Microphase separation in a model graft copolymer

We present a preliminary overview of our work on a series of graft copolymers having poly(ethyl acrylate) backbones with pendant chains of polystyrene (PS). The copolymer system appeared to be in the strong segregation limit and exhibited evidence of ordered structures. The morphology of these structures can apparently be very different from what would be expected. For instance, we observed a lamellar structure in a material containing 28 wt.% PS grafts. Samples under uniaxial strain showed either conventional (i.e., affine deformation) and anomalous ({open_quotes}butterfly{close_quotes} isointensity patterns) behavior in small-angle neutron scattering

Polydisperse star polymer solutions

We analyze the effect of polydispersity in the arm number on the effective interactions, structural correlations and the phase behavior of star polymers in a good solvent. The effective interaction potential between two star polymers with different arm numbers is derived using scaling theory. The resulting expression is tested against monomer-resolved molecular dynamics simulations. We find that the theoretical pair potential is in agreement with the simulation data in a much wider polydispersity range than other proposed potentials. We then use this pair potential as an input in a many-body theory to investigate polydispersity effects on the structural correlations and the phase diagram of dense star polymer solutions. In particular we find that a polydispersity of 10%, which is typical in experimental samples, does not significantly alter previous findings for the phase diagram of monodisperse solutions.Comment: 14 pages, 7 figure

Periodic structures in colloidal crystals with oscillatory flow

We have studied some aspects of the flow of colloidal crystals in a capillary. Applying oscillatory pressure we induce flow in the shear region near where the crystal responds plastically. Under certain conditions, we observe well oriented crystalline regions which alternate orientations in a periodic pattern along the tube. Since the crystals have a lattice spacing of approximately 0.5 μ and Bragg scatter visible light, a periodic colour pattern is observed. We have studied the formation of these cylindrically symmetric striations as a function of shear modulus, frequency and amplitude of oscillations, particle size and tube diameter.Nous avons étudié quelques aspects de l'écoulement de cristaux colloidaux dans un capillaire. En appliquant une pression oscillante nous induisons un écoulement dans la région de cisaillement proche où le cristal a une réponse plastique. Dans certaines conditions nous observons des régions cristallines bien orientées avec un dessin d'orientation périodique le long du tube. Puisque les cristaux ont une périodicité d'environ 0,5 μ, visible en diffraction de lumière, on observe un dessin coloré. Nous avons étudié la formation de ces striures en fonction du module de cisaillement, de la fréquence, de l'amplitude des oscillations, de la taille des particules et du diamètre du tube

SELF DIFFUSION IN INTERACTING COLLOIDAL LIQUIDS

Nous avons mesuré le coefficient de diffusion Ds propre de sphères monodisperses de polystyrène ionisées dans l'eau en utilisant la diffusion Rayleigh forcée. Nous avons observé une décroissance marquée de Ds, de la valeur caractéristique d'une seule particule, quand les interactions entre les particules augmentent. Nous avons également observé un saut d'au moins quatre ordres de grandeur de Ds à la fusion (transition de 1er ordre). Les valeurs obtenues sont en bon accord avec des relations phénoménologiques connues dans les liquides simples, la diffusion propre variant en sens inverse de la viscosité.We have measured the self diffusion coefficient (Ds) of monodisperse charged polystyrene spheres in water by forced Rayleigh scattering. We observed a marked decrease in Ds from the free particle value as the interparticle interactions increase. We also observed a jump of at least four orders of magnitude in Ds at the first order melting transition. The data is found to agree with phenomonological relationships seen in simple liquids with the self diffusion varying inversely with the viscosity

PERIODIC STRUCTURES IN DRIVEN COLLOIDAL CRYSTALS

La grande dimension de la maille des réseaux de cristaux colloïdaux est la cause de la petitesse, de leurs constantes élastiques (1010 fois inférieures à celles des solides usuels). On peut donc aisément étudier des écoulements présentant des rapports contrainte / constante élastique supérieurs à ce que l'on connaissait jusqu'ici. On étudie un écoulement plastique hautement non linéaire en régimes oscillants et stationnaires, ce qui conduit à des structures spatialement périodiques. Ces structures correspondent à des régions alternées de cristallites qui sont l'image par réflection de leurs voisines. La structure observée en régime oscillant dans un tube sort de l'ordinaire en ce que le coeur est liquide alors que contre les parois la structure est solide (ordonnée). Cette configuration, reconnue comme instable jusqu'ici peut être le résultat d'un comportement inhabituel de la relation déformation-contrainte à la frontière solide-fluide. Des expériences en géométrie de Couette produisent des bandes verticales qui correspondent à un mouvement cohérent de dislocations à la frontière de 2 structures images dans un miroir l'une de l'autre. Ces bandes se déplacent en sens inverse de celui de la rotation du cylindre intérieur et à vitesse proche de celle de la propagation des ondes élastiques transverses.The large lattice spacings in colloidal crystals produce elastic constants ~ 1010 less than conventional solids. It is therefore easy to study flow properties at stress/elastic constant ratios higher than previously available. The highly nonlinear plastic flow regime studied in oscillating and steady state flow yields periodic patterns. These patterns correspond to alternating regions of ordered crystallites which are mirror image structures. The pattern observed in oscillatory flow in a tube is also unusual in that the core is liquid while at larger radius one finds a solid on the tube wall. This traditionally unstable configuration may be the result of an anomalous stress-rate relation at the fluid-solid boundary. Experiments in couette geometry produce vertical stripes which correspond to coherent motion of dislocations at the boundaries separating two mirror image structures. The stripes move in the direction opposite from the rotation of the inner cylinder, at velocities close to the transverse sound velocity

Water dynamics in controlled pore silica glasses

Water in porous silica glass is a suitable system for investigating the effect of confinement on translational diffusion. These systems are important because of their relevance in catalytic and separation processes. Here quasi-elastic neutron scattering experiments at room temperature on water-filled controlled-pore glasses with radius of 15, 24 and 32Å, are presented and analyzed using the random-jump diffusion model. Both the average residence time and the mean jump distance increase with decreasing pore radius

Study of short-range motion of atomic hydrogen in amorphous silicon by neutron reflectometry

Preliminary results of neutron reflectometry (NR) measurements on rf sputter-deposited a-Si:H/a-Si:D bilayers indicate that this technique may be used to monitor H and D motions over distances of {approx} 10 to 200 {Angstrom} with a nominal resolution of 5--10 {Angstrom}. In studying rf sputter-deposited thin films containing a high density of microvoids annealed at 270 C, we found that the hydrogen diffused a distance of only {approx} 100 {Angstrom}. Further annealing at 270 and 280 C produced no additional motion. This result is consistent with a model of this system in which the hydrogen is trapped in microvoids after moving a relatively short distance