2,450 research outputs found
Interpretation of the vibrational spectra of glassy polymers using coarse-grained simulations
The structure and vibrational density of states (VDOS) of polymer glasses are
investigated using numerical simulations based on the classical Kremer-Grest
bead-spring model. We focus on the roles of chain length and bending stiffness,
the latter being set by imposing three-body angular potentials along chain
backbones. Upon increasing the chain length and bending stiffness, structural
reorganisation leads to volumetric expansion of the material and build-up of
internal stresses. The VDOS has two dominant bands: a low frequency one
corresponding to inter- and intra-chain non-bonding interactions and a high
frequency one corresponding principally to vibrations of bonded beads that
constitute skeletal chain backbones. Upon increasing the steepness of the
angular potential, vibrational modes associated with chain bending gradually
move from the low-frequency to the high-frequency band. This redistribution of
modes is reflected in a reduction of the so-called Boson peak upon increasing
chain stiffness. Remarkably, the finer structure and the peaks of the
high-frequency band, and their variations with stiffness, can, for short
chains, be explained using an analytical solution derived for a model triatomic
molecule. For longer chains, the qualitative evolution of the VDOS with chain
stiffness is similar, although the distinct peaks observed for short chains
become increasingly smoothed-out. Our findings can be used to guide a
systematic approach to interpretation of Brillouin and Raman scattering spectra
of glassy polymers in future work, with applications in polymer processing
diagnostics.Comment: To appear in Macromolecule
Parameter-free predictions of the viscoelastic response of glassy polymers from non-affine lattice dynamics
We study the viscoelastic response of amorphous polymers using theory and
simulations. By accounting for internal stresses and considering instantaneous
normal modes (INMs) within athermal non-affine theory, we make parameter-free
predictions of the dynamic viscoelastic moduli obtained in coarse-grained
simulations of polymer glasses at non-zero temperatures. The theoretical
results show very good correspondence with rheology data collected from
molecular dynamics simulations over five orders of magnitude in frequency, with
some instabilities that accumulate in the low-frequency part on approach to the
glass transition. These results provide evidence that the mechanical glass
transition itself is continuous and thus represents a crossover rather than a
true phase transition. The relatively sharp drop of the low-frequency storage
modulus across the glass transition temperature can be explained
mechanistically within the proposed theory: the proliferation of
low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy
excitations) is directly responsible for the rapid growth of a negative
non-affine contribution to the storage modulus.Comment: 10 pages, 7 figure
Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation
We present a simultaneous Chandra and XMM-Newton observation of the Castor
sextett, focusing on Castor A and Castor B, two spectroscopic binaries with
early-type primaries. Of the present day X-ray instruments only Chandra can
isolate the X-ray lightcurves and spectra of A and B. We compare the Chandra
observation with XMM-Newton data obtained simultaneously. Albeit not able to
resolve Castor A and Castor B from each other, the higher sensitivity of
XMM-Newton allows for a quantitative analysis of their combined high-resolution
spectrum. He-like line triplets are used to examine the temperature and the
density in the corona of Castor AB. The temporal variability of Castor AB is
studied using data collected with the European Photon Imaging Camera onboard
XMM-Newton. Strong flare activity is observed, and combining the data acquired
simultaneously with Chandra and XMM-Newton each flare can be assigned to its
host. Our comparison with the conditions of the coronal plasma of other stars
shows that Castor AB behave like typical late-type coronal X-ray emitters
supporting the common notion that the late-type secondaries within each
spectroscopic binary are the sites of the X-ray production.Comment: accepted for publication in A&
Nutritional Approaches to Achieve Weight Loss in Nonalcoholic Fatty Liver Disease.
Nonalcoholic fatty liver disease (NAFLD) can range in spectrum from simple hepatic steatosis to nonalcoholic steatohepatitis (NASH), which is characterized by lipotoxicity, hepatocellular ballooning, and inflammation and can progress to cirrhosis. Weight loss is the cornerstone treatment for NAFLD and NASH. Various randomized controlled trials have shown that weight loss of ≥5-10% leads to significant improvements in hepatic steatosis. Diets high in sodium and fructose have been implicated in the pathogenesis of NAFLD. Although some clinical studies suggest that an isocaloric high-fructose diet does not worsen NAFLD, these clinical studies are often short in duration. More recently, the Dietary Approaches to Stop Hypertension diet, a sodium-restricted diet, has been associated with less prevalence of NAFLD and has been shown to improve NAFLD. In addition, the Mediterranean diet has been promising in improving hepatic steatosis, and a larger randomized controlled trial is currently enrolling subjects. For those who are unable to pursue weight loss through dietary approaches, bariatric surgery has been shown to improve hepatic steatosis and steatohepatitis. This method has been variable in improving hepatic fibrosis. In conclusion, weight loss is crucial to the improvement of NAFLD and NASH, and patients should attempt various diets in an attempt to achieve weight loss
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