32 research outputs found
Polymerization of Branched Polyetherimides: Comparison between Monte Carlo Simulation and Flory-Stockmayer Theory
A Monte Carlo (MC) simulation method based on the Gillespie algorithm is
developed for the polymerization of branched polyetherimides from two back-bone
monomers [4,4'-bisphenol A dianhydride (BPADA) and m-phenylenediamine (MPD)], a
chain terminator [phthalic anhydride (PA)], and a branching agent
[tris[4-(4-aminophenoxy)phenyl] ethane (TAPE)]. This polymerization involves 4
reactions that all can be reduced to a condensation reaction between an amine
group and a carboxylic anhydride group. By comparing the MC simulation results
to the predictions of the Flory-Stockmayer theory on the molecular weight
distribution, we show that the rates of the 4 reactions in the MC simulations
should be set based on the concentrations of the functional groups on the
monomers involved in each reaction. Using the MC simulations, we show that the
Flory-Stockmayer theory predicts the molecular weight distribution well for
systems below the gel point that is set by the functionality of the branching
agent but starts to fail for systems around or above the gel point. However,
for all the systems the MC simulations can be used to reliably predict the
molecular weight distribution. Even for a macroscopic system, a converging
distribution can be quickly obtained through the MC simulations on a system of
only a few hundred to a few thousand monomers but with the same molar ratios of
monomers as in the macroscopic system.Comment: 12 pages, 9 figures, 5 table
Building Ubiquitous Computing Environment by Using RFID in Aircraft MRO Process
The implementation of RFID had aroused discussion in every area. Experts believe that the emergence of RFID will cause another business revolution. Many industries had deployed RFID, like aviation industry, in which RFID is used in maintenance materials and baggage management. This paper discusses the implementation of RFID in MRO process and the building of a ubiquitous computing environment. We believe that our proposal has three merits to MRO (1) anti-counterfeit parts (2) MRO liability (3) efficient and effective inspection. The architecture can address the competition pressure that aviation industry faces and consequently enhance competition advantages
Insights into Hydration Dynamics and Cooperative Interactions in Glycerol-Water Mixtures by Terahertz Dielectric Spectroscopy.
We report relaxation dynamics of glycerol-water mixtures as probed by megahertz-to-terahertz dielectric spectroscopy in a frequency range from 50 MHz to 0.5 THz at room temperature. The dielectric relaxation spectra reveal several polarization processes at the molecular level with different time constants and dielectric strengths, providing an understanding of the hydrogen-bonding network in glycerol-water mixtures. We have determined the structure of hydration shells around glycerol molecules and the dynamics of bound water as a function of glycerol concentration in solutions using the Debye relaxation model. The experimental results show the existence of a critical glycerol concentration of ∼7.5 mol %, which is related to the number of water molecules in the hydration layer around a glycerol molecule. At higher glycerol concentrations, water molecules dispersed in a glycerol network become abundant and eventually dominate, and four distinct relaxation processes emerge in the mixtures. The relaxation dynamics and hydration structure in glycerol-water mixtures are further probed with molecular dynamics simulations, which confirm the physical picture revealed by the dielectric spectroscopy
Satellite to buoy IoT communications in the Arctic Ocean
Internet of things (IoT), to Arctic Ocean areas using Low Earth Orbit (LEO) satellites for communication to and from sensor units on the sea, has become increasingly important. It is challenging to close the link budget from a buoy on sea to a LEO satellite due to restrictions on the availability of power in such installations. Phenomena as scattering from the sea surface and small-scale fading, ionospheric scintillations, diffraction loss at low elevation angles, and power availability in small LEO satellites and models for analysis are described. It is of great importance to have a radio wave propagation model that accurately predicts path loss between an installation on the sea and a LEO satellite taking all loss mechanisms into account. In this paper, a path-loss model for satellite to buoy communications over the sea is described. In this model, maritime propagation phenomena in the radio link include free space loss, scintillation loss caused by the ionosphere, diffraction loss at low elevation angle, and scattering at the sea surface depending on the wave height and small-scale fading. Furthermore, a buoy on the sea surface with strong angular movement will cause a varying receive signal level depending on the antenna diagram. These phenomena are assessed. A link budget for the frequencies 433, 868, and 3,400 MHz is calculated for a LEO satellite at a height of 800 km
Interfacial Layers between Ion and Water Detected by Terahertz Spectroscopy
Dynamic fluctuations in hydrogen-bond network of water occur from femto- to
nano-second timescale and provides insights into structural/dynamical aspects
of water at ion-water interfaces. Employing terahertz spectroscopy assisted
with molecular dynamics simulations, we study aqueous chloride solutions of
five monovalent cations, namely, Li, Na, K, Rb and Cs. We show that ions modify
the behavior of surrounding water molecules and form interfacial layers of
water around them with physical properties distinct from that of bulk water.
Small cations with high charge densities influence the kinetics of water well
beyond the first solvation shell. At terahertz frequencies, we observe an
emergence of fast relaxation processes of water with their magnitude following
the ionic order Cs>Rb>K>Na>Li, revealing an enhanced population density of
weakly coordinated water at ion-water interface. The results shed light on the
structure breaking tendency of monovalent cations and provide insights into the
properties of ionic solutions at the molecular level
Binary Star Evolution in Different Environments: Filamentary, Fractal, Halo and Tidal-tail Clusters
Using membership of 85 open clusters from previous studies (Pang et al.
2021a,b, 2022b; Li et al. 2021) based on Gaia DR3 data, we identify binary
candidates in the color-magnitude diagram, for systems with mass ratio q > 0.4.
The binary fraction is corrected for incompleteness at different distances due
to the Gaia angular resolution limit. We find a decreasing binary fraction with
increasing cluster age, with substantial scatter. For clusters with a total
mass > 200, the binary fraction is independent of cluster mass. The
binary fraction depends strongly on stellar density. Among four types of
cluster environments, the lowest-density filamentary and fractal stellar groups
have the highest mean binary fraction: 23.6% and 23.2%, respectively. The mean
binary fraction in tidal-tail clusters is 20.8%, and is lowest in the densest
halo-type clusters: 14.8%. We find clear evidence of early disruptions of
binary stars in the cluster sample. The radial binary fraction depends strongly
on the cluster-centric distance across all four types of environments, with the
smallest binary fraction within the half-mass radius , and increasing
towards a few . Only hints of mass segregation is found in the target
clusters. The observed amount of mass segregation is not significant to
generate a global effect inside the target clusters. We evaluate the bias of
unresolved binary systems (assuming a primary mass of 1) in 1D
tangential velocity, which is 0.1-1. Further studies are
required to characterize the internal star cluster kinematics using Gaia proper
motions
Molecular Weight Distribution of Branched Polymers: Comparison between Monte Carlo Simulation and Flory-Stockmayer Theory
It is challenging to predict the molecular weight distribution (MWD) for a polymer with a branched architecture, though such information will significantly benefit the design and development of branched polymers with desired properties and functions. A Monte Carlo (MC) simulation method based on the Gillespie algorithm is developed to quickly compute the MWD of branched polymers formed through step-growth polymerization, with a branched polyetherimide from two backbone monomers (4,4′-bisphenol A dianhydride and m-phenylenediamine), a chain terminator (phthalic anhydride), and a branching agent (tris[4-(4-aminophenoxy)phenyl] ethane) as an example. This polymerization involves four reactions that can be all reduced to a condensation reaction between an amine group and a carboxylic anhydride group. A comparison between the MC simulation results and the predictions of the Flory-Stockmayer theory on MWD shows that the rates of the reactions are determined by the concentrations of the functional groups on the monomers involved in each reaction. It further shows that the Flory-Stockmayer theory predicts MWD well for systems below the gel point but starts to fail for systems around or above the gel point. However, for all the systems, the MC method can be used to reliably predict MWD no matter if they are below or above the gel point. Even for a macroscopic system, a converging distribution can be quickly obtained through MC simulations on a system of only a few hundred to a few thousand monomers that have the same molar ratios as in the macroscopic system