From seashells to DNA, chirality is expressed at every level of biological structures. In self-assembled structures it may emerge cooperatively from chirality at the molecular scale. Amphiphilic molecules, for example, can form a variety of aggregates and mesophases that express the chirality of their constituent molecules at a supramolecular scale of micrometres. Quantitative prediction of the large-scale chirality based on that at the molecular scale remains a largely unsolved problem. Furthermore, experimental control over the expression of chirality at the supramolecular level is difficult to achieve: mixing of different enantiomers usually results in phase separation. Here we present an experimental and theoretical description of a system in which chirality can be varied continuously and controllably ('tuned') in micrometre-scale structures. We observe the formation of twisted ribbons consisting of bilayers of gemini surfactants (two surfactant molecules covalently linked at their charged head groups). We find that the degree of twist and the pitch of the ribbons can be tuned by the introduction of opposite-handed chiral counterions in various proportions. This degree of control might be of practical value; for example, in the use of the helical structures as templates for helical crystallization of macromolecules.

Programmed self-assembly of well-defined molecular building blocks enables the fabrication of precisely structured nanomaterials. In this work, we explore a new class of giant polymeric surfactants (Mn = (0.7-4.4) × 10(6) g/mol) with bottlebrush architecture and show that their persistent molecular shape leads to the formation of uniform aggregates in a predictable manner. Amphiphilic bottlebrush block copolymers containing polylactide (PLA) and poly(ethylene oxide) (PEO) side chains were synthesized by a grafting-from method, and their self-assembly in aqueous environment was studied by cryogenic transmission electron microscopy. The produced micelle structures with varying interfacial curvatures and core radii (19-55 nm) boasted rod-like hydrophilic PEO brushes protruding from the hydrophobic PLA cores normal to the interface. Highly uniform spherical micelles with low dispersities were obtained from bottlebrush amphiphiles with packing parameters of ∼0.3, estimated from the polymer structural data. Long cylindrical micelles and other nonspherical aggregates were observed for the first time for compositionally less asymmetric bottlebrush surfactants. Critical micelle concentration values of 1 nM, measured for PEO-rich bottlebrush amphiphiles, indicated an enhanced thermodynamic stability of the produced micelle aggregates. Shape-dependent assembly of bottlebrush surfactants allows for the rational fabrication of a range of micelle structures in narrow morphological windows.

No abstract

The direct observation of a shear-induced structure in dilute/semidilute giant micellar solutions in water is reported. At rest, the micelles are randomly oriented and the zero-shear viscosity is concentration dependent and varies between 10 -3 and 10 -2 Pa s. Under shear, the solutions have no measurable anisotropy as long as the applied shear rate is less than a critical value determined from optical birefringence and electric conductivity measurements. Above this "critical shear rate", the viscosity increases, and the solution becomes strongly anisotropic. However, it was found that the "critical shear rate" depends strongly on the gap distance of the Couette cell, thus excluding the possibility of a phase transition at the critical shear rate. Cryo-transmission electron microscopy is used to show shear-induced aggregation of wormlike micelles. It is concluded that shear induces a phase separation between a surfactant rich and a surfactant poor phase at shear rates much lower than the critical value. Initially, the surfactant rich phase forms a network-like superstructure with domain sizes increasing with shear. A strong anisotropy due to the deformation of the network is observed when the domain size reaches the order of the gap size of the Couette. This phenomenon was observed for a large range of concentrations from far below up to several times above the overlap concentration φ*.

The phase behavior of aqueous solutions of several dissymmetric gemini surfactants (with two hydrophobic chains of different lengths) was studied. From rheological and X-ray scattering studies as well as from freeze fracture imaging, some general patterns of the phase behavior were observed which are remarkably different from those observed for monomeric surfactants. The sequence of phases observed with increasing surfactant concentration was an isotropic wormlike micellar phase, multilayered structures which are first isotropic and then organized with orientational ordering, and an inverted hexagonal phase. The multilayered phase showed a scattering peak corresponding to a periodicity of 40 Å indicating the presence of stacks of bilayers without a water layer in between. For some of the samples, cryo-TEM showed that wormlike micelles (WLM) evolve into a ribbonlike structure (elongated bilayer). Upon further increase in concentration, the ribbons transform into multilayered structures with a well-defined width.

Luminescent platinum-terpyridine or orthometalated bipyridine complexes have engendered widespread interest as functional materials, in particular stemming from their intense phosphorescence in the visible region of the electromagnetic spectrum. [1] These stable, neutral or ionic complexes have been widely used as electroluminescent thin films or as dopants in organic light-emitting devices (OLEDs), [2] as DNA intercalators, [3] and molecular probes for biological macromolecules. [4] In some cases, the ligand design, [5] the solvent, [6] or the use of soluble polymers induces metal···metal and p-p stacking interactions, which allow the tuning of the optical properties over a large spectral range. [7] Such intermolecular features are most useful for the engineering of liquid crystals and organogelators, for which intermolecular interactions are critical. [8] Rodlike metal alkynyl complexes of Pd II , Pt II , Rh I , and Hg I exhibit thermotropic liquid-crystalline properties, [9] and metal-poly(yne) polymers form lyotropic liquid crystals. [10] However, as a result of the shape and anisotropy introduced by the alkynyl tethers, most of these complexes exhibit only lamellar (SmA) and nematic mesophases. Also, the absence of p-accepting chromophores results in non-luminescent materials, which notably limits their practical utilization in energy-conversion devices (OLEDs, solar cells, etc.). While phosphorescent organogels of quinolinol platinum(II) complexes have recently been characterized, there was no indication of metal···metal interactions that might provide an additional mechanism for the control of their properties. [11] Thus, the construction of well-organized phosphorescent architectures remains an important objective, with the essential features being the need to incorporate 1) heavy metals to favor spin-orbit coupling for phosphorescence, 2) ligand tailoring to facilitate intermolecular interaction through hydrogen bonding (e.g. amide vectors) or p-p interactions through polyaromatic and/or polyimine fragments, and 3) d 8 -transition metals which are known to favor square-planar structures, thus facilitating metal-metal interactions. Here, we disclose our results on organogels and mesomorphic materials obtained through such a strategy.Syntheses of the pivotal ethynyl platforms 1 a-c were based on gallate-substituted derivatives with methyl, dodecyl (C 12 H 25 ), or phytol-like (C 20 H 41 ) substituents. [12] The final complexes were prepared by a cross-coupling reaction under anaerobic conditions between the terminal ethynyl derivatives and [(terpy)PtCl]BF 4 (terpy = 2,2':6',6''-terpyridine). [13] The reaction is catalyzed by CuI (ca. 6 mol %), and triethylamine is required to quench the nascent HCl. The deep-red complexes 2 a-c were obtained in good yields after column chromatography and crystallization from a mixture of dichloromethane/methanol and further fully characterized by NMR and FTIR spectroscopy, ESI mass spectrometry, and elemental analysis.

The present paper reports the successful elaboration of exfoliated plasticized starch-based nanobiocomposites. This was made possible by using cationic starch as a new clay organomodifier to better match the polarity of the matrix and thus to facilitate the clay exfoliation process. To demonstrate the efficiency of this new approach, either natural (MMT-Na) or organomodified (OMMT-CS) montmorillonite were incorporated into the starch nanobiocomposites by a melt blending process. The morphological analyses (SAXD and TEM) showed that MMT-Na leads to the formation of intercalated nanobiocomposites. On the contrary, OMMT-CS allowed the elaboration of well-exfoliated nanobiocomposites. Tensile tests performed on the obtained nanobiocomposites showed that exfoliated nanobiocomposites display enhanced mechanical properties compared to those of the intercalated nanobiocomposites and neat matrix. These results clearly highlight the great interest in using OMMT-CS to obtain starch-based nanobiocomposites with improved properties.

Polymer vesicles, also named polymersomes, are valuable candidates for drug delivery and micro- or nanoreactor applications. As far as drug delivery is concerned, the shape of the carrier is believed to have a strong influence on the biodistribution and cell internalization. Polymersomes can be submitted to an osmotic imbalance when injected in physiological media leading to morphological changes. To understand these osmotic stress-induced variations in membrane properties and shapes, several nanovesicles made of the graft polymer poly(dimethylsiloxane)-g-poly(ethylene oxide) (PDMS-g-PEO) or the triblock copolymer PEO-b-PDMS-b-PEO were osmotically stressed and observed by light scattering, neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). Hypotonic shock leads to a swelling of the vesicles, comparable to optically observable giant polymersomes, and hypertonic shock leads to collapsed structures such as stomatocytes and original nested vesicles, the latter being only observed for bilayers classically formed by amphiphilic copolymers. Complementary SANS and cryo-TEM experiments are shown to be in quantitative agreement and highlight the importance of the membrane structure on the behavior of these nanopolymersomes under hypertonic conditions as the final morphology reached depends whether or not the copolymers assemble into a bilayer. The vesicle radius and membrane curvature are also shown to be critical parameters for such transformations: the shape evolution trajectory agrees with theoretical models only for large enough vesicle radii above a threshold value around 4 times the membrane thickness.