1,721,030 research outputs found
Kinetics of degradation of non-recycled and recycled contemporary paper
Full text linkPaper is still regarded as the most common carrier of information. Encouraged by environmental policies, the papermaking technology recently changed, resulting in an increased production of recycled paper. Two types of contemporary paper, non-recycled and recycled, were thus investigated to evaluate possible differences in their rate of degradation. The analyses were carried out using unaged paper to evaluate acidity, dry matter and ash content, lignin presence, and fibre furnish. Accelerated ageing experiments were performed at different temperatures (50–80 °C) and water vapour pressures (90.5–227.5 torr). Viscometric measurements were carried out to evaluate the extent of paper degradation as a function of time, upon various accelerated ageing conditions. The results indicate that the rates of degradation follow first-order kinetics with respect to scission of glycosidic bonds, the rate constants for recycled paper being found to be slightly higher than the corresponding constants for non-recycled paper, but comparable within experimental limits. The kinetic constants did not display a monotonic trend with increasing water vapour pressures. The Arrhenius dependence of the rate constants on temperature for both kinds of paper supplied the activation energies, which fall in the typical range expected for paper degradation. Finally, the measured rate constants fit fairly well the recently modelled dose–response function for historic paper. Colour changes were found to be visible to the human eye for samples aged for more than 40 days at 60, 70 and 80 °C
Ultraviolet Photoelectron, Electron Transmission and ab-initio Study of the Factors Determining the Stability of imines
No full textThe electronic structure of several alkyl-, N_bis(trimethylsilyl)methyl- (BTMSM), and C_phenyl-substituted imines 1 - 11 have been determined by ab-initio 3-21G* calculations. The calculations reproduce with good accuracy the available geometry of methyleneimine, 1, obtained by microwaves spectroscopy1 and ab-initio 6-31G** calculations2 as well as the outermost energy levels experimentally determined by UPS for several derivatives. The analysis of the results of the calculations indicate that alkyl substitution of 1 at the nitrogen atom reduces the charge separation at the double bond increasing stabilisation, while substitution at the carbon atom has the opposite effect. Steric hindrance of the substituent(s) and the energy gaps between filled (nitrogen lone pair, HOMO, and πCN) and virtual (π*CN) orbitals play a secondary role on stability.
All the BTMSM and phenyl derivatives studied are stable in air at room temperature irrespective of other substituents present. Our analysis indicates that these substituents change the energy, the ordering and the localisation properties of the valence MOs. In particular, the nitrogen lone pair orbital is no more the HOMO
Investigation of the degradation of contemporary papers aged at different conditions of temperature and relative humidity
No full textAs the technical revolution in the paper-making industry, which took place between
1850 and 1950, led to a dramatic chemical destabilisation of paper materials, there
has been a growing interest in paper degradation studies in the last decades.
Although the mechanism and the rate of degradation have been studied extensively
for historic papers, it seems that less attention has been given to real contemporary
papers so far. Currently, the main structural component of modern paper is a
felt of cellulose fibres from different origins (e.g., hardwood, softwood, grass, bast,
recycled fibres), with the addition of other compounds (sizing, fillers etc.). Due
to environmental issues, there has been a significant increase of recycled paper as
fibrous component of print paper over the last years.
Our research is mainly focused on the degradation of contemporary paper obtained
by accelerated ageing experiments, exposing the samples to extreme conditions in
terms of temperature (T) and relative humidity (RH), in order to develop a dose-response
function.
The samples studied in this research consist of two kinds of contemporary papers,
substantially different from each other: pure cellulose papers that meet the ISO9706
standard specifications, and recycled papers with different percentages of recycled
materials.
Along with the determination of the fibre components of the papers by fibre furnish
analysis and the detection of lignin by phloroglucinol tests, the degradation of the
paper samples during the ageing tests are analysed in terms of pH, measured with
the cold extraction method, and degree of polymerisation (DP), determined by viscometry
in cupriethylenediamine solvent. These data allow to evaluate the extent
of degradation of contemporary papers through a dose-response function. The elaboration
of these results will lead to a comparison with the dose-response functions
already modelled and/or to new dose-response function for contemporary papers
Electronic and geometric structure of methylthiophenes and selected dimethyl-2,2'-bithiophenes
No full textThe ionization energy and the attachment energy values of 4,4’-,3,4’-, and 3,3’-dimethyl-2,2’-bithiophenes have been recorded by means of photoelectron He I and electron transmission spectroscopy, respectively. The data obtained, together with those previously reported for thiophene, 2- and 3-methylthiophene, and 2,2’-bithiophene, are discussed with the help of ab initio 3-21G* and semiempirical calculations. Ab initio data closely reproduce ionization energy trends, while experimental values for virtual molecular orbitals (MO’s) compare better with MIND0/3 results. At the ab inito 3-21G* level, gaseous 2,2’-bithiophene and its 4,4’ and 3,4’derivatives have
two stable rotamers (anti, φ = 140-150°; syn, φ = 40-50°), while in 3,3’-dimethyl-2,2’-bithiophene the two rings are nearly orthogonal to each other. Conformational changes cause variations in the electronic structure clearly visible in the ionization and attachment energy spectra
Ab initio and electron spectroscopy study of carbonyl derivatives
No full textThe bands present in the low ionisation energy region (Ei[less-than-or-eq] 12 eV) of the He I photoelectron spectra, and the low energy resonances of the electron transmission spectra of some substituted carbonyls XC(O)R (X = Et2N, alkyl, EtS and EtO; R = Me and CH2SR) have been assigned to the corresponding molecular orbitals by comparison with the assignments reported for simple related molecules. Fully optimized 3-21G ab initio calculations reproduce the assignment of the spectra and available geometric parameters and therefore have been used to determine the electronic charge distribution among the various atoms and groups in the studied compounds. A combined analysis of the theoretical and experimental data provides a detailed understanding of the electronic interaction among the X, CO and R groups in the molecules studied
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
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