4,608 research outputs found
Bulk and wetting phenomena in a colloidal mixture of hard spheres and platelets
Density functional theory is used to study binary colloidal fluids consisting
of hard spheres and thin platelets in their bulk and near a planar hard wall.
This system exhibits liquid-liquid coexistence of a phase that is rich in
spheres (poor in platelets) and a phase that is poor in spheres (rich in
platelets). For the mixture near a planar hard wall, we find that the phase
rich in spheres wets the wall completely upon approaching the liquid demixing
binodal from the sphere-poor phase, provided the concentration of the platelets
is smaller than a threshold value which marks a first-order wetting transition
at coexistence. No layering transitions are found in contrast to recent studies
on binary mixtures of spheres and non-adsorbing polymers or thin hard rods.Comment: 6 pages, 4 figure
Role of Metastable States in Phase Ordering Dynamics
We show that the rate of separation of two phases of different densities
(e.g. gas and solid) can be radically altered by the presence of a metastable
intermediate phase (e.g. liquid). Within a Cahn-Hilliard theory we study the
growth in one dimension of a solid droplet from a supersaturated gas. A moving
interface between solid and gas phases (say) can, for sufficient (transient)
supersaturation, unbind into two interfaces separated by a slab of metastable
liquid phase. We investigate the criteria for unbinding, and show that it may
strongly impede the growth of the solid phase.Comment: 4 pages, Latex, Revtex, epsf. Updated two reference
Diffusive Evolution of Stable and Metastable Phases II: Theory of Non-Equilibrium Behaviour in Colloid-Polymer Mixtures
By analytically solving some simple models of phase-ordering kinetics, we
suggest a mechanism for the onset of non-equilibrium behaviour in
colloid-polymer mixtures. These mixtures can function as models of atomic
systems; their physics therefore impinges on many areas of thermodynamics and
phase-ordering. An exact solution is found for the motion of a single, planar
interface separating a growing phase of uniform high density from a
supersaturated low density phase, whose diffusive depletion drives the
interfacial motion. In addition, an approximate solution is found for the
one-dimensional evolution of two interfaces, separated by a slab of a
metastable phase at intermediate density. The theory predicts a critical
supersaturation of the low-density phase, above which the two interfaces become
unbound and the metastable phase grows ad infinitum. The growth of the stable
phase is suppressed in this regime.Comment: 27 pages, Latex, eps
Colloidal gelation and non-ergodicity transitions
Within the framework of the mode coupling theory (MCT) of structural
relaxation, mechanisms and properties of non-ergodicity transitions in rather
dilute suspensions of colloidal particles characterized by strong short-ranged
attractions are studied. Results building on the virial expansion for particles
with hard cores and interacting via an attractive square well potential are
presented, and their relevance to colloidal gelation is discussed.Comment: 10 pages, 4 figures; Talk at the Conference: "Unifying Concepts in
Glass Physics" ICTP Trieste, September 1999; to be published in J. Phys.:
Condens. Matte
A cluster mode-coupling approach to weak gelation in attractive colloids
Mode-coupling theory (MCT) predicts arrest of colloids in terms of their
volume fraction, and the range and depth of the interparticle attraction. We
discuss how effective values of these parameters evolve under cluster
aggregation. We argue that weak gelation in colloids can be idealized as a
two-stage ergodicity breaking: first at short scales (approximated by the bare
MCT) and then at larger scales (governed by MCT applied to clusters). The
competition between arrest and phase separation is considered in relation to
recent experiments. We predict a long-lived `semi-ergodic' phase of mobile
clusters, showing logarithmic relaxation close to the gel line.Comment: 4 pages, 3 figure
Controlled Carbon Dioxide Terpolymerizations to Deliver Toughened yet Recyclable Thermoplastics
Using CO2 polycarbonates as engineering thermoplastics has been limited by their mechanical performances, particularly their brittleness. PolyÂ(cyclohexene carbonate) (PCHC) has a high tensile strength (40 MPa) but is very brittle (elongation at break 250 °C). All the polymers are amorphous with a single, high glass transition temperature (96 < T g < 108 °C). The polymer entanglement molar masses, determined using dynamic mechanical analyses, range from 4 < M e < 23 kg mol–1 depending on the polymer composition (PCHC:PCPC). These polymers show superior mechanical performance to PCHC; specifically the lead material (PCHC0.28-grad-PCPC0.72) shows 25% greater tensile strength and 160% higher tensile toughness. These new plastics are recycled, using cycles of reprocessing by compression molding (150 °C, 1.2 ton m–2, 60 min), four times without any loss in mechanical properties. They are also efficiently chemically recycled to selectively yield the two epoxide monomers, CHO and CPO, as well as carbon dioxide, with high activity (TOF = 270–1653 h–1, 140 °C, 120 min). The isolated recycled monomers are repolymerized to form thermoplastic showing the same material properties. The findings highlight the benefits of the terpolymer strategy to deliver thermoplastics combining the beneficial low entanglement molar mass, high glass transition temperatures, and tensile strengths; PCHC properties are significantly improved by incorporating small quantities (23 mol %) of cyclopentene carbonate linkages. The general strategy of designing terpolymers to include chain segments of low entanglement molar mass may help to toughen other brittle and renewably sourced plastics
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