177,022 research outputs found
HPLC method for the determination of monomer conversion and composition during the poly(styrene-r-methylmethacrylate) polymerization
No full textCopolymers of styrene (STY) and methylmetacrylate (MMA) are widely employed due to the low price of the starting monomers coupled with their synthetic versatility. A chromatographic method was developed for the simultaneous determination of the conversion degree of the single components of the polymer during the copolymerization reaction and the composition of the produced copolymer. The crude reaction mixture is directly analyzed during the reaction with only a dilution pretreatment. Accordingly, the method allows quick and simple monitoring of the reaction as it proceeds
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Evidence of Mechanochemical Control in "grafting to" Reactions of Hydroxy-Terminated Statistical Copolymers
No full textPartly deuterated hydroxy-terminated poly(styrene d8-r-methyl methacrylate) statistical copolymers with different molecular weights were grafted to a silicon substrate. Then, fully hydrogenated hydroxy-terminated poly(styrene-r-methyl methacrylate) statistical copolymers were spun over the pregrafted substrate and let to react at 250 °C following the time evolution of the brush thickness and composition. Over time, the grafting density of the deuterated polymers decreases, whereas the grafting density of the hydrogenated polymers increases thus indicating the simultaneous occurrence of grafting and degrafting reactions. Moreover, keeping constant the grafting time, the total grafting density decreases as the molecular weight of the hydrogenated polymers increases, until a limiting value is reached when the molecular weight of the incoming polymer is equal to the one of the preformed brush. The overall picture of the experimental data, further supported by hybrid-particle field coarse-grained simulations, suggests that the reactivity of the system is related to the degree of stretching of the brush layer induced by the entering of additional polymer chains. This new vision of the grafting to mechanism implies that the self-limiting nature of this process derives from a mechanochemical control of the reaction rather than from diffusion/penetration effects
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
IT-assisted Exploration of Excavation Reports. Using Natural Language Processing in the archaeological research process
No full textLetter from R. R. Zellick, Assistant Trust Officer, Anglo California National Bank of San Francisco, to Joseph R. Goodman, October 2, 1942
No full textLetter from R. R. Zellick, Assistant Trust Officer at The Anglo California National Bank of San Francisco, to Joseph R. Goodman, regarding property owned by Dave Tatsuno. Zellick mentions a dispute between current tenants and Tatsuno, and that Tatsuno has asked Goodman to help locate trustworthy tenants.Personal correspondence, organizational records, government documents, publications, and other papers created or collected by Joseph R. Goodman documenting the forced removal and incarceration of Japanese Americans during World War II, as well as organized resistance to incarceration. Included in the collection are records of the Japanese Young Men's Christian Association and the Japanese American Citizens' League in San Francisco, including papers of the Japanese YMCA's executive secretary Lincoln Kanai; Sakai family papers; Goodman's correspondence to and from Japanese American incarcerees, organizations opposing forced removal and incarceration of Japanese Americans, the War Relocation Authority, and others; publications, photographs, and ephemera from the Topaz Relocation Center, where Goodman taught high school; War Relocation Authority records and publications; and newspaper clippings, pamphlets, and reports about forced removal and incarceration created by various government, religious, and civic organizations, in California and nationwide
Thermodynamic vs Kinetic Control of Brush Composition in Grafting to Reactions of Disperse Polymer Systems in Melt
No full textGrafting to reactions are one of the most investigated approaches to producing polymer brushes due to their unique capability to control both the brush thickness and grafting density. Despite their widespread use, some aspects of the grafting to reactions remain not fully understood. Among these, particularly relevant is the preferential grafting of the shortest chains in dispersed polymer systems. This phenomenon is investigated in the present paper by grafting on silicon oxide substrates polystyrene blends containing equimolar amounts of two telechelic polystyrenes with different molecular weights to model disperse polymer systems. Furthermore, for each blend, different end groups, namely, hydroxy or phosphate end groups, are considered. For hydroxy-terminated polystyrene blends, a preferential grafting of the shortest component is observed and derives from the lower entropy loss for short chains when the reactive end group approaches the surface, but the brush composition, in terms of the ratio between short and long chains, is independent of grafting time and temperature. In contrast, at a short time, the grafting of phosphate-terminated polystyrene blends takes place with an increase of short chains definitely higher than the one observed for the hydroxy-terminated blends. In turn, after a sufficiently long time, the brush composition converges to the same final composition of the hydroxy-terminated polystyrenes. This result indicates that at short times the control of the brush composition is dictated by a fast absorption of the short chains on the surface substrate due to the strong polarity of the phosphate group. This adsorption takes place before the grafting to process and results in an observed excess of grafted short chains. In turn, as grafting proceeds, the brush composition evolves toward the thermodynamic composition, which is therefore equal to the one of the hydroxy-terminated polymer blends
Partition by molecular weight of polymer brushes: A combined reactive grand canonical Monte Carlo and self-consistent field investigation of grafting to processes
No full textAn approach that couples reactive Grand Canonical Monte Carlo (rGCMC) and Scheutjens–Fleer Self-Consistent Field (SF-SCF) techniques has been used to draw topological maps of the grafted interface layer between a solid substrate and the melt phase in contact with it. Hybrid rGCMC/SF-SCF has been used to predict morphological features in terms of grafting density, brush thickness and surface roughness of binary blends of random copolymers. Modelling outcomes have been validated on experimental data of binary brushes taken from the literature and obtained by blends of partly deuterated hydroxyl terminated poly (styrene d8-st-methyl methacrylate) and a hydroxy terminated poly (styrene-st-methyl methacrylate) copolymers thermally grafted from melt to a silicon wafer. In this way, it was possible to better elucidate the mechanisms that guide the preferential grafting of “short” or “long” chains during the grafting to process from melt
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