219 research outputs found
Structure and Phase Transitions of Alkyl Chains on Mica
We use molecular dynamics as a tool to understand the structure and phase
transitions [Osman et. al. J. Phys. Chem. B 2000, 104, 4433; 2002, 106, 653] in
alkylammonium micas. The consistent force field 91 is extended for accurate
simulation of mica and related minerals. We investigate mica sheets with 12
octadecyltrimethylammonium (C18) ions or 12 dioctadecyldimethylammonium (2C18)
ions, respectively, as single and layered structures at different temperatures
with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside
preferably above the cavities in the mica surface with an aluminum-rich
boundary. The nitrogen atoms are 380 to 390 pm distant to the superficial
silicon-aluminum plane. With increasing temperature, rearrangements of C18 ions
on the mica surface are found, while 2C18 ions remain tethered due to geometric
restraints. We present basal-plane spacings in the duplicate structures, tilt
angles of the alkyl chains, and gauche-trans ratios to analyze the chain
conformation. Also, the individual phase transitions of the two systems on
heating are explained. Where experimental data are available, the agreement is
very good. We propose a geometric parameter lamba for the saturation of the
surface with alkyl chains, which determines the preferred self-assembly
pattern, i.e., islands, intermediate, or continuous. Lambda also determines the
tilt angles in continuous layers on mica or other surfaces. The thermal
decomposition appears to be a Hofmann elimination with mica as a base-template.Comment: 45 pages with 6 tables and 5 figure
The Melting Point of Crystalline Copolymers – Applying Materials Simulation
The defect Gibbs energy of hydroxyvalerate comonomer inclusions into the crystals made up by random copolymers of poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHB/HV) is calculated by means of the thermodynamic integration approach. The result obtained for a single inclusion is in excellent agreement with those obtained by fitting experimental melting temperature and cocrystal composition data. On decomposing the Gibbs energy, it is found that the crystallization entropy contributes the dominant part of the defect Gibbs energy. Our calculations on multi-inclusion crystals show that the Gibbs energy strongly decreases when the comonomers aggregate in a preferred pattern. Further information to the design of isomorphic copolymers is obtained from these calculations
High refractive index films of polymer nanocomposites
Solutions of PbS particles and gelatin were used for the preparation of nanocomposites by a spin-coating process. This allows for the preparation of nanocomposite films with controlled thickness, e.g., between 40 nm and 2 μm for a film containing 45 wt.% PbS. Surface roughness and film thickness were investigated by surface profilometry and scanning electron microscopy (SEM). The refractive index at 632.8 nm can be expressed by a linear function of the volume fraction of PbS in the range of 0 to 55 vol. % PbS. In this range, the refractive index increases from 1.5 to 2.5 with increasing PbS ratio and belongs, therefore, to the highest refractive indices known for polymeric composite material
Analysis of the Phase Transitions in Alkyl- Mica by Density and Pressure Profiles
In a previous work [H. Heinz, H. J. Castelijns, U. W. Suter, J. Am. Chem.
Soc. 115, 9500 (2003)], we developed an accurate force field and simulated the
phase transitions in C18-mica (octadecyltrimethylammonium-mica) as well as the
absence of such transitions in 2C18-mica (dioctadecyldimethylammonium-mica)
between room temperature and 100 deg C. Here we analyze (i) average z
coordinates of the carbon atoms and interdigitation of the hydrocarbon
bilayers, (ii) density profiles and (iii) pressure profiles of the structures
along all Cartesian axes. In C18-mica, the standard deviation in the z
coordinate for the chain atoms is high and more than doubles in the disordered
phase. The order-disorder transition is accompanied by a change in the
orientation of the ammonium headgroup, as well as decreasing tensile and shear
stress in the disordered phase. In 2C18-mica, the standard deviation in the z
coordinate for the chain atoms is low and does not increase remarkably on
heating. The backbones display a highly regular structure, which is slightly
obscured by rotations in the C18 backbones and minor headgroup displacements at
100 deg C. Close contacts between the bulky headgroups with sidearms cause
significant local pressure which is in part not relieved at 100 deg C. An
increase of the basal-plane spacing at higher temperature is found in both
systems due to larger separation between the two hydrocarbon layers and an
increased z spacing between adjacent chain atoms (=decreased tilt of the chains
relative to the surface normal), and, in C18-mica only, a stronger upward
orientation of the C18 chain at the ammonium headgroup. The likelihood for
chain interdigitation between the two hydrocarbon layers is 0.24 to 0.30 for
C18-mica,and 0.65 to 0.26 for 2C18-mica (for 20 deg C to 100 deg C).Comment: 13 pages, 6 figures (Figs. 5 and 6 only with 150 dpi resolution
High Refractive Index Materials of Iron Sulfides and Poly(ethylene oxide)
High refractive index composites of iron sulfides and poly(ethylene oxide) (PEO) have been prepared by co-precipitation from aqueous solution. Several reaction parameters were varied: inorganic reactants, reactant ratios, reaction temperatures, and reaction times. Selected samples were characterized with organic microelemental analysis, x-ray fluorescence spectroscopy, x-ray diffraction, DSC, and TEM. The nanocomposites with the highest refractive indices have been prepared using PEO, Mohr's salt, and H2S or NaHS. The analyses indicate that the iron sulfides in these materials consist of finely dispersed mackinawite and greigite ("amorphous” FeS) and, partially, also pyrite. The refractive indexes of the resulting composites are clearly above 2 at 632.8 and 1295 nm and can assume values between 2.5 and 2.
Obscured Activity: AGN, Quasars, Starbursts and ULIGs observed by the Infrared Space Observatory
Some of the most active galaxies in the Universe are obscured by large
quantities of dust and emit a substantial fraction of their bolometric
luminosity in the infrared. Observations of these infrared luminous galaxies
with the Infrared Space Observatory (ISO) have provided a relatively unabsorbed
view to the sources fuelling this active emission. The improved sensitivity,
spatial resolution and spectroscopic capability of ISO over its predecessor
Infrared Astronomical Satellite (IRAS), has enabled significant advances in the
understanding of the infrared properties of active galaxies. ISO surveyed a
wide range of active galaxies which, in the context of this review, includes
those powered by intense bursts of star-formation as well as those containing a
dominant active galactic nucleus (AGN). Mid infrared imaging resolved for the
first time the dust enshrouded nuclei in many nearby galaxies, while a new era
in infrared spectroscopy was opened by probing a wealth of atomic, ionic and
molecular lines as well as broad band features in the mid and far infrared.
This was particularly useful since it resulted in the understanding of the
power production, excitation and fuelling mechanisms in the nuclei of active
galaxies including the intriguing but so far elusive ultraluminous infrared
galaxies. Detailed studies of various classes of AGN and quasars greatly
improved our understanding of the unification scenario. Far-infrared imaging
and photometry also revealed the presence of a new very cold dust component in
galaxies and furthered our knowledge of the far-infrared properties of faint
starbursts, ULIGs and quasars. We summarise almost nine years of key results
based upon ISO data spanning the full range of luminosity and type of active
galaxies.Comment: Accepted for publication in 'ISO science legacy - a compact review of
ISO major achievements', Space Science Reviews - dedicated ISO issue. To be
published by Springer in 2005. 62 pages (low resolution figures version).
Higher resolution PDFs available from
http://users.physics.uoc.gr/~vassilis/papers/VermaA.pdf or
http://www.iso.vilspa.esa.es/science/SSR/Verma.pd
As the Walls of Academia are Tumbling Down
http://deepblue.lib.umich.edu/bitstream/2027.42/58008/1/2717844392.pd
The psychological science accelerator’s COVID-19 rapid-response dataset
In response to the COVID-19 pandemic, the Psychological Science Accelerator coordinated three large-scale psychological studies to examine the effects of loss-gain framing, cognitive reappraisals, and autonomy framing manipulations on behavioral intentions and affective measures. The data collected (April to October 2020) included specific measures for each experimental study, a general questionnaire examining health prevention behaviors and COVID-19 experience, geographical and cultural context characterization, and demographic information for each participant. Each participant started the study with the same general questions and then was randomized to complete either one longer experiment or two shorter experiments. Data were provided by 73,223 participants with varying completion rates. Participants completed the survey from 111 geopolitical regions in 44 unique languages/dialects. The anonymized dataset described here is provided in both raw and processed formats to facilitate re-use and further analyses. The dataset offers secondary analytic opportunities to explore coping, framing, and self-determination across a diverse, global sample obtained at the onset of the COVID-19 pandemic, which can be merged with other time-sampled or geographic data
- …