Abstract:Step strain experiments and dynamic light scattering measurements are perfomed to characterize the dynamic behavior of an o/w droplet microemulsion into which is incorporated a telechelic polymer.At sufficient droplet and polymer concentrations, above the percolation threshold, the system is viscoelastic and its dynamic structure factor shows up two steps for the relaxation of concentration fluctuations: the fast one is dominated by the diffusion but the slower one is almost independent of the wave vector. The terminal time of the stress relaxation τ R and the slow time of the dynamic structure factor τ S are both presumably controlled by the residence time of a sticker in a droplet: consistently,

The packing and aggregation of colloidal particles is important for a wide variety of applications, including biological assays, sensors, paints, ceramics, and photonic crystals.[1±4] Over the years, different methods have been developed for controlling the structure and aggregation of large numbers of colloidal particles, thereby enabling the fabrication of coatings, artificial opals, and complex ceramic bodies. By contrast, relatively few methods exist for controlling the aggregation and structure of small numbers of colloidal particles. Motivated by recent interest in colloidal self-assembly for optical applications, a few groups have started developing schemes for making aggregates consisting of a small number of monodisperse colloidal microspheres.[5±7] These small colloidal aggregates, which include dimers, tetrahedra, and more complex polyhedra, possess lower symmetry than the spheres from which they are made and offer the possibility of forming more complex colloidal phases and structures than can be realized using simple spheres, just as molecules form more complex phases and structures than do atoms. It has been suggested, for example, that tetrahedral colloidal clusters might be useful in developing schemes for assembling colloidal crystals in the diamond structure. [5,6] Colloidal crystals with the diamond structure are predicted to exhibit a full photonic bandgap with many desirable properties.[8]We recently demonstrated a process that is capable of making a large number of identical clusters, approximately 10 8 ±10 10 in the original experiments, [6] where the number of spheres (n) in each cluster can be varied between approximately 2 and 15. The process is based on emulsifying a suspension of lightly crosslinked polystyrene microspheres in toluene with an aqueous surfactant solution. This yields a toluene-inwater emulsion with the polystyrene microspheres bound by surface tension to the droplet interfaces. When the toluene is removed by evaporation, the particles form stable clusters of colloidal particles suspended in water. The particles within clusters are strongly bound together by the van der Waals' force, while cluster±cluster aggregation is prevented by the surface charge the clusters acquire when the sulfate groups covalently bonded to the particle surfaces dissociate in water.Clusters of different aggregation number are readily fractionated using density gradient centrifugation to produce monodisperse suspensions of clusters. The shapes of the different aggregates for 2 £ n £ 11 correspond to compact packings that minimize the second moment of the mass distribution, defined aswhere r i is the position of the center of the a sphere and r cm is the center of mass of a given cluster configuration.[9] The packings of larger clusters do not minimize the second moment of the mass distribution, but yield unique clusters with values of the second moment that are close to the minimal values. The process as originally demonstrated exploits certain unique properties of the polystyrene microspheres th...

The directed three-dimensional self-assembly of microstructures and nanostructures through the selective hybridization of DNA is the focus of great interest toward the fabrication of new materials. Single-stranded DNA is covalently attached to polystyrene latex microspheres. Single-stranded DNA can function as a sequence-selective Velcro by only bonding to another strand of DNA that has a complementary sequence. The attachment of the DNA increases the charge stabilization of the microspheres and allows controllable aggregation of microspheres by hybridization of complementary DNA sequences. In a mixture of microspheres derivatized with different sequences of DNA, microspheres with complementary DNA form aggregates, while microspheres with noncomplementary sequences remain suspended. The process is reversible by heating, with a characteristic "aggregate dissociation temperature" that is predictably dependent on salt concentration, and the evolution of aggregate dissociation with temperature is observed with optical microscopy.

In order to study the viscoelastic properties of certain complex fluids which are described in terms of a multiconnected transient network we have developed a convenient model system composed of microemulsion droplets linked by telechelic polymers. The phase behavior of such systems has two characteristic features: a large monophasic region which consists of two sub-regions (a fluid sol phase and a viscoelastic gel phase) separated by a percolation line and a two phase region at low volume fraction with separation into a dilute sol phase and a concentrated gel phase. From the plausible origin of these features we expect them to be very similar in different systems. We describe here the phase behavior of four different systems we prepared in order to vary the time scale of the dynamical response of the transient network; they consist of the combination of two oil(decane) in water microemulsions differing by the stabilizing surfactant monolayer (Cetyl pyridinium chloride/ octanol or TX100/TX35) and of two telechelic polymers which are end-grafted poly (ethylene oxide) chains, differing by the end-grafted hydrophobic aliphatic chains (C 12 H 25 or C 18 H 37 ). We first summarize the characterization of the structure of the four systems by small angle neutron scattering: the size of the microemulsion droplets is found to be constant in a given system upon addition of a telechelic polymer.In the CPCl systems we find a mean radius of the microemulsion droplets = 62± 1Å and a very narrow size distribution and in the TX systems we find a mean radius = 84± 2Å and a somewhat larger size ____________________________________________________________________ 2 06/01 Paper "Robust phase behaviour..." Filali et al distribution. We can then calculate precisely the number of polymers per microemulsion droplet and compare the phase behavior of the four systems in consistent units. As expected we find very similar phase behavior in the four systems.