RESUMEN
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.
RESUMEN
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.
Asunto(s)
Liposomas/química , Fluidez de la Membrana , Modelos Moleculares , Poliestirenos/química , Espectrometría por Rayos X/métodos , Simulación por Computador , Cinética , Sustancias Macromoleculares/análisis , Sustancias Macromoleculares/química , Conformación Molecular , Movimiento (Física) , Polímeros/análisis , Polímeros/química , Poliestirenos/análisisAsunto(s)
Antibacterianos/farmacología , Sangre/microbiología , Ceftizoxima/análogos & derivados , Ceftizoxima/farmacología , Ácido Clavulánico/farmacología , Bacterias Gramnegativas/efectos de los fármacos , beta-Lactamasas/biosíntesis , Bacteriemia/microbiología , Medios de Cultivo , Bacterias Gramnegativas/clasificación , Bacterias Gramnegativas/enzimología , Bacterias Gramnegativas/aislamiento & purificación , Infecciones por Bacterias Gramnegativas/microbiología , Humanos , Pruebas de Sensibilidad Microbiana/métodos , Factores de Tiempo , CefpodoximaRESUMEN
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.
Asunto(s)
Silicatos de Aluminio/química , Coloides/química , Modelos Químicos , Modelos Moleculares , Nanotubos/química , Arcilla , Simulación por Computador , Cinética , Modelos Estadísticos , Conformación Molecular , Tamaño de la Partícula , Electricidad Estática , Factores de TiempoRESUMEN
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.
RESUMEN
The dynamics of compositional fluctuations in a miscible, entangled homopolymer blend of poly(ethylene oxide) and poly(methyl methacrylate) were studied on length scales smaller than the polymer radii of gyration, and for times comparable to the polymers' disentanglement time. The measured relaxation rates are consistent with predictions of the reptation model, as expressed via the dynamic random-phase approximation. Moreover, the observed mode amplitudes allow for an estimate of the entanglement length in the blend.
RESUMEN
X-ray photon correlation spectroscopy and small-angle scattering measurements are presented of the dynamics and structure of concentrated suspensions of charge-stabilized polystyrene latex spheres dispersed in glycerol, for volume fractions from 3% to 52%. The static structures of the suspensions show essentially hard-sphere behavior, and the short-time dynamics shows good agreement with predictions for the wave-vector-dependent collective diffusion coefficient. However, the intermediate scattering function is found to violate a scaling behavior found previously for a sterically stabilized hard-sphere suspension.
