Topological Connection between Vesicles and Nanotubes in Single-Molecule Lipid Membranes Driven by Head-Tail Interactions.

Lipid nanotube-vesicle networks are important channels for intercellular communication and transport of matter. Experimentally observed in neighboring mammalian cells but also reproduced in model membrane systems, a broad consensus exists on their formation and stability. Lipid membranes must be composed of at least two molecular components, each stabilizing low (generally a phospholipid) and high curvatures. Strong anisotropy or enhanced conical shape of the second amphiphile is crucial for the formation of nanotunnels. Anisotropic driving forces generally favor nanotube protrusions from vesicles. In this work, we report the unique case of topologically connected nanotubes-vesicles obtained in the absence of directional forces, in single-molecule membranes, composed of an anisotropic bolaform glucolipid, above its melting temperature, Tm. Cryo-TEM and fluorescence confocal microscopy show the interconnection between vesicles and nanotubes in a single-phase region, between 60 and 90 °C under diluted conditions. Solid-state NMR demonstrates that the glucolipid can assume two distinct configurations, head-head and head-tail. These arrangements, seemingly of comparable energy above the Tm, could explain the existence and stability of the topologically connected vesicles and nanotubes, which are generally not observed for classical single-molecule phospholipid-based membranes above their Tm.

Unexpected Slow Kinetics of Poly(Methacrylic Acid) Phase Separation in the Semi-Dilute Regime.

Poly (methacrylic acid) (PMAA) solutions are known to exhibit a lower critical solution temperature (LCST). A temperature-composition phase diagram of PMAA has been constructed by standard cloud point determination through transmittance measurements, and also by studying the steady states reached under phase separation. This allows us to reconstruct the binodal curve describing the phase behavior of PMAA for both low and high concentration regimes, and to determine accurately the LCST temperature. In a second step, the structures formed following a temperature jump above the cloud point and their evolution in time have been investigated at the nanoscale using small angle neutron scattering (SANS). This approach shows that the formation of phase-separated nanostructures is a slow process, requiring more than 12 h. The formed structures are then shown to depend on the amplitude of the temperature jump above the cloud point. An original mechanism of phase separation is identified in the semi-dilute regime. The growth of micrometric-size droplets with an inner structure displaying the rheological properties of a gel leads to the formation of a percolating network which hinders the influence of gravity. Such a result can explain the slow kinetics of the PMAA LCST transition.

Colored Janus Nanocylinders Driven by Supramolecular Coassembly of Donor and Acceptor Building Blocks.

Janus nanocylinders exhibit nanometric dimensions, a high aspect ratio, and two faces with different chemistries (Janus character), making them potentially relevant for applications in optics, magnetism, catalysis, surface nanopatterning, or interface stabilization, but they are also very difficult to prepare by conventional strategies. In the present work, Janus nanocylinders were prepared by supramolecular coassembly in water of two different polymers functionalized with complementary assembling units. The originality of our approach consists in combining charge transfer complexation between electron-rich and electron-poor units with hydrogen bonding to (1) drive the supramolecular formation of one-dimensional structures (cylinders), (2) force the two polymer arms on opposite sides of the cylinders independently of their compatibility, resulting in Janus nanoparticles, and (3) detect coassembly through a color change of the solution upon mixing of the functional polymers.

Author Correction: Straightforward preparation of supramolecular Janus nanorods by hydrogen bonding of end-functionalized polymers.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Straightforward preparation of supramolecular Janus nanorods by hydrogen bonding of end-functionalized polymers.

Janus cylinders are one-dimensional colloids that have two faces with different compositions and functionalities, and are useful as building blocks for advanced functional materials. Such anisotropic objects are difficult to prepare with nanometric dimensions. Here we describe a robust and versatile strategy to form micrometer long Janus nanorods with diameters in the 10-nanometer range, by self-assembly in water of end-functionalized polymers. The Janus topology is not a result of the phase segregation of incompatible polymer arms, but is driven by the interactions between unsymmetrical and complementary hydrogen bonded stickers. Therefore, even compatible polymers can be used to form these Janus objects. In fact, any polymers should qualify, as long as they do not prevent co-assembly of the stickers. To illustrate their applicative potential, we show that these Janus nanorods can efficiently stabilize oil-in-water emulsions.

Reversible Assembly of Microgels by Metallo-Supramolecular Chemistry.

The combination of supramolecular chemistry and soft colloids as microgels represents an ambitious way to develop multi-versatile colloidal assemblies. Hereafter, terpyridine-functionalized poly(N-isopropylacrylamide) (PNiPAM) microgel building blocks are shown to undergo an assemble-freeze-disassemble process. The microgel assemblies, which are controlled by monitoring the attractive and repulsive potentials between the soft colloidal particles, are then frozen by forming inter-particle metal-terpyridine bis-complexes upon addition of the metallic cation (such as FeII , CoII ). By oxidation of the metal-terpyridine bis-complex links, the aggregates open up, which is due to the complex dissociation releasing the connected particles in the form of single microgels. We extended our work to the development of 1D filaments and 2D membranes materials made of soft particles connected via supramolecular chemistry.

PEGylated NiPAM microgels: synthesis, characterization and colloidal stability.

The objective of this work is to synthesize highly stable thermoresponsive microgels that could be used in diverse applications. To achieve this, N-isopropylacrylamide (NiPAM) based microgels were first synthesized by surfactant-free precipitation polymerization of NiPAM in the presence of poly(ethylene glycol)methacrylate (PEG) as a macro-comonomer and methylenebisacrylamide (MBA) as a chemical crosslinker. By combining a complete set of techniques such as dynamic light scattering (DLS), scanning electron microscopy (SEM), zetametry, 1H NMR and micro-differential scanning calorimetry (µDSC), we clearly demonstrate that (i) the incorporation of the PEG chains controls the size and the polydispersity of the NiPAM-based microgels, whereas the thermal behavior in solution (enthalpy, volume phase transition temperature (VPTT)) remains almost the same as for pure NiPAM microgels; (ii) the PEG chains are mainly located on the microgel periphery; and (iii) the presence of the PEG chains strongly increases the colloidal stability of microgels in electrolyte solutions at high temperatures.

Substituent Effects on the Inclusion of 1-Alkyl-6-alkoxy-quinolinium in 4-Sulfonatocalix[n]arenes.

The effect of the chain length of the alkyl and alkoxy substituents on the binding characteristics of 1-alkyl-6-alkoxy-quinolinium cations was studied using 4-sulfonatocalix[4]arene (SCX4) and 4-sulfonatocalix[6]arene (SCX6) in neutral aqueous solutions at 298 K. Isothermal calorimetric titrations showed enthalpy-controlled inclusion with 1:1 stoichiometry. The equilibrium constants of complexation were always larger for the confinement in SCX4 than in its SCX6 homologue because the better matching between the host and guest sizes allowed more exothermic interaction. The binding affinity diminished with the lengthening of the aliphatic chain of the guests in the case of the association with SCX4, but insignificant change was found for SCX6 complexes. The most substantial change in the enthalpic and entropic contributions to the driving force of complex production occurred when the alkyl chain was linked to the heterocyclic nitrogen and the number of its carbon atoms varied between 1 and 4. 1H NMR spectra evidenced that in SCX6, the 1-alkyl-6-alkoxy-quinolinium cations could be included within the macrocycle cavity. In the case of SCX4, the quinolinium ring is always inside the host, but the alkyl chain is included within SCX4 only for a short chain length (n up to 4). In contrast, the alkoxy chain displays a very weak interaction with the cavity irrespective of the length. Because of the outward orientation from the host, the lengthening of the alkoxy substituent of the quinolinium moiety barely influenced the thermodynamics of inclusion in SCX4. Distinct linear enthalpy-entropy correlations were found for the encapsulation in SCX4 and SCX6.

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.

Cyclodextrin/dextran based hydrogels prepared by cross-linking with sodium trimetaphosphate.

Novel ßCD-based hydrogels have been synthesized using sodium trimetaphosphate (STMP) as non-toxic reagent. Straightforward mixing of ßCD with dextran and STMP in basic aqueous media led to hydrogels incorporating dextran chains, phosphate groups and ßCD units. The hydrogels have been characterized by swelling measurements, XPS and (31)P NMR. The swelling ratio was correlated to the content in phosphated groups, which give a polyelectrolyte character to these hydrogels. The significant rise of the swelling ratio with the ßCD content increase has been attributed to a decrease of the number of phosphate-based crosslinks, the ßCD units playing the role of dangling ends in the tridimensional network. Their loading capacity and their release properties have been investigated for methylene blue and benzophenone in order to demonstrate their potentiality for drug delivery. Through different interaction mechanisms, electrostatic and inclusion complex interactions, these compounds are loaded with different efficiencies. The release involves deswelling, diffusion mechanisms and partition equilibrium.

Effect of torsional isomerization and inclusion complex formation with cucurbit[7]uril on the fluorescence of 6-methoxy-1-methylquinolinium.

Inclusion of 6-methoxy-1-methylquinolinium (C1MQ) in the cavity of cucurbit[7]uril (CB7) was studied by absorption, fluorescence, NMR and isothermal calorimetric methods in aqueous solution at 298 K. The free C1MQ exhibited dual-exponential fluorescence decay kinetics due to the two torsional isomers differing in the orientation of the methoxy moiety relative to the heterocyclic ring. The enthalpy-driven encapsulation of the heterocycle in CB7 led to a very stable 1 : 1 complex with a binding constant of (2.0 ± 0.4) × 10(6) M(-1). The rate of C1MQ-CB7 complex dissociation was found to be comparable to the NMR timescale. Because the methoxy moiety is oriented outward from the host, its s-cis-s-trans isomerization is slightly affected by the confinement. Inclusion complex formation significantly slowed down the photoinduced electron transfer from I(-) and N3(-) to the singlet-excited C1MQ, but did not preclude the reaction because long distance electron transfer occurred through the wall of the CB7 macrocycle. Due to the large difference in the quenching rate constant for free and encapsulated forms, C1MQ is an excellent probe for the study of the inclusion of nonfluorescent compounds in CB7 in the presence of Cl(-) or Br(-).

Some aspects of the orientational order distribution of flexible chains in a diblock mesophase.

The segmental motions of flexible chains in the lamellar structure of a strongly segregated poly(styrene)-poly(dimethylsiloxane) (PS-PDMS) diblock were investigated over a time scale of a few tens of microseconds. (2)H NMR experiments were performed on the PDMS block, selectively perdeuterated. Transverse relaxation measurements show that the main part of the PDMS repeat units display anisotropic reorientational motions within the diblock lamellae and such a segmental ordering essentially results from interchain steric repulsions. (2)H double quantum-based experiments evidenced a non-uniform local stretching of PDMS chains and enabled the underlying distribution of the orientational order parameter to be determined quantitatively. Besides, a fraction of the PDMS chain segments, about 14%, were found to display isotropic - or nearly isotropic - reorientations, which could be assigned to repeat units located within a thin sublayer (about 1-2 nm) at the lamellae midplane, but also deeper in the lamellae, close to folded parts of the chains. These experimental results were confronted to theoretical descriptions of opposing polymer brushes and, in particular, to the strong-stretching theory (SST) including the entropic contribution of free chain ends.

Segmental motions of poly(ethylene glycol) chains adsorbed on Laponite platelets in clay-based hydrogels: a NMR investigation.

The segmental dynamics of poly(ethylene glycol) (PEG) chains adsorbed on the clay platelets within nanocomposite PEG/Laponite hydrogels was investigated over the tens of microseconds time scale, using combined solution and solid-state NMR approaches. In a first step, the time evolution of the molecular mobility displayed by the PEG chains following the addition to a Laponite aqueous dispersion was monitored during the aggregation of the clay disks and the hydrogel formation, by means of (1)H solution-state NMR. Part of the PEG repeat units were found to get strongly constrained during the gelation process. Comparisons between this time evolution of the PEG local dynamics in the PEG/Laponite/water systems and the increase of the macroscopic storage shear modulus, mainly governed by the assembling of the Laponite disks, indicate that the slowing down of the segmental motions arises from adsorbed PEG repeat units or chain portions strongly constrained between aggregated clay layers. In a second step, after completion of the gelation process, the molecular motions of the adsorbed PEG chains were probed by (1)H solid-state NMR spectroscopy. (1)H double-quantum experiments indicate that the adsorbed PEG repeat units, though reported to be frozen over a few tens of nanoseconds, still display significant reorientational motions over the tens of microseconds time scale. Using a comparison with a model system of amorphized PEG chains, the characteristic frequency of these segmental motions was found to range between 78.0 kHz and 100.7 MHz at 300 K. Interestingly, at this temperature, the level of reorientational motions detected for these adsorbed PEG chain portions was found to be as restricted as the one of bulk amorphous PEG chains, cooled at a slightly lower temperature (about 290 K).

Thermal aging of interfacial polymer chains in ethylene-propylene-diene terpolymer/aluminum hydroxide composites: solid-state NMR study.

The possible influence of micrometric-size filler particles on the thermo-oxidative degradation behavior of the polymer chains at polymer/filler interfaces is still an open question. In this study, a cross-linked ethylene-propylene-diene (EPDM) terpolymer filled by aluminum trihydrate (ATH) particles is investigated using (1)H solid-state NMR. The time evolution of the EPDM network microstructure under thermal aging at 80 °C is monitored as a function of the exposure time and compared to that of an unfilled EPDM network displaying a similar initial structure. While nearly no variations of the topology are observed on the neat EPDM network over 5 days at 80 °C, a significant amount of chain scission phenomena are evidenced in EPDM/ATH. A specific surface effect induced by ATH on the thermodegradative properties of the polymer chains located in their vicinity is thus pointed out. Close to the filler particles, a higher amount of chain scissions are detected, and the characteristic length scale related to these interfacial regions displaying a significant thermo-oxidation process is determined as a function of the aging time.

Out of equilibrium dynamics of poly(vinyl methyl ether) segments in miscible poly(styrene)-poly(vinyl methyl ether) blends.

The local dynamics of the low-T(g) component in a polymer blend, dynamically asymmetric poly(styrene)-poly(vinyl methyl ether) (PS-PVME), is studied below the glass transition, via dielectric relaxation spectroscopy. A particular attention has been paid to blends with a high PS content (PS weight fraction higher than 50%). A relaxation process, slower than the localized motions inducing the PVME secondary relaxations, is detected. Even though these blends fall out of equilibrium in this temperature regime, the structural recovery process is not efficient on the time scale of this PVME motional process. This relaxation is attributed to rather localized, weakly cooperative PVME motions resulting from the topological constraints imposed by the frozen PS chains.