N.A.M. Besseling

Dynamics of reversible supramolecular polymers : Independent determination of the dependence of linear viscoelasticity on concentration and chain length by using chain stoppers

The linear viscoelasticity of solutions of a hydrogen bonded reversible supramolecular polymer in the presence of a chain stopper was studied by rheometry and by dynamic light scattering using probe particles. The use of chain stoppers enabled the independent variation of the degree of polymerisation and the monomer concentration, and the effect of both parameters on rheology was investigated. Scaling exponents were obtained for the chain length and concentration dependence of the zero-shear viscosity and the terminal relaxation time, and these were compared to theoretical values. The results indicate that the reversible supramolecular polymer is semiflexible, and that both breaking and reptation of chains contribute to the stress relaxation. The parameters from macroscopic rheometry were compared to microscopic values obtained from probe particle diffusion. The particles probe the macroscopic viscoelastic parameters if their size is large compared to the correlation length in the system and to the (persistence) length of the polymer chains.

Rheology of a reversible supramolecular polymer studied by comparison of the effects of temperature and chain stoppers

The rheology of a reversible supramolecular polymer is studied by comparing the effects of an increase in temperature and the addition of chain stoppers. The dependence of the zero-shear viscosity and the terminal relaxation time on temperature is exponential, and the activation energy for viscous flow can be calculated. Above a critical stopper fraction, power laws describe the stopper dependence of the viscosity and relaxation time. A simple model for the effect of the addition of chain stoppers on the average degree of polymerization adequately describes the results. A comparison of flow curves at several temperatures and stopper fractions reveals considerable differences between solutions with the same zero-shear viscosity. These are mainly associated with differences in the terminal relaxation time. A mechanism of shear-induced alignment and subsequent elongation of chains is proposed, with which the experimental results are consistent.

Redox responsive molecular assemblies based on metallic coordination polymers

Redox responsive molecular self-assemblies are very important in a wide range of applications including bioengineering, nanotechnology, and medicine. We demonstrate here a redox responsive micellar system based on metal–ligand coordination and electrostatic interaction. The micelles are spontaneously formed in the mixed solutions of a block polyelectrolyte and an oppositely charged iron–bisligand coordination polyelectrolyte, where both Fe2+ and Fe3+ ions are applicable. Switching between these two oxidation states does not trigger any decomposition of the micelles, but changes the charge composition and the magnetic properties of the system, which enables the uptake of extra, oppositely charged species into the micelles. Moreover, this process triggers spectral and morphological changes of the micelles. These results open a new vista for making redox responsive smart molecular assemblies.

Self-consistent field theory for the nucleation of micelles

We report the first molecular model for the transition states (critical nuclei) that determine the rate of micelle formation and breakdown. These processes, and homogeneous nucleation of macroscopic phase separation are handled in a unified way. It turns out that with micelle-forming amphiphiles several new features appear that do not occur with ordinary phase separation. We predict that the activation free-energy barrier for micelle formation is maximal but finite at the critical micelle concentration (CMC) and decreases with concentration. The barrier for micelle breakdown vanishes at the CMC and increases with concentration. Furthermore, our results indicate formation of transient large aggregates upon quenches far beyond the CMC.

Promoted formation of coordination polyeletrolytes by layer-by-layer assembly

Metal mediated coordination polymers offer the opportunity to fabricate devices and materials that are equally important for fundamental research and new technologies. Most coordination polymers were synthesized at the desired stoichiometry. We demonstrate that coordination polyelectrolytes can form in layer-by-layer (LBL) assembled films regardless of the initial metal to bisligand ratio in solution. Upon dipping a substrate alternately into a covalent polylectrolyte solution and then into the mixed solution of Fe3+ and bisligands, the Fe3+ to ligand ratio in the LBL film tends to be 1:1 and forms polymeric structures even if the ratios in the bulk solution strongly deviate from unity. Our results suggest that layer-by-layer assembly promotes the formation of coordination polyelectrotyes.

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...

Depletion interaction measured by colloidal probe atomic force microscopy

We investigated the depletion interaction between stearylated silica surfaces in cyclohexane in the presence of dissolved polydimethylsiloxane by means of colloidal probe atomic force microscopy. We found that the range of the depletion interaction decreases with increasing concentration. Furthermore the depletion interaction in this system is much weaker than predicted by theories assuming a hard-wall type interaction between polymer segments and surface. We conclude that the interaction between the polymer segments and the surface is not zero, which weakens the depletion interaction.

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.

On the curvature dependence of the interfacial tension in a symmetric three-component interface

We consider a symmetric interface between two polymers A(N) and B(N) in a common monomeric solvent S using the mean-field Scheutjens-Fleer self-consistent field theory and focus on the curvature dependence of the interfacial tension. In multi-component systems there is not one unique scenario to curve such an interface. We elaborate on this by keeping either the chemical potential of the solvent or the bulk concentration of the solvent fixed, that is we focus on the semi-grand canonical ensemble case. Following Helfrich, we expand the surface tension as a Taylor series in the curvature parameters and find that there is a non-zero linear dependence of the interfacial tension on the mean curvature in both cases. This implies a finite Tolman length. In a thermodynamic analysis we prove that the non-zero Tolman length is related to the adsorption of solvent at the interface. Similar, but not the same, correlations between the solvent adsorption and the Tolman length are found in the two scenarios. This result indicates that one should be careful with symmetry arguments in a Helfrich analysis, in particular for systems that have a finite interfacial tension: one not only should consider the structural symmetry of the interface, but also consider the constraints that are enforced upon imposing the curvature. The volume fraction of solvent, the chain length N as well as the interaction parameter chi(AB) in the system can be used to take the system in the direction of the critical point. The usual critical behavior is found. Both the width of the interface and the Tolman length diverge, whereas the density difference between the two phases, adsorbed amount of solvent at the interface, interfacial tension, spontaneous curvature, mean bending modulus as well as the Gaussian bending modulus vanish upon approach of the critical point.