Biomedical subjects

S G J Mochrie

Publications and source records attributed to S G J Mochrie.

6 recordsLinked to original sources

PubMed2006

Crossover from stretched to compressed exponential relaxations in a polymer-based sponge phase.

X-ray photon correlation spectroscopy was used to characterize the wave vector- and temperature-dependent dynamics of spontaneous thermal fluctuations in a sponge (L3) phase that occurs in a blend of a symmetric poly(styrene-ethylene/butylene-styrene) triblock copolymer with a polystyrene homopolymer. Measurements of the intermediate scattering function reveal a crossover from stretched- to compressed-exponential relaxations as the temperature is lowered from 180 to 120 degrees C.

Journal Article↗

PubMed2006

Optimizing the signal-to-noise ratio for X-ray photon correlation spectroscopy.

An analysis is presented of how to optimize the experimental beamline configuration for achieving the best possible signal-to-noise ratio (SNR) in X-ray photon correlation spectroscopy experiments using area detectors. It is shown that there exists an optimum detector distance; namely, the highest SNR is achieved by matching the angular pixel size with the angular source size. Binning several pixels together can increase the SNR by permitting to match the shape of a detector pixel to the shape of the source. It is also shown that collimating slits several times wider than the effective transverse coherence length are optimal; further, it is demonstrated that the energy dependence of the SNR is dictated by the energy dependence of detector efficiency and source brilliance. Ultimately the effects of focusing and low longitudinal coherence are discussed.

Algorithms↗

PubMed2005

Fluctuation dynamics of block copolymer vesicles.

X-ray photon correlation spectroscopy was used to characterize the wave-vector- and temperature-dependent dynamics of spontaneous thermal fluctuations in a vesicle (L4) phase that occurs in a blend of a symmetric poly(styrene-ethylene/butylene-styrene) triblock copolymer with a polystyrene homopolymer. Measurements of the intermediate scattering function reveal stretched-exponential behavior versus time, with a stretching exponent slightly larger than 2/3. The corresponding relaxation rates show an approximate q(3) dependence versus wave vector. Overall, the experimental measurements are well described by theories that treat the dynamics of independent membrane plaquettes.

Computer Simulation↗

PubMed2004

Evolution of particle-scale dynamics in an aging clay suspension.

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a suspension of highly charged, nanometer-sized disks. At wave vectors q corresponding to interparticle length scales, the dynamic structure factor follows a form f(q,t) approximately exp([-(t/tau)(beta)], where beta approximately 1.5. The relaxation time tau increases with the sample age t(a) approximately as tau approximately t(1.8)(a) and decreases with q as tau approximately q(-1). Such behavior is consistent with models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The measured amplitude of f(q,t) varies with q in a manner that implies caged particle motion. The decrease in the range of this motion and an increase in suspension conductivity with increasing t(a) indicate a growth in interparticle repulsion as the mechanism for internal stress development implied by these models.

Aluminum Silicates↗

PubMed2004

Symmetric-to-asymmetric transition in triblock copolymer-homopolymer blends.

In blends of a symmetric poly(styrene-ethylene/butylene-styrene) tri-block-copolymer with a polystyrene homopolymer, small-angle x-ray scattering and cryotransmission electron microscopy measurements reveal a microstructure consisting of a disordered arrangement of poly(ethylene/butylene) membranes suspended in polystyrene. For triblock volume fractions less than 0.22, the membranes form an asymmetric sponge or L4 phase, consisting predominantly of equilibrium vesicles. For volume fractions greater than 0.22, they form a symmetric sponge-phase (L3 phase), separated from the L4 phase by a first-order transition.

Journal Article↗

PubMed2003

Surface dynamics of polymer films.

The dynamics of supported polymer films were studied by probing the surface height fluctuations as a function of lateral length scale using x-ray photon correlation spectroscopy. Measurements were performed on polystyrene (PS) films of thicknesses varying from 84 to 333 nm at temperatures above the PS glass transition temperature. Within a range of wave vectors spanning 10(-3) to 10(-2) nm(-1), good agreement is found between the measured surface dynamics and the theory of overdamped thermal capillary waves on thin films. Quantitatively, the data can be accounted for using the viscosity of bulk PS.

Journal Article↗