Erwan Nicol

Structure and Viscoelasticity of Mixed Micelles Formed by Poly(ethylene oxide) End Capped with Alkyl Groups of Different Length

Poly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect of varying the length of the alkyl end group on the structure and viscoelasticity was investigated for pure and mixed micelle suspensions. The aggregation number (p) of the micelles increased and the critical association concentration (CAC) decreased with increasing the length of the end group. At high concentrations a discontinuous reversible liquid-solid transition was observed below a critical temperature (Tc) that increased with increasing length of the end group. Mixing end-capped PEO with different alkyl lengths led first to formation of the micelles by polymers with the lowest CAC into which the other polymers were incorporated when the concentration was increased. The viscoelastic properties at high concentrations are the same for pure systems and mixtures with the same average length of the alkyl end group.

Transient Gelation and Glass Formation of Reversibly Cross-linked Polymeric Micelles

Poly(ethylene oxide) (PEO) end-capped with a hexadecyl group at one (alpha-PEO) or both ends (alpha,omega-PEO) are highly asymmetric diblock or triblock copolymers that form spherical micelles in aqueous solution. alpha,omega-PEO can bridge between two micelles leading to reversible association of the micelles. At a first critical concentration (C(p)), the micelles percolate and a transient network is formed with an elastic modulus determined by the concentration of alpha,omega-PEO. C(p) increases with increasing fraction of alpha-PEO and is insensitive to the temperature. At a second critical concentration (C(c)), a liquid-solid transition occurs. C(c) is independent of the fraction of alpha-PEO and increases with increasing temperature. There are indications that the solid state is formed by nucleation and growth of domains of dynamically arrested micelles. The properties of the transient network are almost the same in the liquid and in the solid state.

Effect of arm exchange on the liquid-solid transition of dense suspensions of star polymers.

Star polymers with dynamic arm exchange are formed in water by self-assembly of amphiphilic diblock copolymers based on poly(ethylene oxide) end capped with a small hydrophobic block. The arm exchange was arrested in situ by photo-cross-linking of the core. The effect of dynamic arm exchange on the osmotic compressibility and viscosity was investigated systematically as a function of the concentration and temperature. The discontinuous liquid-solid transition reported for dense polymeric micelle suspensions was found to be preserved after dynamic arm exchange was arrested in situ. The effect of cross-linking and aggregation number on the liquid-solid transition was investigated.

Fast and effective quantum-dots encapsulation and protection in PEO based photo-cross-linked micelles

Cadmium-based quantum dots (QDs) were easily, quickly and efficiently transferred from an organic medium to water without modification of their surface chemistry by the simple emulsion/solvent evaporation technique using micelles of amphiphilic diblock copolymers based on poly(ethylene oxide) and poly(2-methacryloyloxyethyl acrylate) (PEO-b-PMEA) as hosts. The resulting hybrid micelles were stabilized very rapidly by photo-cross-linking the hydrophobic core around the QDs. The encapsulation and photo-cross-linking process were shown to barely affect the photoluminescence properties. Grafting a short octyl chain at the end of the hydrophobic block enhanced both the colloidal stability of the QDs dispersed in water and prevented the quenching of their fluorescence by copper ions. Grafting a longer hexadecyl chain at the end of the PMEA block decreased the efficiency of the corona cross-linking and led to poorer stabilization and protection.

Patchy Supramolecular Bottle-Brushes Formed by Solution Self-Assembly of Bis(urea)s and Tris(urea)s Decorated by Two Incompatible Polymer Arms

In an attempt to design urea-based Janus nanocylinders through a supramolecular approach, nonsymmetrical bis(urea)s and tris(urea)s decorated by two incompatible polymer arms, namely, poly(styrene) (PS) and poly(isobutylene) (PIB), were synthesized using rather straightforward organic and polymer chemistry techniques. Light scattering experiments revealed that these molecules self-assembled in cyclohexane by cooperative hydrogen bonds. The extent of self-assembly was limited for the bis(urea)s. On the contrary, reasonably anisotropic 1D structures (small nanocylinders) could be obtained with the tris(urea)s (Nagg ∼ 50) which developed six cooperative hydrogen bonds per molecule. (1)H transverse relaxation measurements and NOESY NMR experiments in cyclohexane revealed that perfect Janus nanocylinders with one face consisting of only PS and the other of PIB were not obtained. Nevertheless, phase segregation between the PS and PIB chains occurred to a large extent, resulting in patchy cylinders containing well separated domains of PIB and PS chains. Reasons for this behavior were proposed, paving the way to improve the proposed strategy toward true urea-based supramolecular Janus nanocylinders.