Polymer brush-based nanostructures: from surface self-assembly to surface co-assembly

Dipolar Brush Polymers: A Numerical Study of the Force Exerted onto a Penetrating Colloidal Particle Under an External Field

Langevin Dynamics numerical simulations have been used to compute the force profiles that dipolar polymer brushes exert onto a penetrating colloidal particle. It has been observed that force profiles are strongly influenced by externally applied fields: at large distances from the grafting surface, a force barrier appears, and at shorter distances a region with lower repulsive forces develops. Furthermore, with the right combination of polymer grafting density, polymer chain length and strength of the external field, it is possible to observe in this intermediate region both the existence of net attractive forces onto the penetrating particle and the emergence of a stationary point. The existence of these regions of low repulsive or net attractive forces inside the dipolar brushes, as well as their dependence on the different parameters of the system can be qualitatively reasoned in terms of a competition between steric repulsion forces and Kelvin forces arising from the dipolar mismatch between different regions of the system. The possibility to tune force profile features such as force barriers and stationary points via an external field paves the way for many potential surface-particle-related applications.

Design of an efficient magnetic brush solid acid and its catalytic use in organic reactions

In this research, with the Green Chemistry approach, to load more sulfonic acid active sites on catalyst surfaces, a nanocomposite material based on core-shell magnetite coated with vinyl silane and a sulfonated polymeric brush-like structure is designed and synthesized as a new class of efficient solid acid catalysts, referred to as Fe3O4@VS-APS brush solid acid. The synthesized catalyst was comprehensively characterized by a range of instrumental techniques, including XRD, SEM, TEM, FT-IR, EDX, TGA, and VSM. The activity of the catalyst was evaluated in Biginelli, Strecker, and esterification reactions. The Fe3O4@VS-APS brush solid acid has special features, such as easy reusability when a simple magnet is used for four reaction runs, an appropriate balance between hydrophobic and hydrophilic properties on the catalyst surface, and effective catalytic performance in the production of 3,4-dihydropyrimidin-2-one(thione) derivatives, 2-phenyl-2-(phenylamino)acetonitrile and octyl acetate.

Polymer Brushes and Surface Nanostructures: Molecular Design, Precise Synthesis, and Self-Assembly

For over two decades, polymer brushes have found wide applications in industry and scientific research. Now, polymer brush research has been a significant research focus in the community of polymer science. In this review paper, we give an introduction to the synthesis, self-assembly, and applications of one-dimensional (1D) polymer brushes on polymer backbones, two-dimensional (2D) polymer brushes on flat surfaces, and three-dimensional (3D) polymer brushes on spherical particles. Examples of the synthesis of polymer brushes on different substrates are provided. Studies on the formation of the surface nanostructures on solid surfaces are also reviewed in this article. Multicomponent polymer brushes on solid surfaces are able to self-assemble into surface micelles (s-micelles). If the s-micelles are linked to the substrates through cleavable linkages, the s-micelles can be cleaved from the substrates, and the cleaved s-micelles are able to self-assemble into hierarchical structures. The formation of the surface nanostructures by coassembly of polymer brushes and "free" polymer chains (coassembly approach) or polymerization-induced surface self-assembly approach, is discussed. The applications of the polymer brushes in colloid and biomedical science are summarized. Finally, perspectives on the development of polymer brushes are offered in this article.

Comparing polymer-surfactant complexes to polyelectrolytes

HYPOTHESIS

Understanding the complex interactions between polymers and surfactants is required to optimise commercially relevant systems such as paint, toothpaste and detergent. Neutral polymers complex with surfactants, forming 'pearl necklace' structures that are often conceptualised as pseudo-polyelectrolytes. Here we pose two questions to test the limits of this analogy: Firstly, in the presence of salt, do these polymer-surfactant systems behave like polyelectrolytes? Secondly, do polymer-surfactant complexes resist geometric confinement like polyelectrolytes?

EXPERIMENTS

We test the limits of the pseudo-polyelectrolyte analogy through studying a poly(N-isopropylacrylamide) (PNIPAM) brush in the presence of sodium dodecylsulfate (SDS). Brushes are ideal for interrogating pseudo-polyelectrolytes, as neutral and polyelectrolyte brushes exhibit distinct and well understood behaviours. Spectroscopic ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and neutron reflectometry (NR) were used to monitor the behaviour and structure of the PNIPAM-SDS system as a function of NaCl concentration. The ability of the PNIPAM-SDS complex to resist geometric confinement was probed with NR.

FINDINGS

At a fixed SDS concentration below the zero-salt CMC, increasing NaCl concentration <100 mM promoted brush swelling due to an increase in osmotic pressure, not dissimilar to a weak polyelectrolyte. At these salt concentrations, the swelling of the brush could be described by a single parameter: the effective CMC. However, at high NaCl concentrations (e.g., 500 mM) no brush collapse was observed at all (non-zero) concentrations of SDS studied, contrary to what is seen for many polyelectrolytes. Study of the polymer-surfactant system under confinement revealed that the physical volume of surfactant dominates the structure of the strongly confined system, which further differentiates it from the polyelectrolyte case.

Mechanical response of networks formed by end-functionalised spherical polymer grafted nanoparticles

Via computer simulations we examine the mechanical response of hybrid polymer-particle networks composed of rigid spherical nanoparticles with long flexible polymer chains grafted onto their surface. The canopy of grafted polymer arms are end-functionalised such that interacting polymer-grafted nanoparticles (PGNs) form labile bonds when their coronas overlap. In the present study, the number of grafted arms, f, are such that the PGN brushes are in the small (f = 600) and intermediate curvature (f = 900 and 1200) regime with stable bonded interactions. To investigate the mechanical response of networks formed by these PGNs, controlled uniaxial elongation at a specified pulling rate is imposed on a 2-D network of PGNs placed on a hexagonal lattice. In the simulations, the force required to deform the network is measured as a function of the elongation and pulling rate imposed on the network until the network fails. By analysis of the force-strain curves and the rearrangement of the PGNs in the network we show that an increase in the number of grafted arms, pulling velocity and energy of the bonded interactions alters both the toughness and the mode of failure of the networks. In particular, we show that an increase in the number of grafted arms results in a reduction of toughness. Furthermore, analysis of the simulations of force relaxation after rapid extension indicates that the relaxation in deformed networks can be characterised by one or two time scales that depend on the number of grafted arms. The analysis of force-strain curves and force relaxation demonstrate the role of Deborah number, De, and the limitations in the use of a unique De in understanding the mechanical response of the networks respectively.

Packing core-corona particles on a spherical surface

We explore the non-trivial structures that can be obtained by the assembly of repulsive core-corona particles confined on a spherical surface. Using Monte Carlo simulations, we study the low-temperature equilibrium configurations as a function of the size of the confining (spherical) surface for a small number of particles (N ≤ 12) and obtain a large variety of minimal-energy arrangements including anisotropic and chiral structures. For a small cluster (N = 4), we construct a phase diagram in the confining surface radius vs corona range plane that showed regions where configurations with a certain energy are not accessible. Also, a phase diagram in the temperature and confining surface radius plane showed the presence of reentrant phases. The assembly of Platonic and Archimedean solids and the emergence of helical structures are also discussed. When the number of particles is large (N ≥ 100), apart from the appearance of defects, the overall configurations correspond closely to the ones formed in an unconfined two-dimensional case. Interestingly, the present model reproduces the symmetry of experimentally obtained small clusters of colloidal spheres confined at the surface of evaporating liquid droplets which cannot be explained in terms of packing of hard spheres. Thus, our simulations provide insight on the role that the softness of the particles may have in the assembly of clusters of nanoparticles.