Multiple imputation: current perspectives

This paper provides an overview of multiple imputation and current perspectives on its use in medical research. We begin with a brief review of the problem of handling missing data in general and place multiple imputation in this context, emphasizing its relevance for longitudinal clinical trials and observational studies with missing covariates. We outline how multiple imputation proceeds in practice and then sketch its rationale. We explore the problem of obtaining proper imputations in some detail and distinguish two main classes of approach, methods based on fully multivariate models, and those that iterate conditional univariate models. We show how the use of so-called uncongenial imputation models are particularly valuable for sensitivity analyses and also for certain analyses in clinical trial settings. We also touch upon other forms of sensitivity analysis that use multiple imputation. Finally, we give some open questions that the increasing use of multiple imputation has thrown up, which we believe are useful directions for future research.     

The continuum mechanical theory of multicomponent diffusion in fluid mixtures

The continuum mechanical approach for deriving the generalized equations of multicomponent diffusion in fluids is described here in detail, which is based on application of the principle of linear momentum balance to a species in a mixture, resulting in the complete set of diffusion driving forces. When combined with the usual constitutive equations including the continuum friction treatment of diffusion, the result is a very complete and clear exposition of multicomponent diffusion that unifies previous work and includes all of the various possible driving forces as well as the generalized Maxwell–Stefan form of the constitutive equations, with reciprocal diffusion coefficients resulting from Newton’s third law applied to individual molecular encounters. This intuitively appealing and rigorous approach, first proposed over 50 years ago, has been virtually ignored in the chemical engineering literature, although it has a considerable following in the mechanical engineering literature, where the focus, naturally, has been physical properties of multiphase fluid and solid mixtures. The described approach has the advantages of transparency over the conventional approach of non-equilibrium thermodynamics and of simplicity over those based on statistical mechanical or kinetic theory of gases or liquids. We provide the general derivation along with some new results in order to call attention of chemical engineers to this comprehensive, attractive, and accessible theory of multicomponent diffusion in fluids.     

On the use of acceleration waves to characterize the dynamic response of non-linear viscoelastic materials

We consider the analysis of data obtained in one-dimensional acceleration wave experiments on a non-linear viscoelastic material and discuss how such data can be used to characterize the dynamic response of the material. To begin, we review briefly the general theory of one-dimensional motions in viscoelastic materials and the methods employed to generate and observe acceleration waves in a material sample. We then discuss two methods of data analysis, a wave front analysis and a wave profile analysis, and indicate the type of information each analysis provides with regard to the dynamic properties of the material. Finally, using a known model for poly(methyl methacrylate) in a finite-difference, Lagrangian wave-propagation code, we calculate acceleration wave profiles at several locations in the sample and, treating these profiles as “experimental” data, we then illustrate how data from three or more acceleration wave experiments can be used to formulate a specific viscoelastic constitutive model for the material.     

多相流测量中涡轮流量计的影响因素分析

阐述了涡轮流量计的工作原理和动态特性,建立了涡轮流量计的多相流测量模型,并在多相流模拟装置中进行了实验验证,得出了流体密度是涡轮流量计在测量多相流的流量时的影响因子,并且讨论了流体密度影响多相流的流量测量的规律。     

A NOTE ON USING THRESHOLD AUTOREGRESSIVE MODELS FOR MULTI-STEP-AHEAD PREDICTION OF CYCLICAL DATA

Abstract. We give a brief account of how the class of threshold autoregressive time series models may be used to make short, medium and long range predictions of cyclical data.     

Multiphase monolith reactors: Chemical reaction engineering of segmented flow in microchannels

The use of segmented flow in capillaries, also known as Taylor flow, for reaction engineering purposes has soared in recent years. On the one hand, Taylor flow has been used in honeycomb monolith catalyst supports. On the other hand, Taylor flow is the common flow pattern in multiphase microchannel reactors. This contribution reviews the fluid mechanical aspects of this flow pattern in quite general terms, with an emphasis on the underlying principles. From very simple analysis, design estimates for mass transfer, pressure drop and residence time distribution may be obtained with relative ease and—for multiphase reactors—surprising accuracy.     

Expression and Role of Ubiquitin-Specific Peptidases in Osteoblasts

The ubiquitin-proteasome system regulates biological processes in normal and diseased states. Recent investigations have focused on ubiquitin-dependent modifications and their impacts on cellular function, commitment, and differentiation. Ubiquitination is reversed by deubiquitinases, including ubiquitin-specific peptidases (USPs), whose roles have been widely investigated. In this review, we explore recent findings highlighting the regulatory functions of USPs in osteoblasts and providing insight into the molecular mechanisms governing their actions during bone formation. We also give a brief overview of our work on USP53, a target of PTH in osteoblasts and a regulator of mesenchymal cell lineage fate decisions. Emerging evidence addresses questions pertaining to the complex layers of regulation exerted by USPs on osteoblast signaling. We provide a short overview of our and others’ understanding of how USPs modulate osteoblastogenesis. However, further studies using knockout mouse models are needed to fully understand the mechanisms underpinning USPs actions.     

Charge transport and its characterization using photo‐CELIV in bulk heterojunction solar cells

This mini‐review gives a simple overview of the workings of organic photovoltaic (OPV) devices, the way in which charge transfer occurs through the active layers, and then introduces how photo‐induced charge carrier extraction by linearly increasing voltage (photo‐CELIV) and time of flight (TOF) techniques can be employed to give comprehensive indications of charge carrier mobility, density and recombination in OPVs. It is shown how photo‐CELIV and TOF characterizations, using extraction current transients, can give an understanding of degradation mechanisms through observation of the trapping of charge carriers and bimolecular recombination. Examples of deployment and the interpretation of the results are given. It is hoped that this brief introduction will serve as a stepping stone into more in‐depth papers and books and encourage wider use of photo‐CELIV and TOF technologies which can be employed with whole devices. © 2016 Society of Chemical Industry     

Quantitative-structure-effect relationship for some technical nonionic surfactants

The detergency effect has been examined for a series of technical nonionic surfactants with the use of statistical experimental designs and revealed a plateau in each of the response surfaces obtained. The surfactant concentrations and washing temperatures, needed to reach the edge of each detergency effect plateau, were also determined. These conditions, which define the edge of the plateau, could be well modeled from the physicochemical properties of the surfactants with the use of partial least squares of latent structures. It was also possible to point out the importance of the different physicochemical properties. If an experimental design has been utilized, the detergency effect of a nonionic surfactant can be modeled from multiple linear regression as a function of surfactant concentration, washing time, and washing temperature. We have shown how these regression coefficients can be modeled from the physicochemical properties of the surfactants. Partial least squares of latent structures were used to estimate these models as well. We also demonstrated how these models can be used to predict the regression coefficients of a surfactant not included in the model estimations. The resultant regression coefficients can then be used to predict the detergency effects of this surfactant at different variable settings. The detergency effects thus obtained are in good agreement with measured data acquired under corresponding conditions.     

A Survey of Computational Models for Adhesion

This work presents a survey of computational methods for adhesive contact focusing on general continuum mechanical models for attractive interactions between solids that are suitable for describing bonding and debonding of arbitrary bodies. The most general approaches are local models that can be applied irrespective of the geometry of the bodies. Two cases can be distinguished: local material models governing the constitutive behavior of adhesives, and local interface models governing adhesion and cohesion at interfaces in the form of traction–separation laws. For both models various sub-categories are identified and described, and used to organize the available literature that has contributed to their advancement. Due to their popularity and importance, this survey also gives an overview of effective adhesion models that have been formulated to characterize the global behavior of specific adhesion problems.     

Ultrasonic Evaluation of Initiation and Development of Oxidation Damage in Ceramic-Matrix Composites

In this paper we report on the development of a method for ultrasonic nondestructive characterization of oxidation damage in ceramic-matrix composites. The method is based on ultrasonic measurement of elastic moduli of the composite, which are then used to determine the elastic moduli of the fiber-matrix interphase. Thus the interphasial damage may be estimated quantitatively. As a model system we used, to demonstrate applicability of the method, a unidirectional SiC-fiber-reinforced reaction-bonded silicon nitride matrix composite (SiC/RBSN). The composite samples were oxidized in flowing oxygen for 0.1, 1, 10, and 100 h at 600°, 900°, 1200°, and 1400°C. The ultrasonic phase velocity in the composite was measured at room temperature before and after oxidation; the data were then used to find the composite moduli, which quantify the induced damage. Significant changes in ultrasonic velocities and composite moduli were found as a result of oxidation. Fiber-matrix interphasial moduli were determined by multiphase micromechanical analysis. We found that oxidation of the carbon interphasial layer is the dominant mechanism in decreasing the elastic moduli of the composite. The critical exposure time for transition from the nondamaged to the damaged state at different oxidation temperatures has been determined.     

Analytik der morphologie von Mehrphasensystemen

Polymer multiphase systems are classified according to various points of view at first and the impact of supermolecular structure on physical and chemical properties is pointed out. Methods of analysis of the supermolecular structure are then discussed with emphasis on those in which chemical reactions are employed. Such methods are based on differences in either reactivity or accessibility of the different phases for specific reagents in a multiphase system. The following materials and methods are used as examples: selective dying of styrene-butadiene block copolymers and semicrystalline pol(ethylene) for electron microscopy; ordering phenomena and their characterization by mechanical spectroscopy and selective hydrolysis in segmented polyesters; chemical etching and GPC-analysis of the reaction products in order to elucidate the fine structure of the fold surfaces in semicrystalline polymers. The importance of such reactions for the present status of knowledge on the nature of supermolecular structure is extensively and critically discussed using poly(ethylene) as an example. Poly(ethylene terephthalate) is reviewed as an example for a more complicated and not fully understood case of a multiphase system. The last section deals with the problem of the distribution of comonomer units between crystalline and amorphous zones in random copolymers which have been partially crystallized It is deduced from experiments that comonomer units are built into the lattice to a fair amount. Theoretical understanding of this effect is not yet well developed.     

A new multiscale modeling approach for the prediction of mechanical properties of polymer-based nanomaterials

A detailed knowledge about the physics and chemistry of multiphase materials on different length and time scales is essential to tailor their macroscopic physical and mechanical properties. A better understanding of these issues is also highly relevant to optimize their processing and, thus, their elucidation can be decisive for their final industrial application. In this paper, we develop a new multiscale modeling method, which combines the self-consistent field theory approach with the kinetic Monte Carlo method, to simulate the structural–dynamical evolution taking place in thermoplastic elastomers, where hard glassy and soft rubbery phases alternate. Since the early seventies, it is well established that the properties of the core nanophases in these multiphase materials considerably affect their overall mechanical properties. However, recent experimental studies have clearly demonstrated that, besides the efficient handling of the core nanophases, the appropriate treatment of their interfacial region is another major challenge one has to face on the way of target-oriented development of these materials. In this work, we set a particular focus on the complex structural–dynamical processes occurring at the interphases, and study their influence on the local structural and mechanical properties. To reach our objectives, we apply the new methodology on a thermoplastic elastomer composed of ABA triblock copolymers, subjected to a sizeable external perturbation, and determine its time-averaged internal stress and composition profile. We deduce from this investigation that, to obtain the correct local mechanical properties of these multiphase materials, their structure and dynamics need to be taken into account on an equal footing. Finally, our investigation also provides an explanation and confirms the importance of the chain-pullout mechanism in the viscoelastic and stress relaxation behavior of these materials.     

How good are the Electrodes we use in PEFC?

Basically, companies and laboratories implement production methods for their electrodes on the basis of experience, technical capabilities and commercial preferences. But how does one know whether they have ended up with the best possible electrode for the components used? What should be the (i) optimal thickness of the catalyst layer? (ii) relative amounts of electronically conducting component (catalyst, with support – if used), electrolyte and pores? (iii) “particle size distributions” in these mesophases? We may be pleased with our MEAs, but could we make them better? The details of excellently working MEA structures are typically not a subject of open discussion, also hardly anyone in the fuel cell business would like to admit that their electrodes could have been made much better. Therefore, we only rarely find (far from systematic) experimental reports on this most important issue. The message of this paper is to illustrate how strongly the MEA morphology could affect the performance and to pave the way for the development of the theory. Full analysis should address the performance at different current densities, which is possible and is partially shown in this paper, but vital trends can be demonstrated on the linear polarization resistance, the signature of electrode performance. The latter is expressed through the minimum number of key parameters characterizing the processes taking place in the MEA. Model expressions of the percolation theory can then be used to approximate the dependence on these parameters. The effects revealed are dramatic. Of course, the corresponding curves will not be reproduced literally in experiments, since these illustrations use crude expressions inspired by the theory of percolation on a regular lattice, whereas the actual mesoscopic architecture of MEA is much more complicated. However, they give us a flavour of reserves that might be released by smart MEA design.     

The influence of additive property on performance of organic bulk heterojunction solar cells

The performance of bulk heterojunction organic solar cells based upon blends of donor and acceptor materials has been shown to be highly dependent on the microstructure and photoelectric properties of active layer. Recently, various methods, such as post-annealing, microwave annealing and control in the film-forming rate, and so on, have been used to modify the morphology to achieve high device performance. Among these methods, adding additives is a simple and promising approach, which can not only control the morphology but also improve the photon absorption or energy-level distribution of the active layer. In this review, we will introduce the additives that used widely in recent from following aspects: species, mechanism, and performance. First, the additive species and its selection principle according to special donor and acceptor system will be concluded. Then, the mechanisms of improved morphology and photoelectric properties by adding different kinds of additives will be illustrated in brief. At last, we will discuss the influences of additives on device performance.     

Multiphase catalytic reactor engineering and design for pharmaceuticals and fine chemicals

A review of recent developments in multiphase catalytic processes for the manufacture of pharmaceutical and fine chemicals, and an overview of reaction engineering principles needed for analysis of the local and overall reaction rate for reactor design and interpretation of performance is presented. The first section gives an overview of recent applications in pharmaceuticals and fine chemicals where heterogeneous and homogeneous catalyzed multiphase chemistries have been identified that are more efficient and represent safer operation with decreased environmental impact when compared to existing processes. The next three sections describe a scheme for classification of the various types of reactions that are typically encountered, along with distinguishing features of these reactions and commonly used multiphase reactor types. This is followed by a review of reaction engineering principles needed for describing the local overall rate of reaction, including a summary of typical models for evaluation of the intrinsic reaction kinetics, incorporation of transport-kinetic interactions, methods for identification of the controlling reaction regime and assessment of the relative contribution of transport effects. The next two sections set forth basic reactor models for commonly used reactor types, including mechanically agitated reactors and bubble column reactors. A brief summary of commonly used correlations for estimation of mass transfer coefficients in these reactors for gas-liquid and liquid-liquid systems is also given. The final section is devoted to a summary of key reaction engineering issues that occur in pharmaceutical and fine chemical multiphase catalytic processes, along with some thoughts on future needs and challenges.     

A class of exact solutions for population balances with arbitrary internal coordinates

We develop a novel transformation that maps the linear, nonhomogeneous, multidimensional population balance equation (PBE) into an advection equation that is readily solved using the method of characteristics. The PBEs targeted by this transformation exclude aggregation, breakage, and time dependent birth and death rates. In addition, internal coordinates are assumed to grow independently of each other. The ensuing general formulation is then used to recover closed‐form analytical solutions for problems with monosurface and bulk‐diffusion growth‐rates as well as Gaussian‐type nucleation. For completeness, we derive the multidimensional Green's functions for our approach. This is followed by a brief discussion on how the proposed framework may be used for code verification of moment methods such as the quadrature method of moments and the direct quadrature method of moments. Finally, a sample Mathematica code is provided to derive analytical solutions for the single‐internal‐coordinate case given user‐specified growth, birth, and death rates. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1691–1698, 2015     

Recent progress in the use of in situ X-ray methods for the study of heterogeneous catalysts in packed-bed capillary reactors

Synchrotron-based X-ray techniques, such as Diffraction and Absorption Spectroscopy (XAS), can be readily employed to study catalysts in action, thereby offering great potential for revealing the mechanism and behaviour of catalytic solids both during preparation and reaction. The continued advancement of X-ray generation and collection mean that it is now possible to obtain high quality data from catalysts under reaction conditions with second/sub-second time resolution. In this paper, we describe in detail a specific setup which can be used to obtain transmission in situ data. It is able to mimic industrial preparation/reaction environments and has been developed to such an extent that such measurements are now routine. To illustrate its applicability, we present time-resolved diffraction data obtained from an iron molybdate-based catalyst under pseudo industrial operating conditions revealing its bulk solid-state chemistry and stability, and further show how the catalyst behaviour changes as a function of the Fe/Mo ratio. We also illustrate the versatility of this setup in obtaining data using simultaneous multiple techniques (including non-X-ray-based methods, e.g. UV–vis) from an iron molybdate catalyst under methanol/air-flow. We conclude with a brief outlook towards the future, in which we identify new possibilities for studying catalysts in action which could yield insight into their preparation and behaviour.     

Enthalpic and entropic origins of nucleation barriers during polymer crystallization: the Hoffman-Lauritzen theory and beyond

In the search for a greater understanding of polymer crystallization, numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century. There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth, i.e. molecular dynamic/scattering and structural/morphological. Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process, structural/morphological analysis of polymer crystals reveal information recorded during this process. Namely, the final structure and morphology of polymer crystals have atomic, stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process. It is commonly understood that polymer crystallization, from the thermodynamic perspective, is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals. This process is controlled by a free energy barrier. A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization. The Hoffman-Lauritzen (HL) theory, which was put forward more than 40 years ago, was one of the first analytical theories to illustrate how polymers crystallize. Since then, modifications to the HL theory and suggestions for new approaches have been reported, but the core physical picture of the HL theory has largely remained intact. This article consists of four major parts: (1) we will compare the crystallization of small molecules and long chain molecules, and the relationship between crystallization and crystal habits. The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question: what is the nucleation barrier? Specifically, we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results. This barrier, from our perspective, consists of selection processes taking place in different length- and time-scales (4) finally, there is a brief discussion on what issues remain, in particular, in the areas of undercooled liquid structures and the initial stages of crystallization.