Biomedical subjects

B Farago

Publications and source records attributed to B Farago.

At least 19 recordsLinked to original sources

PubMed2006

Molecular observation of constraint release in polymer melts.

The dynamics of binary polymer blends of few labeled long chains in successively shorter matrix chains has been investigated by neutron spin echo (NSE) spectroscopy. For the first time the effect of constraint release on the chain relaxation has been directly observed on a microscopic scale. Decreasing the matrix chain length reduces the topological confinement until unconfined Rouse motion is observed, when the matrix chains are too short to confine the long chain in a tube. Whereas an analytical description of the effect is not yet available, a new simulation based on the slip-link model shows perfect agreement with the NSE data over the full range of matrix molecular weights.

Computer Simulation↗

PubMed2004

Hydrogen motions in the alpha-relaxation regime of poly(vinyl ethylene): a molecular dynamics simulation and neutron scattering study.

The hydrogen motion in poly(vinyl ethylene) (1,2-polybutadiene) in the alpha-relaxation regime has been studied by combining neutron spin echo (NSE) measurements on a fully protonated sample and fully atomistic molecular dynamics simulations. The almost perfect agreement between experiment and simulation results validates the simulated cell. A crossover from Gaussian to non-Gaussian behavior is observed for the intermediate scattering function obtained from both NSE measurements and simulations. This crossover takes place at unusually low Q values, well below the first maximum of the static structure factor. Such anomalous deviation from Gaussian behavior can be explained by the intrinsic dynamic heterogeneity arising from the differences in the dynamics of the different protons in this system. Side group hydrogens show a markedly higher mobility than main chain protons. Taking advantage of the simulations we have investigated the dynamic features of all different types of hydrogens in the sample. Considering each kind of proton in an isolated way, deviations from Gaussian behavior are also found. These can be rationalized in the framework of a simple picture based on the existence of a distribution of discrete jumps underlying the atomic motions in the alpha process.

Journal Article↗

PubMed2003

Experimental evidence by neutron scattering of a crossover from Gaussian to non-Gaussian behavior in the alpha relaxation of polyisoprene.

We report incoherent quasielastic neutron scattering experiments exploring the alpha-relaxation range in polyisoprene over an unprecedented range in momentum transfer Q. The data corroborate and validate earlier molecular dynamics simulations and reveal the existence of a crossover from Gaussian to non-Gaussian character of the main chain protons self-correlation function in the alpha-relaxation regime. The real challenge of the experiment was to push the neutron techniques to cover a Q range as wide as possible. By combining two neutron spin echo spectrometers and a backscattering instrument, we have been able to study the dynamics in a Q range of 0.1 < or = Q < or = 4.7 A(-1). In the low-Q regime the shape of the relaxation function was found to be related to the dispersion of the relaxation times as predicted by the Gaussian assumption. At short distances or large Q, this relationship is strongly violated indicating a non-Gaussian regime. We have performed a detailed comparison between the experiments and simulations at different temperatures and found, apart from a temperature shift, complete agreement. Combining experiments and simulations led to a consistent interpretation in terms of a distribution of jumps underlying the diffusive motion of protons in the alpha process. This model leads to a time-dependent non-Gaussianity parameter that agrees nearly quantitatively with the simulations and exhibits all features resolved so far from various simulations.

Journal Article↗

PubMed2003

Steric interactions in cellular structures formed in a water/oil/surfactant/cosurfactant mixture.

Dynamics of dense microemulsion droplets forming ordered cellular phases as temperature increases are investigated by neutron spin echo spectroscopy (NSE). The shape fluctuations of droplets are shown using a specific contrast. Their relaxation time tau(f) is obtained by analysis of the NSE curves, which also reveals the short-range Brownian motion (diffusion constant D0) of the "caged" droplets. The thermal dependence of D0 and tau(f) supports the notion of entropically driven interdroplet steric repulsion stabilizing the cellular phases.

Journal Article↗

PubMed2002

Non-Gaussian nature of the alpha relaxation of glass-forming polyisoprene.

We report quasielastic neutron scattering experiments exploring the alpha relaxation in polyisoprene over an unprecedented range in momentum transfer. Corroborating and validating earlier molecular dynamics simulations, the measurements reveal a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. We show that a consistent interpretation in terms of a distribution of finite jumps underlying the alpha process is possible. This model leads to a time-dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations.

Journal Article↗

PubMed2002

Intermediate length scale dynamics of polyisobutylene.

We report on a neutron spin echo investigation of the intermediate scale dynamics of polyisobutylene studying both the self-motion and the collective motion. The momentum transfer (Q) dependences of the self-correlation times are found to follow a Q(-2/beta) law in agreement with the picture of Gaussian dynamics. In the full Q range of observation, their temperature dependence is weaker than the rheological shift factor. The same is true for the stress relaxation time as seen in sound wave absorption. The collective times show both temperature dependences; at the structure factor peak, they follow the temperature dependence of the viscosity, but below the peak, one finds the stress relaxation behavior.

Journal Article↗

PubMed2002

Molecular observation of contour-length fluctuations limiting topological confinement in polymer melts.

In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time.

Journal Article↗

PubMed2000

Measurement of vortex motion in a type-II superconductor: A novel use of the neutron spin-echo technique

We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity v(L) was given as expected by the values of electric and magnetic fields, via E = -v(L)wedgeB. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.

Journal Article↗

PubMed2000

Origin of dynamic heterogeneities in miscible polymer blends: A quasielastic neutron scattering study

In order to investigate the origin of the often invoked nanoheterogeneities in miscible polymer blends, we have performed quasielastic neutron scattering experiments on the component dynamics within the miscible polymer blend polyisoprene/polyvinyl ether including the pure components as a reference. We find that the apparent local heterogeneities observed by spectroscopic techniques originate from the chain specific crossover properties between entropy driven and local chain dynamics and are, thus, a purely dynamical phenomenon.

Journal Article↗

PubMed1996