38,399 research outputs found
Synthesis, thermal behavior, and aggregation in aqueous solution of poly(methyl methacrylate)-b-poly(2-hydroxyethyl methacrylate)
IndexaciĂłn: ScieloABSTRACT
Amphiphilic block copolymers of poly(methyl methacrylate) PMMA and poly(2-hidroxyethyl methacrylate) PHEMA were synthesized by a two-step atom transfer radical polymerization (ATRP). Copolymers with various degrees of polymerization and different relative block sizes were obtained. The structure of the resulting polymers have been characterized and verified by FT-IR and 1H-NMR, molecular weight were determined by size exclusion chromatography analyses. The thermal properties of these polymers were investigated by differential scanning calorimetry DSC and thermogravimetric analysis TGA. The glass transition temperature of mono halogenated PMMA increases from 116 °C to 123 °C with increasing molecular weight, whereas the glass transition temperature of block copolymers depends slightly on polymer structure. The derivatives of TGA curves indicate that thermal degradation occurs in one stage. The self-assembly of PMMA-b-PHEMA in aqueous solution have been investigated by fluorescence probing methods. The critical micelle concentrations are in the range 10-6 - 10-7 M. The micropolarity sensed by pyrene is higher than in aggregates formed by block copolymers based on polystyrene.
Keywords: Block copolymers, glass transition temperature, thermogravimetric analysis, critical micelle concentration, fluorescence probing methods
Can ultrastrong coupling change ground state chemical reactions?
Recent advancements on the fabrication of organic micro- and nanostructures
have permitted the strong collective light-matter coupling regime to be reached
with molecular materials. Pioneering works in this direction have shown the
effects of this regime in the excited state reactivity of molecular systems and
at the same time has opened up the question of whether it is possible to
introduce any modifications in the electronic ground energy landscape which
could affect chemical thermodynamics and/or kinetics. In this work, we use a
model system of many molecules coupled to a surface-plasmon field to gain
insight on the key parameters which govern the modifications of the
ground-state Potential Energy Surface (PES). Our findings confirm that the
energetic changes per molecule are determined by single-molecule-light
couplings which are essentially local, in contrast with those of the
electronically excited states, for which energetic corrections are of a
collective nature. Still, we reveal some intriguing quantum-coherent effects
associated with pathways of concerted reactions, where two or more molecules
undergo reactions simultaneously, and which can be of relevance in low-barrier
reactions. Finally, we also explore modifications to nonadiabatic dynamics and
conclude that, for this particular model, the presence of a large number of
dark states yields negligible changes. Our study reveals new possibilities as
well as limitations for the emerging field of polariton chemistry
Efficient memory management in VOD disk array servers usingPer-Storage-Device buffering
We present a buffering technique that reduces video-on-demand server memory requirements in more than one order of magnitude. This technique, Per-Storage-Device Buffering (PSDB), is based on the allocation of a fixed number of buffers per storage device, as opposed to existing solutions based on per-stream buffering allocation. The combination of this technique with disk array servers is studied in detail, as well as the influence of Variable Bit Streams. We also present an interleaved data placement strategy, Constant Time Length Declustering, that results in optimal performance in the service of VBR streams. PSDB is evaluated by extensive simulation of a disk array server model that incorporates a simulation based admission test.This research was supported in part by the National R&D Program of Spain, Project Number TIC97-0438.Publicad
Implementation of a Hardware/Software Platform for Real-Timedata-Intensive Applications in Hazardous Environments
Real-Time Technology and Applications Symposium. Brookline, MA, USA, 10-12 Oct. 1996In real-time data-intensive applications, the simultaneous achievement of the required performance and determinism is a difficult issue to address, mainly due to the time needed to perform I/O operations, which is more significant than the CPU processing time. Additional features need to be considered if these applications are intended to perform in hostile environments. In this paper, we address the implementation of a hardware/software platform designed to acquire, transfer, process and store massive amounts of information at sustained rates of several MBytes/sec, capable of supporting real-time applications with stringent throughput requirements under hazardous environmental conditions. A real-world system devoted to the inspection of nuclear power plants is presented as an illustrative examplePublicad
Coherent caloritronics in Josephson-based nanocircuits
We describe here the first experimental realization of a heat interferometer,
thermal counterpart of the well-known superconducting quantum interference
device (SQUID). These findings demonstrate, on the first place, the existence
of phase-dependent heat transport in Josephson-based superconducting circuits
and, on the second place, open the way to novel ways of mastering heat at the
nanoscale. Combining the use of external magnetic fields for phase biasing and
different Josephson junction architectures we show here that a number of heat
interference patterns can be obtained. The experimental realization of these
architectures, besides being relevant from a fundamental physics point of view,
might find important technological application as building blocks of
phase-coherent quantum thermal circuits. In particular, the performance of two
different heat rectifying devices is analyzed.Comment: 34 pages, 15 figures, review article for Ultra-low temperatures and
nanophysics ULTN2013. Microkelvin Proceeding
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