room-temperature mobilities are in reasonable agreement with experimental values.     

Thermodynamic prediction of polymer retention in temperature-programmed HPLC

Temperature programming has been used increasingly in liquid chromatography in recent years. In particular, temperature gradient elution has shown great potential in the analysis of complex polymers. In this study, the polymer retention behavior in temperature gradient interaction chromatography is investigated based on thermodynamic consideration of the retention factor. The polymer retention predicted by the model calculation is in good agreement with the experimental results, and the model allows devising a temperature program for designed retention behaviors such as a linear dependence of retention volume on log(molecular weight) of polymers. In addition, the migration behavior of polymeric solute along the separation column can be simulated, which shows strong molecular weight dependence. The migration behavior is also confirmed experimentally by employing different length columns or delayed injection.     

Thermal stability and melt rheology of poly(<Emphasis Type="Italic">p</Emphasis>-dioxanone)

Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO’s thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples’ viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (M _w = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox–Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.     

Poly-para-phenylene-ethynylene assemblies for a potential molecular nanowire: an SFM study

The self-assembly of a poly-para-phenylene-ethynylene (PPE) derivative cast onto muscovite mica is investigated by Tapping Mode Scanning Force Microscopy. The film morphology depends on the concentration of the PPE in the casting solution. At high concentration a grain morphology with some linear aggregates is observed. At low concentration PPE self-assembles into well defined and stable needles with a cross section with molecular dimensions. A model for the packing of the molecules in these supramolecular structures is suggested. Properly functionalised needles exhibit the molecular architecture of a molecular nanowire ready to be interfaced by metallic nanoelectrodes.     

Nonlinear Viscoelastic Behaviour of Thermorheologically Complex Materials

In previous work, a phenomenologically constitutive model was presented describing the finite, nonlinear, viscoelastic behaviour of polymer glasses up to yield. This model was, however, restricted to thermorheologically simple materials. In this paper this restriction is removed, thus extending the model to materials behaving thermorheologically complex. Based on linear viscoelasticity, this extension can be achieved by either adding a process in parallel, or in series. Experiments in the plastic range suggested an approach based on stress additivity, i.e. two processes in parallel. The resulting model consists of two linear relaxation time spectra in parallel, each having its own characteristic stress and temperature dependence. Whereas in the case of a single process the influence of stress and temperature is comparable, this is no longer valid for two processes since the molecular processes depend on a part of the applied stress rather than on the total applied stress itself. Numerical predictions using the extended representation showed that the model correctly describes the yield behaviour observed in practice. Simulations of creep experiments at various stress levels and temperatures showed a good qualitative agreement with experimental observations in literature.     

Multiscale modeling of progressive failure in polymer nanocomposites using nanoscale informed damage mechanics

In this article, a novel technique to model damage and damage evolution in polymer nanocomposites (PNCs) using the internal state variable approach is proposed. The multiscale aspects of the nanocomposite are captured by embedding local inhomogeneities and the localized nanoparticle (nanographene) and polymer atom interactions at the interface into a continuum scale damage model. This approach assumes that the damage evolution is primarily due to changes in nonbonded interactions at the nanoscale, hence the moniker nanoscale-informed damage mechanics (NIDM) model. The unknown coefficients in the NIDM model are obtained by a linear regression fit of the polymer and the PNC stress-strain behavior using molecular dynamics simulation. It is envisioned that the resulting nonlinear constitutive model can be readily incorporated into a hierarchical multiscale finite element model and used by the aerospace industry for structural scale applications to model progressive failure.     

The influence of poly(acrylic acid) molecular weight on the fracture toughness of glass-ionomer cements

A linear elastic fracture mechanics approach has been used to characterize failure in glass-ionomer cements. The toughness, fracture toughness, flexural strength and inherent flaw size increase with the molecular weight of the poly(acrylic acid), whilst the Young's modulus remains approximately constant. The dependence of toughness on poly(acrylic acid) molecular weight is not as large as predicted by a reptation/chain pull out model for fracture and this is thought to be a result of the weak ionic crosslinks formed during the cement reaction.     

A promising approach for modeling biological fibers

The final-to-initial stiffness ratio is very large (>100) for many biological fibers, and as such, these materials have been modeled as being strain limiting. We propose an unconventional structure for a stored energy function that leads to a constitutive relation capable of describing this observed strain-limiting behavior. The model can attain infinite stress at a finite strain while storing a finite amount of internal energy. Many biological fibers have a mechanical response that starts out as being compliant and nonlinear, and transitions into one that is stiff and linear. We present a biological fiber model comprised of a strain-limiting fiber (strain being attributed to molecular reconfiguration) loaded in conjunction with a Hookean fiber (strain being attributed to molecular stretch). The model’s parameters are physical, intuitive and readily extracted from a stress/strain curve. Chordæ tendineæ data are used to demonstrate the efficacy of the model.     

线性压电马达的动态模拟与性能评估

提出了一个模拟线性压电马达运动特性的动态模型.该模型考虑了马达压电元件的运动顺序,将压电马达视为一个具有变接触刚度与变阻尼系数的两自由度动态系统.其本构方程考虑了压电元件的机-电耦合效应.该模型使用Co-quad图、波特图和根轨迹图进行表述以及实验验证.结果表明,该模型能够预测线性压电马达的运动特性,并可用于改进或设计新的线性压电马达.性能评估表明,实验中使用的线性压电马达具有5 nm的分辨率、20mm/s的行走速度以及220 N的驱动力.     

Strategy and its implications of protein bioanalysis utilizing high-resolution mass spectrometric detection of intact protein

Currently, mass spectrometry-based protein bioanalysis is primarily achieved through monitoring the representative peptide(s) resulting from analyte protein digestion. However, this approach is often incapable of differentiating the measurement of protein analyte from its post-translational modifications (PTMs) and/or potential biotransformation (BTX) products. This disadvantage can be overcome by direct measurement of the intact protein analytes. Selected reaction monitoring (SRM) on triple quadrupole mass spectrometers has been used for the direct measurement of intact protein. However, the fragmentation efficiency though the SRM process could be limited in many cases, especially for high molecular weight proteins. In this study, we present a new strategy of intact protein bioanalysis by high-resolution (HR) full scan mass spectrometry using human lysozyme as a model protein. An HR linear ion-trap/Orbitrap mass spectrometer was used for detection. A composite of isotopic peaks from one or multiple charge states can be isolated from the background and used to improve the signal-to-noise ratio. The acquired data were processed by summing extracted ion chromatograms (EIC) of the 10 most intense isotopic ions of octuply protonated lysozyme. Quantitation of the plasma lysozyme was conducted by utilizing high resolving power and an EIC window fitting to the protein molecular weight. An assay with a linear dynamic range from 0.5 to 500 μg/mL was developed with good accuracy and precision. The assay was successfully employed for monitoring the level of endogenous lysozyme and a potential PTM in human plasma. The current instrumentation limitations and potential advantages of this approach for the bioanalysis of large proteins are discussed.     

Rheological Behaviour for Polymer Melts and Concentrated Solutions Part Ⅲ: A New Multiple Entanglement Model to Predict the Dependence of Linear Viscoelastic Function (η_0, Ψ_(10)~0,η_(ext)~0) on the Ranges of Primary Molecular Weights and the

It is shown theoretically that the viscoelasticity of polymer melts is determined by three combining factorst they are the primary molecular weight and its distribution, the number of entanglement sites on polymer chain and the sequence distribution of constituent chains in entanglement spacings. A unified quantity for the three combing factors is the average constrained dimensional number of constituent chains in the long entanglement spacings (v). A new relation of v to the primary molecular weight and the number of testing polymers were derived from the multiple entanglement and reptation model, and a new method for determining v was proposed. The dependences of linear viscoelastic functions on the primary molecular weight and its distribution were derived by the statistical method. When Mn=6Me to 18 Me, the values of (v) can range from 3.33 to 3.70. Their values are in a good agreement with the experiment data, and it can slightjy vary with the different species of polymers and the different ranges of molecular weight of polymers     

Expert judgement combination using moment methods

Moment methods have been employed in decision analysis, partly to avoid the computational burden that decision models involving continuous probability distributions can suffer from. In the Bayes linear (BL) methodology prior judgements about uncertain quantities are specified using expectation (rather than probability) as the fundamental notion. BL provides a strong foundation for moment methods, rooted in work of De Finetti and Goldstein. The main objective of this paper is to discuss in what way expert assessments of moments can be combined, in a non-Bayesian way, to construct a prior assessment. We show that the linear pool can be justified in an analogous but technically different way to linear pools for probability assessments, and that this linear pool has a very convenient property: a linear pool of experts’ assessments of moments is coherent if each of the experts has given coherent assessments. To determine the weights of the linear pool we give a method of performance based weighting analogous to Cooke's classical model and explore its properties. Finally, we compare its performance with the classical model on data gathered in applications of the classical model.     

线型高分子无规交联和裂解的一个动力学模型

建立了线型高分子无规交联和裂解的一个动力学模型。通过模型,推导了一级反应的数均分子量的计算公式,探讨了交联和裂解速率对最终产物数均分子量的影响,以及在纯交联过程中,起始分子量分布对凝胶化时间的影响。文中指出,无规裂解的Simha模型仅仅是本模型中的一个特例。     

A general solution to the necklace model problem in the rheology of macromolecules

Summary A very general solution is given of a problem which plays a vital role in certain molecular theories on the deformation of macromolecules. In these theories use is made of the necklace model, in which the macromolecule in solution is supposed to be built up of submolecules or segments. The total hydrodynamic resistance of the monomers in a segment is assumed to be concentrated in the end points of the segments: the beads. By applying a macroscopic field of flow to the solution, forces are exerted on the beads which are compensated by entropy-elastic forces in the segment. The configuration of the beads is described by a distribution function which must satisfy an equation of continuity. This equation is now solved by means of Fourier transformation for any time-dependent field of flow for which the velocities are linear functions of the coordinates. The solution appears to be a Gaussian distribution whose second moments have to satisfy a system of linear first-order time-dependent differential equations. Once the distribution function is known, all kinds of macroscopically measurable quantities of the solution can in principle be calculated.