Eduardo Mendes

Responsive biomimetic networks from polyisocyanopeptide hydrogels

Mechanical responsiveness is essential to all biological systems down to the level of tissues and cells. The intra- and extracellular mechanics of such systems are governed by a series of proteins, such as microtubules, actin, intermediate filaments and collagen. As a general design motif, these proteins self-assemble into helical structures and superstructures that differ in diameter and persistence length to cover the full mechanical spectrum. Gels of cytoskeletal proteins display particular mechanical responses (stress stiffening) that until now have been absent in synthetic polymeric and low-molar-mass gels. Here we present synthetic gels that mimic in nearly all aspects gels prepared from intermediate filaments. They are prepared from polyisocyanopeptides grafted with oligo(ethylene glycol) side chains. These responsive polymers possess a stiff and helical architecture, and show a tunable thermal transition where the chains bundle together to generate transparent gels at extremely low concentrations. Using characterization techniques operating at different length scales (for example, macroscopic rheology, atomic force microscopy and molecular force spectroscopy) combined with an appropriate theoretical network model, we establish the hierarchical relationship between the bulk mechanical properties and the single-molecule parameters. Our results show that to develop artificial cytoskeletal or extracellular matrix mimics, the essential design parameters are not only the molecular stiffness, but also the extent of bundling. In contrast to the peptidic materials, our polyisocyanide polymers are readily modified, giving a starting point for functional biomimetic hydrogels with potentially a wide variety of applications, in particular in the biomedical field.

Catalytic control over supramolecular gel formation

Low-molecular-weight gels show great potential for application in fields ranging from the petrochemical industry to healthcare and tissue engineering. These supramolecular gels are often metastable materials, which implies that their properties are, at least partially, kinetically controlled. Here we show how the mechanical properties and structure of these materials can be controlled directly by catalytic action. We show how in situ catalysis of the formation of gelator molecules can be used to accelerate the formation of supramolecular hydrogels, which drastically enhances their resulting mechanical properties. Using acid or nucleophilic aniline catalysis, it is possible to make supramolecular hydrogels with tunable gel-strength in a matter of minutes, under ambient conditions, starting from simple soluble building blocks. By changing the rate of formation of the gelator molecules using a catalyst, the overall rate of gelation and the resulting gel morphology are affected, which provides access to metastable gel states with improved mechanical strength and appearance despite an identical gelator composition.

Enhanced hardening of soft self-assembled copolymer gels under homogeneous magnetic fields

The rheological response of highly swollen physical gels obtained by self-assembling of triblock copolymers containing low remanence ferromagnetic particles was investigated in the presence of external homogeneous magnetic fields. Three different types of sample geometries with distinctive magnetic particle orderings were investigated: isotropic (no magnetic field present during synthesis), parallel to the plane of the gel film and perpendicular to the plane of the gel film. Both the storage and loss moduli exhibit a strong increase with magnetic field strength for all geometries. Dependence of the rheological response on particle volume fraction was also investigated. The strength of such rheological hardening, as well as its saturation behaviour, depend strongly on the relative orientation between particle strings, shear and external field. In some cases a very strong relative increase of storage modulus, up to 6000% was obtained. Further transient rheological studies suggest that strong rearrangement of the particle network is largely responsible for the enormous increase in elastic modulus. Parallel to that, a maximum in the loss factor was observed as a function of particle volume fraction and field strength and it was interpreted in terms of a competition between an increase in string (clusters) hardening and a decrease in their ability to deform and flow. These results suggest that magnetorheological gels are an intermediate system between magnetorheological elastomers (MREs) and magnetorheological fluids (MRFs) with directional dependent rheological response and partial rearrangement of the particle network.

Non-equilibrium dynamics of block copolymer micelles in solution: recent insights and open questions

The increasing variety and complexity of block copolymers leads to a large morphological diversity of self-assembled structures. This, in turn, opens the way for numerous new applications in fields spanning from drug delivery and templated synthesis of nanoporous materials to oil extraction. However, unlike low-molecular weight surfactants, the dynamics of block copolymer systems can be very slow, which has important consequences for carrying out reliable experiments and for their use in practical applications. This paper highlights the mechanism of non-equilibrium block copolymer dynamics, and discusses micellization and transition kinetics between spherical and rod-like micelles by examining the present knowledge and posing some open questions.

Effects of Salts and Ethanol on the Population and Morphology of Triblock Copolymer Micelles in Solution

The morphological changes of micelles composed of triblock copolymer of ethylene oxide and propylene oxide (EO20PO70EO20) in the presence of different inorganic salts and ethanol have been investigated using dynamic light scattering (DLS), rheometry, and cryogenic transmission electron microscopy (cryo-EM). The following salts were studied: KF, KCl, KI, LiCl, and CsCl. In the presence of KF, KCl, and CsCl, spherical and wormlike micelles coexist. LiCl and KI have little influence on the morphology of the micelles, whereas KF has the most pronounced effect. In agreement with the well-known Hoffmeister anion salt series, F- has the strongest effect of the three anions studied (F-, Cl-, I-). In contrast, the effectiveness of the cation type does not follow the original Hoffmeister cation series. The addition of ethanol to the KCl micellar solutions leads to the formation of more or longer wormlike micelles, which start to interact at certain copolymer concentrations depending on the volume fraction of ethanol added. Both the dilute and the semidilute regimes of the wormlike micelles were studied. The length of the micelles reaches a maximum value at around 8-10 vol % ethanol, after which it decreases again. At higher ethanol concentrations (18 vol %), spherical micelles are formed. Conclusions from this study enhance our understanding of the role played by ethanol and salts in the formation of micelle-templated mesoporous materials, such as SBA-15.

Scattering from solutions of star polymers

Abstract:We study the scattering intensity of dilute and semi-dilute solutions of star polymers. The star conformation is described by a model introduced by Daoud and Cotton. In this model, a single star is regarded as a spherical region of a semi-dilute polymer solution with a local, position dependent screening length. For high enough concentrations, the outer sections of the arms overlap and build a semi-dilute solution (a sea of blobs) where the inner parts of the actual stars are embedded. The scattering function is evaluated following a method introduced by Auvray and de Gennes. In the dilute regime there are three regions in the scattering function: the Guinier region (low wave vectors,

Spatial and directional control over self-assembly using catalytic micropatterned surfaces

qR \ll 1

Complex morphologies of self-assembled block copolymer micelles in binary solvent mixtures: the role of solvent–solvent correlations

) from where the radius of the star can be extracted; the intermediate region (