1,721,097 research outputs found
Polymeric substrates and polymeric catalysts in oscillatory carbonylation reactions
No full textAiming to produce fully self-oscillatory materials [1], materials that act life-like, materials that are able to carry and dispense their content in predefined rhythm we have made number of advancement in the area of polymeric oscillators in lab conditions. In addition to demonstrating polymeric alkyne-terminated substrates as viable in the oscillatory palladium-catalysed carbonylation reaction [2] we have now shown that polymer-bound catalytic species can also yield oscillations in pH. [3] Moving towards autonomous rhythmic materials we now developed chitosan-based palladium catalyst fully capable of delivering oscillatory pH behaviour in a batch-like carbonylation reaction set-up.1. Isakova A, Novakovic K. Oscillatory Chemical Reactions In The Quest For Rhythmic Motion of Smart Materials. European Polymer Journal 2017, 95, 430-439.2. Donlon L, Novakovic K. Oscillatory carbonylation using alkyne-functionalised poly(ethylene glycol). Chemical Communications 2014, 50, 15506-15508.3. Isakova A, Murdoch B, Novakovic K. From small molecules to polymeric catalysts in the oscillatory carbonylation reaction: multiple effects of adding HI. Physical Chemistry Chemical Physics, 2018, 20, 9281-928
Genipin-crosslinked chitosan-poly(vinyl pyrrolidone) hydrogel for coupling with pH oscillators
No full textSmart polymer gels are macromolecular networks that can exhibit significant and reversible conformational rearrangements upon variable changes in the local environment. Hydrogels are examples of such materials that can swell or deswell in aqueous solutions upon application of an external stimulus such as temperature or pH.1 Oscillatory chemical reactions (e.g. Belousov Zhabotinsky) exhibit the relevant stimuli to induce a simultaneous volume change within a responsive hydrogel if both environments are compatible.2 Pronounced oscillations in reaction heat output (e.g. 0.6 kJ/oscillation at 40\ub0C) and pH (1-6) are exhibited by the phenylacetylene oxidative carbonylation (PCPOC) reaction studied in our group.3,4 It is therefore desirable to couple the PCPOC reaction with a cationic hydrogel able to swell in acidic media. Chitosan, a cationic polysaccharide, is known to be biocompatible, non-toxic, biodegradable, antimicrobial, mucoadhesive and a strong candidate for drug release. However, owing to its fragility and uncontrollable porosity, it has limited use. To overcome this, it is often chemically crosslinked with agents such as glutaraldehyde, formaldehyde or epoxy compounds, although all are cytotoxic and are inappropriate for biomedical applications. Genipin is an alternative natural crosslinking agent that is 10,000 times less cytotoxic than glutaraldehyde, has a slower degradation rate and fluoresces upon forming crosslinks. Poly(vinyl pyrrolidone) (PVP) is a haemocompatible, water soluble polymer that can control the porosity of chitosan structures. In this work, chitosan has been crosslinked with PVP using genipin to form smart hydrogels. Following initial experimental studies, the gel composition that produces stable structures is established. The degree of swelling and the rate at which equilibrium is reached are experimentally studied as a function of polymerisation temperature, polymerisation time and pH. Polymerisation temperatures considered were 37 and 50 \ub0C with polymerisation times ranging from 24 to 72 h. In all experiments acidic environments were investigated. (1) Tanaka, T.; Fillmore, D.; Sun, S. T.; Nishio, I.; Swislow, G.; Shah, A. Physical Review Letters 1980, 45, 1636. (2) Yoshida, R. Adv. Mater. 2010, 22, 3463. (3) Novakovic, K.; Grosjean, C.; Scott, S. K.; Whiting, A.; Willis, M. J.; Wright, A. R. Chemical Physics Letters 2007, 435, 142. (4) Novakovic, K.; Mukherjee, A.; Willis, M.; Wright, A.; Scott, S. Physical Chemistry Chemical Physics 2009, 11, 9044
Smart hydrogels for biomedical use with oscillatory reactions
No full textSmart polymer gels can exhibit significant and reversible conformational rearrangements upon variation in an applied stimulus such a pH or temperature. These networks are ideally suited for biomedical use as drug delivery systems or scaffolds for tissue engineering. Effective distribution of imbibed constituents requires a controlled and sustained therapeutic release regime. This can be realised by coupling responsive hydrogels to oscillatory reactions producing oscillations in the appropriate stimuli to induce a simultaneous volume change within a smart polymer gel.1 One such reaction is the palladium catalysed phenylacetylene oxidative carbonylation (PCPOC) reaction which generates pronounced oscillations in heat output (e.g. 0.6 kJ/oscillation at 40 \ub0C) and pH (1-6) over a wide temperature range (10-40 \ub0C).2, 3 The present work focuses on determining and optimising the suitability of pH-sensitive genipin-crosslinked chitosan-poly(vinyl pyrrolidone) hydrogels for coupling to chemical oscillators. This semi-interpenetrating polymer network combines properties such as biocompatibility, low toxicity, biodegradability and haemocompatibility using genipin, a natural crosslinking agent with low cytotoxicity that fluoresces upon crosslink formation. The effects of polymerisation time, temperature and composition on the structure and swelling behaviour of these cationic hydrogels are investigated. Fluorescence intensity is found to increase with extent of crosslinking in all cases. This is corroborated qualitatively using scanning electron microscopy (SEM) where two different drying techniques are employed and quantitatively following porosity analysis. Equilibrium swelling behaviour is determined using a range of buffers (pH 2, 4, 7 and 10) both gravimetrically and via fluorescence visualisation. The accuracy of these techniques is compared. Variation in the porous microstructure of the gel during swelling is also studied for the first time via confocal microscopy where three-dimensional images of the network morphology and changes in surface topography are examined (Figure 1). Compared to SEM, this is a non-destructive technique for analysing gel microstructure. Information acquired from these studies is used to determine the optimum formulation for prospective combination with the oscillatory PCPOC reaction. Subsequently, to assess hydrogel response dynamics, oscillatory behaviour has been simulated by immersing the hydrogel in pH 2 and pH 4 buffers in an alternate fashion to coincide with the time period of the oscillatory reaction and the swelling behaviour is determined. The ease at which these networks can be prepared coupled to their autofluorescent nature aids development towards a prospective application in controlled; pulsatile and sustained release, distinguishing this system from current delivery modes. a b c Figure 1 Variation in porous architecture of smart hydrogel (a) during swelling via three-dimensional structure (b) and topographical (c) evaluation. 1. Yoshida, R. Advanced Materials 2010, 22, 3463. 2. Novakovic, K.; Grosjean, C.; Scott, S. K.; Whiting, A.; Willis, M. J.; Wright, A. R. Chemical Physics Letters 2007, 435, 142. 3. Novakovic, K.; Mukherjee, A.; Willis, M.; Wright, A.; Scott, S. Physical Chemistry Chemical Physics 2009, 11, 9044
The effect of temperature on selectivity and the dynamics of product formation during the oscillatory mode of the phenylacetylene oxidative carbonylation reaction
No full textThe dynamics of reactant consumption and product formation in the oscillatory mode of the palladium-catalysed phenylacetylene oxidative carbonylation reaction over the temperature range of 0–40 \ub0C were studied. Experiments were performed isothermally in a reaction calorimeter to correlate reactant consumption and product formation with the occurrence of oscillations and the heat released by the reaction. Reactant and products were quantified using GC-MS analysis. The dynamics of reactant and product formation throughout the course of the reaction (several days) were recorded. Oscillations in pH and accompanying pulses of heat were captured. The increase in period and amplitude of the pH oscillations that occurs as temperature decreases was found to continue at 0 \ub0C.[1] The reaction heat was released in a pulsatile manner, resulting in a stepwise increase in the total energy released with the steps in energy being correlated with the pH oscillations. As the size of the pH oscillations increased, the amount of energy released during each step increased. It was also found that product selectivity changed with temperature: at 40 \ub0C dimethyl (2Z)-2-phenyl-2-butenedioate was the major product whereas at 0 \ub0C the major product was 5,5-dimethoxy-3-phenyl-2(5H)-furanone. The clear temperature dependence of the conversion rates as well as an increase in conversion as oscillations in pH developed were documented and an explanation for this behaviour is offered. In addition, a reaction pathway associated with selective formation of the observed products is proposed and aligned with the observed oscillatory phenomena. 1. Novakovic, K., et al., The influence of reaction temperature on the oscillatory behaviour in the palladium-catalysed phenylacetylene oxidative carbonylation reaction. Physical Chemistry Chemical Physics, 2009: p. 9044-9049
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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