F.A.M. Leermakers

Self-Assembled Structures of Amphiphilic Ionic Block Copolymers: Theory, Self-Consistent Field Modeling and Experiment

We present an overview of statistical thermodynamic theories that describe the self-assembly of amphiphilic ionic/hydrophobic diblock copolymers in dilute solution. Block copolymers with both strongly and weakly dissociating (pH-sensitive) ionic blocks are considered. We focus mostly on structural and morphological transitions that occur in self-assembled aggregates as a response to varied environmental conditions (ionic strength and pH in the solution). Analytical theory is complemented by a numerical self-consistent field approach. Theoret- ical predictions are compared to selected experimental data on micellization of ionic/hydrophobic diblock copolymers in aqueous solutions.

Self-consistent-field modelling of casein adsorption: comparison of results for ?s1-casein and ?-casein.

The theoretical adsorption behaviour of the milk proteins, α s1 - and β-casein, has been studied using a self-consistent-field (SCF) model. Previously published results for β-casein on the effects of ionic strength and pH on protein conformation are compared with those for α s1 -casein. We find a lower adsorbed amount for α s1 -casein, and a more complex adsorbed conformation because of its more heterogeneous primary structure. The predominant conformation appears to involve a substantial loop for α s1 -casein, producing a thinner adsorbed layer than is predicted for β-casein, which has, predominantly, a long tail extending away from the surface into the aqueous region. The overall layer structure for both proteins is shown to consist of a combination of many coexisting protein conformations. The relative proportion of the different conformations controls the overall layer properties and their variation with pH and ionic strength.

Adsorption of Molecular Brushes with Polyelectrolyte Backbones onto Oppositely Charged Surfaces : A Self-Consistent Field Theory

The two-gradient version of the Scheutjens-Fleer self-consistent field (SF-SCF) theory is employed to model the interaction between a molecular bottle brush with a polyelectrolyte backbone and neutral hydrophilic side chains and an oppositely charged surface. Our system mimics graft-copolymers with a cationic main chain (among which poly( L-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG) or poly( L-lysine)- graft-polyoxazoline are well-known examples) interacting with negatively charged (metal oxide) solid surfaces. We aim to analyze the copolymer-surface interaction patterns as a function of the molecular architecture parameters. Two regimes are investigated: First, we compute the effective interaction potential versus the distance from the surface for individual bottle brush macromolecules. Here, depending on the grafting ratio and the degree of polymerization of the side chains, the interplay of electrostatic attractions of the main chain to the surface and the steric repulsion of the grafts results in different patterns in the interaction potential and, therefore, in qualitatively different adsorption behavior. In particular, we demonstrate that, at high side chain grafting density and short grafts, the molecular brushes are strongly adsorbed electrostatically onto negatively charged substrates, whereas, in the opposite case of low grafting ratio and high molecular weight of grafts, the steric repulsion of the side chains from the surface dominates the polymer-surface interaction. At intermediate grafting ratios, the adsorption/depletion scenario depends essentially on the ratio between the electrostatic screening length and the thickness of the molecular bottle brush. We further have analyzed the equilibrium adsorbed amount as a function of the macromolecular architecture. Our results are based on a detailed account of attractive and repulsive (including intermolecular in-plane) interactions, and suggest a nonmonotonic dependence of the adsorbed amount on the grafting ratio, in good agreement with the experimental studies for PLL- g-PEG adsorption onto Nb2O5 surfaces. The results of the theoretical modeling are discussed in the context of the optimization of the PLL-g-PEG molecular design parameters in order to create protein-resistant surfaces.

Statistical thermodynamics of association colloids: V. critical micelle concentration, micellar size and shape

Abstract Recently, we generalized the lattice theory of Scheutjens and Fleer (1) for chain molecules in inhomogeneous systems to amphiphilic molecules in systems of nonplanar geometries (2–4). This theory is used here to study surfactant micelle systems. In this paper it is shown that the critical micelle concentration for surfactant micelles is theoretically well defined. The fact that in most experimental systems the transition is not clearly observable is explained. The relation between chain architecture and overall surfactant concentration on the size and shape of surfactant micelles is studied. Segment density profiles through cross sections of spherical micelles are presented and are in accord with recent Molecular Dynamics and Monte Carlo simulations. However, long range interactions seem not to be essential for the stabilization of micelles if the interactions between head groups and tails are sufficiently unfavorable.

Adhesion and friction properties of polymer brushes: fluoro versus nonfluoro polymer brushes at varying thickness.

A series of different thicknesses of fluoro poly(2,2,2-trifluoroethyl methacrylate) and its analogous nonfluoro poly(ethyl methacrylate) polymer brushes were prepared via surface-initiated ATRP (SI-ATRP) on Si(111) surfaces. The thiol-yne click reaction was used to immobilize the SI-ATRP initiator with a high surface coverage, in order to achieve denser polymer brushes (grafting density from ~0.1 to 0.8 chains/nm(2)). All polymer brushes were characterized by static water contact angle measurements, infrared absorption reflection spectroscopy, and X-ray photoelectron spectroscopy. Adhesion and friction force measurements were conducted with silica colloidal probe atomic force microscopy (CP-AFM) under ambient and dry (argon) conditions. The fluoro poly(2,2,2-trifluoroethyl methacrylate) polymer showed a decrease in adhesion and friction with increasing thickness. The analogous nonfluoro poly(ethyl methacrylate) polymer brushes showed high adhesion and friction under ambient conditions. Friction coefficients down to 0.0057 (ambient conditions) and 0.0031 (dry argon) were obtained for poly(2,2,2-trifluoroethyl methacrylate) polymer brushes with 140 nm thickness, which are the lowest among these types of polymer brushes.

Self-consistent-field modeling of complex molecules with united atom detail in inhomogeneous systems. Cyclic and branched foreign molecules in dimyristoylphosphatidylcholine membranes

We have developed a detailed self-consistent-field model for studying complex molecules in inhomogeneous systems, in which all the molecules are represented in a detailed united atom description. The theory is in the spirit of the approach developed by Scheutjens and co-workers for polymers at interfaces and self-assembly of surfactants and lipids into association colloids. It is applied to lipid membranes composed of dimyristoylphosphatidylcholine (DMPC). In particular, we looked at the incorporation of linear, branched, and cyclic molecules into the lipid bilayers being in the liquid phase. Detailed information on the properties of both the lipids and the additives is presented. For the classes of linear and branched alcohols and phenol derivatives we find good correspondence between calculated partition coefficients for DMPC membranes and experimental data on egg-yolk PC. The calculated partitioning of molecules of isomers, containing a benzene ring, two charged groups (one positive and one negative) and 16 hydrocarbon segments, into DMPC membranes showed variations of the partition coefficient by a factor of 10 depending on the molecular architecture. For zwitterionic additives we find that it is much more difficult to bring the positive charge into the membrane core than the negative one. This result can be rationalized from information on the electrostatic potential profile of the bare membrane, being positive in both the core and on the membrane surface but negative near the position of the phosphate groups. For several tetrahydroxy naftalenes we found that, although the partition coefficient is barely influenced, the average orientation and position of the molecule inside the membrane is strongly dependent on the distribution of the hydroxyl groups on the naphthalene rings. The orientation changes from one where the additive spans the membrane when the hydroxyls are positioned on (2,3,6,7) positions, to an orientation with the rings parallel to the membrane surface and located near the head group–hydrophobic core interface for the hydroxyls at the (1,3,5,7) positions. We propose that, when our model is used in combination with octanol/water partitioning data, a very accurate prediction is possible of the affinity of complex molecules for lipid membranes.

Chain stiffness and bond correlations in polymer brushes

We have incorporated chain stiffness and correlations between neighboring bonds into a self‐consistent field (SCF) lattice model for end‐attached polymer layers (commonly known as ‘‘brushes’’). An increase in the chain stiffness leads to an increasing brush height. This increase is directly related to the change of the length of a Kuhn segment in the polymer chain. Introducing correlations between neighboring bonds gives a higher density of the brush, corresponding to a decrease of the brush height. For not too stiff chains these two effects virtually compensate each other. Hence, the volume fraction profile of ‘‘real’’ grafted chains is nearly identical to that of a polymer brush consisting of freely jointed chains.

Pickering emulsions : wetting and colloidal stability of hairy particles - a self-consistent field theory

The assembly of sterically stabilized colloids at liquid-liquid interfaces is studied with the self-consistent field (SCF) theory using the discretization scheme that was developed by Scheutjens, Fleer, and co-workers. The model is based on a poly(methyl methacrylate) (pMMA) particle with poly(isobutylene) (pIB) grafted to the surface. The stabilizing groups on the particle surface have a significant effect on the interfacial assembly and, therefore, also on the formation and properties of Pickering emulsions. The wetting behavior of the particle is altered by the presence of the stabilizing groups, which affects the equilibrium position of the particles at the interface. The stabilizing groups can also lead to an activation barrier before interfacial adsorption, analogous to the steric repulsion between two particles. These effects are numerically solved with the SCF theory. It is commonly known that flocculating conditions enhance the interfacial adsorption and yield stable Pickering emulsions, which is confirmed in this work. Additionally, it is concluded that those conditions are not an absolute requirement. There is a window of stabilizer concentrations Γ(pIB), 2.2-3.3 mg/m(2) pIB, that shows both partial wetting and colloidal stability. The activation barrier for interfacial assembly is 140-550 k(B)T and is an order of magnitude higher than the colloidal stability. The difference can be attributed to the unfavorable interaction of pIB with water and a difference in geometry (plate-sphere vs sphere-sphere). This study demonstrates the interplay and provides a quantitative comparison between the wetting behavior and the colloidal stability, and it gives a better understanding of the colloidal assembly at soft interfaces and formation of Pickering emulsions in general.

Counterion localization in solutions of starlike polyelectrolytes and colloidal polyelectrolyte brushes: a self-consistent field theory.

A quantitative analysis of the distribution of counterions in salt-free solutions of colloidal polyelectrolyte brushes and starlike polyelectrolytes is performed on the level of the Poisson-Boltzmann approximation. Exact numerical solutions are obtained for starlike polyelectrolyte molecules composed of f = 20, . . ., 50 arms with a fixed fractional charge alpha per segment by applying the self-consistent field method of Scheutjens and Fleer (SF-SCF). The Wigner-Seitz cell dimension defines the concentration of polyelectrolyte stars in the system. The numerical results are compared to predictions of an analytical mean field theory and related to experimental observations on the osmotic pressure in solutions of starlike polyelectrolytes and colloidal polyelectrolyte brushes.

Polyelectrolytes tethered to a similarly charged surface

Conformations of weakly charged quenched polyelectrolyte chains tethered to a similarly charged planar surface are analyzed on the basis of a combination of scaling, analytical, and numerical self-consistent field (SCF) approaches. Scaling theory predicts universal power law dependences of the large-scale conformational properties (like the end-to-end distance) of grafted chains on the overall surface charge per unit area. The SCF approach allows analysis of the detailed conformational structure of grafted polyions as a function of the distribution of immobilized charges between the surface and grafted chains. The analytical solution is only available in the limiting cases of sparse grafting of polyions to the charged plane and sufficiently dense grafting of polyions to a neutral surface. In the intermediate case when both interchain interactions and interaction of grafted chains with the surface are important, only numerical solutions can be obtained. We consider various ways to distribute the charges be...