1,720,958 research outputs found
Coupling of thermal field-flow fractionation and FTIR spectroscopy for the fractionation and analysis of complex polymers
Full text linkThesis (MSc)--Stellenbosch University, 2017.ENGLISH ABSTRACT: The fast-paced growth and production of complex polymers has accelerated the need for advancement in characterization techniques. The structural complexity of polymers is described using the concept of molecular heterogeneity in molar mass, chemical composition, molecular architecture. The task of characterizing complex polymeric materials is a challenge, and has accelerated the need for hyphenated techniques that provide adequate information regarding the different microstructure distributions. The increased use of hyphenated techniques has prompted the development of online coupling of thermal field flow fractionation (ThFFF) and Fourier transform infrared spectroscopy (FTIR). ThFFF is a powerful channel-based fractionation tool for microstructure analysis of complex polymers as it has the ability to separate according to both molar mass and composition in a single experiment. FTIR, coupled to a separation technique, provides concentration profiles as well as detailed chemical structure information of macromolecules as a function of ThFFF elution volume.
The presented coupled method will significantly decrease the time needed for ThFFF-FTIR as compared to collecting fractions from ThFFF separation and subsequent offline analysis by FTIR. The inherent problem with the online coupling of ThFFF to FTIR is the combination of strong solvent signals and relatively low sample concentration necessary for ThFFF separation. To overcome this, a specialized flow cell was constructed and a mathematical solvent suppression routine was used to subtract the solvent signals. In the present study, the setup of the method as well as data treatment and example measurements are presented. Blends of polystyrene (PS) and poly (methyl methacrylate) (PMMA) as well as copolymers of styrene and methyl methacrylate (SMA) are analyzed and quantified using the method. By selecting spectral bands that are unique to each analyte, the distribution of the individual analyte components across the elution profile are measured and presented. Even when quantification is not possible, the technique can be used as a means of qualitative analysis, as seen for styrene-acrylonitrile (SAN) copolymers. In one integrated procedure, ThFFF -FTIR is shown to provide detailed microstructural characterization of complex multicomponent samples.AFRIKAANSE OPSOMMING: Die vinnige groei en produksie van komplekse polimere het die behoefte aan vooruitgang in karakteriseringstegnieke versnel. Die strukturele kompleksiteit van polimere word beskryf deur gebruik te maak van die molekulêre heterogeniteit konsept in molêre massa, chemiese samestelling, molekulêre argitektuur. Die taak om hierdie polimeerstowwe te karakteriseer, is 'n uitdaging en het die behoefte aan koppeltekstegnieke versnel wat voldoende inligting verskaf oor die verskillende mikrostruktuurverdelings. Die toenemende gebruik van koppeltekens het daartoe gelei dat die ontwikkeling van aanlynkoppeling van termiese veldstroomfraktasie (ThFFF) en Fourier-transform infrarooi spektroskopie (FTIR) ontwikkel is. ThFFF is 'n kragtige kanaalgebaseerde breukinstrument vir mikrostrukture-analise van komplekse polimere, aangesien dit die vermoë het om volgens 'n enkele eksperiment volgens beide molêre massa en samestelling te skei. FTIR, gekoppel aan 'n skeidingstegniek, verskaf konsentrasieprofiele sowel as gedetailleerde makromolekule chemiese struktuurinligting as 'n funksie van elueringsvolume
Development of Rheo-IR: Combination of a Strain-Controlled Rheometer with an IR Spectrometer for in-situ Mechanical and Chemical Analysis of Cement Paste
Full text linkFourier-transform infrared (FTIR) spectroscopy is a common tool for determining the chemical composition of a material in the solid, liquid, or gas phases, both qualitatively and quantitatively. It is also employed as a technique for monitoring the rate of chemical changes as a function of time, concentration, temperature etc. These chemical changes can have rheological implications, such as polymerization kinetics, rubber crosslinking, or epoxy curing, to name a few. In this thesis, a unique set-up that is able to simultaneously measure rheology and IR spectra with an improved sensitivity, to correlate mechanical behavior with microstructural changes, was developed. The stepwise development of chemically and mechanically sensitive Rheo-IR set-up is presented. This includes the design of an IR transparent upper-plate rheological geometry used as an attenuated total reflectance (ATR) sampling tool and a description of the technical and methodological adaption of the ATR crystal into an ARES G2 rheometer. In this new set-up, a strain-controlled rheometer is combined in a novel configuration with an ATR crystal and the IR beam is guided through two off-axis parabolic mirrors to the quasi-static upper plate of the rheometer to gain maximum IR sensitivity. Thereby online and directly correlated real-time FTIR spectra can be acquired whilst simultaneously conducting rheological measurements. This allows for \u27in-situ\u27 correlation of macroscopic rheological properties with microscopic, molecular chemical changes. These experiments are conducted for a material under controlled conditions having exactly the same sample time evolution for the simultaneous measurement. In addition, this set-up allows to study the effect of shear under steady state and oscillatory shear conditions, both in the linear (SAOS) and nonlinear regime (steady shear and LAOS).
As a proof of concept and to demonstrate its potential, this newly developed method was applied to correlate the polymer network formation for a free radical co-polymerization of acrylic acid and methylenebis(acrylamide) as a crosslinking agent via IR spectroscopy and the respective mechanical time evolution, in a dilute water-based solution. In addition, the newly developed Rheo-IR technique was applied to cement paste hydration and structural build-up. The IR results were compared to those from offline FTIR. Along with the Rheo-IR and FTIR measurements other rheological techniques, including FT-Rheology, Rheo-NMR and Rheo-Dielectrics were applied to study early cement paste hydration and structural build-up. The strain deformation and flow of fresh cement paste on the basis of microscale processes during early hydration was of interest. Applying FT-rheology, the intensity at mechanical higher harmonics are quantified and normalized to the fundamental intensity. A model with a quadratic scaling in the strain amplitude is used to predict two critical strains, that can be associated with the solid () and mobile () parts of cement paste. Interestingly, the effect of hydration time has minimal relevance on these critical strains. The influence of the microstructure seen through the mechanical evolution is attributed to the formation of hydrates, as determined spectroscopically through FTIR. Although, the hydration products (i.e. ettringite) do form within the first hour of hydration, the relative amount seems not to be the main contributing factor to structural build up. The structural build-up as a function of applied strain as investigated by Rheo-NMR, shows that although a relatively large strain for cement () is applied, the molecular mobility is the same as that of a lower strain (), within the first hour of hydration. Through Rheo-Dielectrics, a similar trend is observed for the dc-conductivity. However, after 2 to 5 hours a 100 to 1000 times decrease in conductivity is seen, due to setting. The sample exposed to a higher strain, sets faster. Based on the observations from the Rheo-combined methods as a well as FT-Rheology the assumption is that physical interactions such as colloidal interactions could be more dominant than chemical bond formation for structural build-up in the first few minutes to the first hour of cement paste hydration
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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