Experimental and computational diagnosis of the fluxional nature of a benzene-1,3,5-tricarboxamide-based hydrogen-bonded dimer

Precise characterization of the hydrogen bond network present in discrete self-assemblies of benzene-1,3,5-tricarboxamide monomers derived from amino-esters (ester BTAs) is crucial for the construction of elaborated functional co-assemblies. For all ester BTA dimeric structures previously reported, ester carbonyls in the side chain acted as hydrogen bond acceptors, yielding well-defined dimers stabilized by six hydrogen bonds. The ester BTA monomer derived from glycine (BTA Gly) shows a markedly different self-assembly behaviour. We report herein a combined experimental and computational investigation aimed at determining the nature of the dimeric species formed by BTA Gly. Two distinct dimeric structures were characterized by single-crystal X-ray diffraction measurements. Likewise, a range of spectroscopic and scattering techniques as well as molecular modelling were employed to diagnose the nature of dynamic dimeric structures in toluene. Our results unambiguously establish that both ester and amide carbonyls are involved in the hydrogen bond network of the discrete dimeric species formed by BTA Gly. The participation of roughly 4.5 ester carbonyls and 1.5 amide carbonyls per dimer as determined by FT-IR spectroscopy implies that several conformations coexist in solution. Moreover, NMR analysis and modelling data reveal rapid interconversion between these different conformers leading to a symmetric structure on the NMR timescale. Rapid hydrogen bond shuffling between conformers having three (three), two (four), one (five) and zero (six) amide carbonyl groups (ester carbonyl groups, respectively) as hydrogen bond acceptors is proposed to explain the magnetic equivalence of the amide N-H on the NMR timescale. When compared to other ester BTA derivatives in which only ester carbonyls act as hydrogen bond acceptors, the fluxional behaviour of the hydrogen-bonded dimers of BTA Gly likely originates from a larger range of energetically favorable conformations accessible through rotation of the BTA side chains.

Effect of the Strength of Stickers on Rheology and Adhesion of Supramolecular Center-Functionalized Polyisobutenes

In order to systematically investigate the effect of the strength of the supramolecular interactions on the debonding properties of associative polymers, a series of model systems have been characterized by probe-tack tests. These model materials, composed of linear and low dispersity poly(isobutylene) chains ( M_n ≈ 3 kg/mol) center-functionalized by a single bis-urea sticker, are able to self-assemble by four hydrogen bonds. Three types of stickers are used in the present study: a bis-urea with a methylene diphenyl (MDI) spacer, a bis-urea with a tolyl (TOL) spacer, and a bis-urea with a xylyl (XYL) spacer. In order to investigate the influence of stickers in depth, both the nanostructure of the materials and the linear rheology were investigated by small-angle X-ray scattering (SAXS) and oscillatory shear, respectively. For two types of stickers (TOL and XYL), the association of polymers via hydrogen bonds induces the formation of bundles of rodlike aggregates at room temperature and the behavior of a soft elastic material was observed. For bis-urea MDI, no structure is detected by SAXS and a Newtonian behavior is observed at room temperature. In probe-tack experiments, all these materials show a cohesive mode of failure, a signature of flowing materials as previously observed for tri-urea center-functionalized poly(butylacrylate) (PnBA3U). However, XYL center-functionalized polyisobutene shows much higher debonding energies than PnBA3U, revealing the importance of the strength of noncovalent bonds in the scission/recombination dynamics. On the basis of the analysis of the debonding images, this effect is discussed via the mechanical behavior at large deformation.

Probing halogen-halogen interactions in solution

Halogen-halogen interactions are a particularly interesting class of halogen bonds that are known to be essential design elements in crystal engineering. In solution, it is likely that halogen-halogen interactions also play a role, but the weakness of this interaction makes it difficult to characterize or even simply detect. We have designed a supramolecular balance that allows detecting BrBr interactions between CBr₃ groups in solution and close to room temperature. The sensitivity and versatility of the chosen platform have allowed accumulating consistent data. In halogenoalkane solvents, we propose estimates for the free energy of these weak halogen bond interactions. In toluene solutions, we show that the interactions between Br atoms and the solvent aromatic groups dominate over the BrBr interactions.

Effects of multifunctional cross-linkers on rheology and adhesion of soft nanostructured materials

We investigate the nanostructure, the rheology and the adhesion of soft supramolecular materials elaborated by blending monofunctional and multifunctional poly(isobutene) (PIB) chains. Monofunctional PIB chains (PIBUT) are linear and unentangled polymer chains (M_n ≈ 3 kg mol^-1) functionalized in the middle by a bis-urea interacting moiety, able to self-associate by four hydrogen bonds. Covalent coupling of monofunctional PIB allows us to synthesize longer chains bearing two or three interacting moieties. These chains are then added to monofunctional PIB to prepare blends containing up to 10% of multifunctional PIB (M-PIBUT). The influence of M-PIBUT on the supramolecular nanostructure, which results from the self-assembly of stickers, is studied by Atomic Force Microscopy and Small Angle X-ray Scattering at room temperature. Multifunctional and monofunctional chains are shown to interact with each other to form bundles of rod-like aggregates. The consequences of these interactions on the rheology of the blends were studied by shear tests in the linear and non linear regimes, below and above the order-disorder transition temperature. A pronounced strengthening effect of M-PIBUT is observed at room temperature: the supramolecular blends become more elastic and are more resistant to creep with increasing concentration of M-PIBUT. The effects of M-PIBUT on the nanostructure and the rheology suggest that M-PIBUT, which can link with more than one supramolecular aggregate, plays the role of a physical cross-linker. The impact of these supramolecular cross-linkers on the adhesion of the blends is studied by probe-tack tests and discussed by analyzing the in situ deformation through the debonding images.

Adhesion and non-linear rheology of adhesives with supramolecular crosslinking points

Soft supramolecular materials are promising for the design of innovative and highly tunable adhesives. These materials are composed of polymer chains functionalized by strongly interacting moieties, sometimes called "stickers". In order to systematically investigate the effect of the presence of associative groups on the debonding properties of a supramolecular adhesive, a series of supramolecular model systems has been characterized by probe-tack tests. These model materials, composed of linear and low dispersity poly(butylacrylate) chains functionalized in the middle by a single tri-urea sticker, are able to self-associate by six hydrogen bonds and range in molecular weight (Mn) between 5 and 85 kg mol(-1). The linear rheology and the nanostructure of the same materials (called "PnBA3U") were the object of a previous study. At room temperature, the association of polymers via hydrogen bonds induces the formation of rod-like aggregates structured into bundles for Mn < 40 kg mol(-1) and the behavior of a soft elastic material was observed (G'≪G'' and G'∼ω(0)). For higher Mn materials, the filaments were randomly oriented and the polymers displayed a crossover towards viscous behavior although terminal relaxation was not reached in the experimental frequency window. All these materials show, however, similar adhesive properties characterized by a cohesive mode of failure and low debonding energies (Wadh < 40 J m(-2) for a debonding speed of 100 μm s(-1)). The debonding mechanisms observed during the adhesion tests have been investigated in detail with an Image tools analysis developed by our group. The measure of the projected area covered by cavities growing in the adhesive layer during debonding can be used to estimate the true stress in the walls of the cavities and thus to characterize the in situ large strain deformation of the thin layer during the adhesion test itself. This analysis revealed in particular that the PnBA3U materials with Mn < 40 kg mol(-1) soften very markedly at large deformation like yield stress fluids, explaining the low adhesion energies measured for these viscoelastic gels.

Rate of permeabilization of giant vesicles by amphiphilic polyacrylates compared to the adsorption of these polymers onto large vesicles and tethered lipid bilayers

We examined by fluorescence microscopy the permeabilization of giant vesicles by hydrophobically modified polyacrylates (called amphipols). Amphipols trigger permeabilization to FITC-dextran of egg-PC/DPPA vesicles with no breakage of the lipid bilayers. The polyanionic amphipols were passing through bilayers as shown by permeabilization of multilamellar vesicles. Remarkably, the vesicles were not simultaneously permeable but became leaky one after the other. Altogether, our observations suggest a random formation of pores having diameters above a few nanometers. Decreasing pH and increasing ionic strength and polymer concentration were increasing the rate of permeabilization. The rate and efficiency of permeabilization was compared to the rate and density of adsorption of amphipols onto lipid membranes (as estimated by titration calorimetry onto large unilamellar vesicles and neutron reflectivity measurements on tethered bilayers). The polymer adsorption layer is built up in a few minutes. We conclude that the rate-limiting step for permeabilization is not the adsorption from the bulk solution but relates to slow intramembrane reorganizations.

Cooperative rearrangements leading to long range order in monolayers of supramolecular polymers

Using scanning tunneling microscopy (STM), we followed the self-organization process of a supramolecular polymer monolayer deposited on a gold surface. During the growth of ordered domains from small to large scales, the molecule-molecule interactions were found to overrule the coupling to the substrate, causing a reorientation of the monolayer. The flexibility at the molecular level, due to reversible hydrogen bonds, was directly visualized by STM. The supramolecules were able to slide and insert between neighboring molecules, allowing the annihilation of domain boundaries and improving long range order. Large domains were found to cross monoatomic steps on the substrate without perturbation of their order.

Evolution of multilevel order in supramolecular assemblies

The process of self-assembly at multiple length scales of bis-urea substituted toluene on a Au(111) surface was studied by low temperature scanning tunneling microscopy. Pattern formation is controlled by specific hydrogen bonds between these molecules but also by significantly weaker lateral coupling between the resulting supramolecular polymers and a quasiepitaxial interlocking with the substrate. The ordered assemblies exhibit a tunnel transparency. Our experiments indicate the necessity of multiple interactions of different strengths for obtaining ordered structures with hierarchical levels of organization.

Brownian particles in supramolecular polymer solutions

The Brownian motion of colloidal particles embedded in solutions of hydrogen-bonded supramolecular polymers has been studied using dynamic light scattering. At short times, the motion of the probe particles is diffusive with a diffusion coefficient equal to that in pure solvent. At intermediate time scales the particles are slowed down as a result of trapping in elastic cages formed by the polymer chains, while at longer times the motion is diffusive again, but with a much smaller diffusion coefficient. The influence of particle size and polymer concentration was investigated. The experimental data are compared to a theoretical expression for the mean-square displacement of an embedded particle in a viscoelastic medium, in which the solvent is explicitly taken into account. Differences between the friction and elastic forces experienced by the particle and the macroscopic viscosity and elasticity are explained by the inhomogeneity of the medium on the length scale of the particle size.