Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.     

Synthesis of polymer/Laponite nanocomposite latex particles via emulsion polymerization using silylated and cation-exchanged Laponite clay platelets

We report the synthesis and characterization of polymer/Laponite nanocomposite latex particles through emulsion polymerization using organically modified Laponite clay platelets as seeds. Two approaches were adopted for the organic modification of Laponite. The first one is based on the grafting of either γ-methacryloyloxy propyl dimethyl-methoxysilane (γ-MPDES) or γ-methacryloyloxy propyl triethoxysilane (γ-MPTES) on the clay edges. The other strategy consists in exchanging the clay interlayer sodium ions by either a free radical initiator, 2,2-azobis(2-methyl propionamidine)hydrochloride (AIBA) or a cationic vinyl monomer, 2-(methacryloyloxy)ethyl trimethyl ammonium chloride (MADQUAT). The grafting was characterized both qualitatively using FTIR and quantitatively using elemental analysis or UV analysis. The results show that the degree of functionalization depends on the nature of the organic modifier. Before performing the emulsion polymerization reaction, the functionalized clay platelets were successfully dispersed in water. Nanocomposite latexes were then synthesized using a mixture of styrene (Styr) and butyl acrylate (BA) and sodium dodecyl sulphate (SDS) as anionic surfactant. An important result of the present work is that clay redispersion in water is a key step of the overall process. The larger the size of the clay aggregates, the poorer the stability of the resulting latex suspension. The morphology and mechanism of formation of the nanocomposite particles are discussed.     

Effects of the environmental factors on the casein micelle structure studied by cryo transmission electron microscopy and small-angle x-ray scattering/ultrasmall-angle x-ray scattering

Casein micelles are colloidal protein-calcium-transport complexes whose structure has not been unequivocally elucidated. This study used small-angle x-ray scattering (SAXS) and ultrasmall angle x-ray scattering (USAXS) as well as cryo transmission electron microscopy (cryo-TEM) to provide fine structural details on their structure. Cryo-TEM observations of native casein micelles fractionated by differential centrifugation showed that colloidal calcium phosphate appeared as nanoclusters with a diameter of about 2.5 nm. They were uniformly distributed in a homogeneous tangled web of caseins and were primarily responsible for the intensity distribution in the SAXS profiles at the highest q vectors corresponding to the internal structure of the casein micelles. A specific demineralization of casein micelles by decreasing the pH from 6.7 to 5.2 resulted in a reduced granular aspect of the micelles observed by cryo-TEM and the existence of a characteristic point of inflection in SAXS profiles. This supports the hypothesis that the smaller substructures detected by SAXS are colloidal calcium phosphate nanoclusters rather than putative submicelles.     

Hierarchical Self-Assembly of Amphiphilic β-C-Glycosylbarbiturates into Multiresponsive Alginate-Like Supramolecular Hydrogel Fibers and Vesicle Hydrogel

Ordered molecular self-assembly of glycoamphiphiles has been regarded as an attractive, practical and bottom-up approach to obtain stable, structurally well-defined, and functional mimics of natural polysaccharides. This study describes a versatile and rational design of carbohydrate-based hydrogelators through N,N’-substituted barbituric acid-mediated Knoevenagel condensation onto unprotected carbohydrates in water. Amphiphilic N-substituted β-C-maltosylbarbiturates self-assembled into pH- and calcium-triggered alginate-like supramolecular hydrogel fibers with a multistimuli responsiveness to temperature, pH and competitive metal chelating agent. In addition, amphiphilic N,N’-disubstituted β-C-maltosylbarbiturates formed vesicle gels in pure water that were scarcely observed for glyco-hydrogelators. Finally, barbituric acid worked as a multitasking group allowing chemoselective ligation onto reducing-end carbohydrates, structural diversity, stimuli-sensitiveness, and supramolecular interactions by hydrogen bonding.     

High Solids Content,Soap‐Free,Film‐Forming Latexes Stabilized by Laponite Clay Platelets

High solids content film‐forming poly[styrene‐co‐(n‐butyl acrylate)] [poly(Sty‐co‐BuA)] latexes armored with Laponite clay platelets have been synthesized by soap‐free emulsion copolymerization of styrene and n‐butyl acrylate. The polymerizations were performed in batch in the presence of Laponite and a methyl ether acrylate‐terminated poly(ethylene glycol) macromonomer in order to promote polymer/clay association. The overall polymerization kinetics showed a pronounced effect of clay on nucleation and stabilization of the latex particles. Cryo‐transmission electron microscopy observation confirmed the armored morphology and indicated that the majority of Laponite platelets were located at the particle surface. The resulting nanostructured films displayed enhanced mechanical properties.

     

Hybrid Organic-Inorganic Colloids with a Core-Corona Structure: A Transmission Electron Microscopy Investigation

The present paper describes a simple route to prepare organic-inorganic colloids with a core-corona structure. Colloids are obtained by complexation of multivalent metal cations by anionic-neutral double hydrophilic block copolymers leading to hybrid polyion complex micelles. The hybrid colloids can then be mineralized in suspension by performing hydrolysis of metal cations within the micelles. The present study focuses on the structural characterization of the colloidal complexes and of the mineralized nanoparticles using a combination of transmission electron microscopy (TEM) techniques including conventional bright-field imaging (with and without negative staining) and cryo-TEM observations. It is shown that the different types of TEM observations are complementary and necessary to get a complete picture of the core-corona colloids.