J. van der Gucht

Stuffed brushes: theory and experiment*

The interaction between polymer brushes and mesoscopic particles is investigated both theoretically and experimentally. We present an analytical mean-field theory for a polymer brush (a layer of long polymer chains end-grafted to a substrate) with varying excluded volume interactions between monomer units. This system mimics the reversible adsorption of mesoscopic particles, such as surfactant micelles or proteins, on the grafted chains. The equilibrium structural properties of the brush (the brush thickness and overall degree of complexation) as well as the number of adsorbed particles per unit area, G, are analysed as functions of the affinity between particle and chain, grafting density s and excluded volume interactions. In our model G is found to have a maximum as a function of s. Experimentally the adsorption of BSA on a hydrophobic substrate with grafted PEO chains is measured with reflectometry. In the case of short grafted chains the adsorbed amount of BSA, G, decreases continuously with increasing s, which agrees with previous results and model calculations in the literature. In the case of long PEO chains, however, G is found to have a maximum as a function of s. Qualitatively the experimental dependence of G on s is found to agree with the results of our mean-field model. PEO chains show no affinity for BSA in the bulk, whereas in a grafted conformation an effective attraction is found. Some comments are made on the nature of this affinity, which is not yet fully understood.

On the stability and morphology of complex coacervate core micelles : from spherical to wormlike micelles

We present a systematic study of the stability and morphology of complex coacervate core micelles (C3Ms) formed from poly(acrylic acid) (PAA) and poly(N-methyl-2-vinylpyridinium)-b-poly(ethylene oxide) (PM2VP-b-PEO). We use polarized and depolarized dynamic and static light scattering, combined with small-angle X-ray scattering, to investigate how the polymer chain length and salt concentration affect the stability, size, and shape of these micelles. We show that C3Ms are formed in aqueous solution below a critical salt concentration, which increases considerably with increasing PAA and PM2VP length and levels off for long chains. This trend is in good agreement with a mean-field model of polyelectrolyte complexation based on the Voorn-Overbeek theory. In addition, we find that salt induces morphological changes in C3Ms when the PAA homopolymer is sufficiently short: from spherical micelles with a diameter of several tens of nanometers at low salt concentration to wormlike micelles with a contour length of several hundreds of nanometers just before the critical salt concentration. By contrast, C3Ms of long PAA homopolymers remain spherical upon addition of salt and shrink slightly. A critical review of existing literature on other C3Ms reveals that the transition from spherical to wormlike micelles is probably a general phenomenon, which can be rationalized in terms of a classical packing parameter for amphiphiles.

Controlled mixing of lanthanide(III) ions in coacervate

This article presents a facile strategy to combine Eu(3+) and Gd(3+) ions into coacervate core micelles in a controlled way with a statistical distribution of the ions. Consequently, the formed micelles show a high tunability between luminescence and relaxivity. These highly stable micelles present great potential for new materials, e.g. as bimodal imaging probes.

Coalescence, Cracking, and Crack Healing in Drying Dispersion Droplets

The formation of a uniform film from a polymer dispersion is a complex phenomenon involving the interplay of many processes: evaporation and resulting fluid flows through confined geometries, particle packing and deformation, coalescence, and cracking. Understanding this multidimensional problem has proven challenging, precluding a clear understanding of film formation to date. This is especially true for drying dispersion droplets, where the particular geometry introduces additional complexity such as lateral flow toward the droplet periphery. We study the drying of these droplets using a simplified approach in which we systematically vary a single parameter: the glass transition temperature (Tg) of the polymer. We combine optical with scanning electron microscopy to elucidate these processes from the macroscopic down to the single-particle level, both qualitatively and quantitatively, over times ranging from seconds to days. Our results indicate that the polymer Tg has a marked influence on the time evolution of particle deformation and coalescence, giving rise to a distinct and sudden cracking transition. Moreover, in cracked droplets it affects the frequently overlooked time scale of crack healing, giving rise to a second transition from self-healing to permanently cracked droplets. These findings are in line with the classical Routh-Russel model for film formation yet extend its scope from particle-level dynamics to long-range polymer flow.

Coil size oscillatory packing in polymer solutions near a surface

The theory developed by Scheutjens and Fleer to describe polymer adsorption and depletion is used to calculate the density profile of nonadsorbing polymers near a surface. The theory predicts damped oscillations in the segment density profile with a wavelength of about the coil size. As a consequence, the interaction energy between two surfaces immersed in a solution of nonadsorbing polymers is an oscillatory function of the separation distance, too. The decay length of the oscillations is proportional to the coil size and independent of the polymer concentration. The oscillations are associated with a liquid-like layering of polymer coils near the surface. An increase in concentration or chain length causes a decrease in the amplitude of the oscillations, because the stronger interpenetration of the coils suppresses inhomogeneities. In dilute solutions no oscillations are observed, because the decay length of the oscillations is smaller than the depletion correlation length, in analogy with the Fisher–Wi...

Physical Gels Based on Charge-Driven Bridging of Nanoparticles by Triblock Copolymers

We have prepared an aqueous physical gel consisting of negatively charged silica nanoparticles bridged by ABA triblock copolymers, in which the A blocks are positively charged and the B block is neutral and water-soluble. Irreversible aggregation of the silica nanoparticles was prevented by precoating them with a neutral hydrophilic polymer. Both the elastic plateau modulus and the relaxation time increase slowly as the gel ages, indicating an increase both in the number of active bridges and in the strength with which the end blocks are adsorbed. The rate of this aging process can be increased significantly by applying a small shear stress to the sample. Our results indicate that charge-driven bridging of nanoparticles by triblock copolymers is a promising strategy for thickening of aqueous particle containing materials, such as water-based coatings.

Multi-responsive physical gels formed by a biosynthetic asymmetric triblock protein polymer and a polyanion

We report the design, production and characterization of a biosynthetic asymmetric triblock copolymer which consists of one collagen-like and one cationic block spaced by a hydrophilic random coiled block. The polymer associates into micelles when a polyanion is added due to the electrostatic interaction between the cationic block and the polyanion. The collagen-like block self-assembles into thermo-responsive triple helices upon cooling. When both end blocks are induced to self-assemble, a physical gel is formed via thermo-responsive association of the charge-driven micelles. The self-assembly of both end blocks and the effects of salt and temperature thereon were characterized by light scattering and rheology.

Synergistic stiffening in double-fiber networks.

Many biological materials are composite structures, interpenetrating networks of different types of fibers. The composite nature of such networks leads to superior mechanical properties, but the origin of this mechanical synergism is still poorly understood. Here we study soft composite networks, made by mixing two self-assembling fiber-forming components. We find that the elastic moduli of the composite networks significantly exceed the sum of the moduli of the two individual networks. This mechanical enhancement is in agreement with recent simulations, where it was attributed to a suppression of non-affine deformation modes in the most rigid fiber network due to the reaction forces in the softer network. The increase in affinity also causes a loss of strain hardening and an increase in the critical stress and stain at which the network fails.

Coarse-grained simulations for flow of complex soft matter fluids in the bulk and in the presence of solid interfaces

We present a coarse-grained particle-based simulation technique for modeling flow of complex soft matter fluids such as polymer solutions in the presence of solid interfaces. In our coarse-grained description of the system, we track the motion of polymer molecules using their centers-of-mass as our coarse-grain co-ordinates and also keep track of another set of variables that describe the background flow field. The coarse-grain motion is thus influenced not only by the interactions based on appropriate potentials used to model the particular polymer system of interest and the random kicks associated with thermal fluctuations, but also by the motion of the background fluid. In order to couple the motion of the coarse-grain co-ordinates with the background fluid motion, we use a Galilean invariant, first order Brownian dynamics algorithm developed by Padding and Briels [J. Chem. Phys. 141, 244108 (2014)], which on the one hand draws inspiration from smoothed particle hydrodynamics in a way that the motion of the background fluid is efficiently calculated based on a discretization of the Navier-Stokes equation at the positions of the coarse-grain coordinates where it is actually needed, but also differs from it because of the inclusion of thermal fluctuations by having momentum-conserving pairwise stochastic updates. In this paper, we make a few modifications to this algorithm and introduce a new parameter, viz., a friction coefficient associated with the background fluid, and analyze the relationship of the model parameters with the dynamic properties of the system. We also test this algorithm for flow in the presence of solid interfaces to show that appropriate boundary conditions can be imposed at solid-fluid interfaces by using artificial particles embedded in the solid walls which offer friction to the real fluid particles in the vicinity of the wall. We have tested our method using a model system of a star polymer solution at the overlap concentration.

Interactions between surfaces in the presence of nonadsorbing equilibrium polymers

The behaviour of a solution of equilibrium polymers (or living polymers) between two surfaces is studied using a Bethe–Guggenheim lattice model for molecules with orientation-dependent interactions. The average monomer concentration, the average length of the chains and the interaction between the surfaces are calculated as a function of the separation distance between the surfaces. When the gap is in full equilibrium with a homogeneous bulk solution, the equilibrium polymers cause a depletion attraction, which becomes stronger with increasing bulk monomer concentration. The range of the interaction passes through a maximum as a function of the concentration. In dilute solutions the range of the interaction increases and the strength decreases with increasing bonding energy, while above the overlap concentration the bonding energy is irrelevant. For restricted equilibrium between the gap and the bulk, when the amount of polymer in the gap is determined by the flow of fluid out of the gap upon compression, the interaction becomes repulsive. This repulsion becomes stronger with increasing concentration and depends only very weakly on the bonding energy. Two limiting cases for the fluid flow were considered: (i) perfect-slip conditions at the surfaces, resulting in a constant monomer concentration in the gap and (ii) no-slip conditions at the surfaces, resulting in a parabolic flow profile of solution out of the gap.