Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Controlled Arrangement of Gold Nanoparticles on Planar Surfaces via Constrained Dewetting of Surface-Grafted RAFT Polymer

Linear and four-arm star polystyrene samples prepared by RAFT polymerization were grafted to gold surfaces directly via their thiocarbonylthio-end groups. Nanoscale polymer patterns were subsequently formed via constrained dewetting. The patterned polymer films then served as a template for the precise arrangement of gold nanoparticles in a monolayer with a well-defined and regular structure. Using star polymers as a linker between the planar gold surface and the particles, the structural stability of the arranged particles can be further enhanced. The surface-bound nanocomposite films made of polymer and nanoparticles can also reversibly switch their nanostructures by simple wetting or dewetting treatment.

Lamellae-Parking Garage Structure-Lamellae Transition in Densely Grafted Layers of Amphiphilic Homopolymers: Impact of Polymerization Degree

By means of computer modeling, the self-organization of densely grafted macromolecules with amphiphilic monomer units as a function of macromolecular polymerization degree and solvent quality was studied and a diagram of state was constructed. The diagram contains fields of disordered distribution of monomer units and of prolonged aggregates, regions of lamellae with small and big domain spacing, and transition region. Within the transition region, the lamellae with different spacing coexist: the lamellae with big domain spacing are on the top of the grafting layer and the lamellae with small domain spacing are close to the grafting surface. The lamellae are connected with each other and form bicontinuous parking garage structure joining all side groups into a single cluster. The domain spacing of lamellae does not depend on the macromolecular length, but the width of the transition region decreases with the decrease of polymerization degree until total vanishing at relatively short macromolecules. The sharp switch between lamellae and bicontinuous structure opens the perspective for practical applications of densely grafted layers with amphiphilic monomer units.

Thermoreversible Surface Polymer Patches: A Cryogenic Transmission Electron Microscopy Investigation

Hybrid core-shell type nanoparticles from gold nanoparticle cores and poly( N-isopropylacrylamide) shells were investigated with regard to their structural plasticity. Reversible addition-fragmentation chain transfer polymerization was used to synthesize well-defined polymers that can be readily anchored onto the gold nanoparticle surface. The polymer shell morphologies were directly visualized in their native solution state at high resolution by cryogenic transmission electron microscopy, and the microscopic results were further corroborated by dynamic light scattering. Different environmental conditions and brush architectures are covered by our experiments, which leads to distinct thermally induced responses. These responses include constrained dewetting of the nanoparticle surface at temperatures above the lower critical solution temperature of poly( N-isopropylacrylamide), leading to surface polymer patches. This effect provides a novel approach toward breaking the symmetry of nanoparticle interactions, and we show first evidence for its impact on the formation of colloidal superstructures.

Homopolymer Nanolithography

Future progress in nanoscience and nanotechnology necessitates further development of versatile, labor-, and cost-efficient surface patterning strategies. A new approach to nanopatterning is reported, which utilizes surface segregation of a smooth layer of an end-grafted homopolymer in a poor solvent. The variation in polymer grafting density yields a range of surface nanostructures, including randomly organized pinned spherical micelles, worm-like structures, networks, and porous films. The capability to use the polymer patterns for site-specific deposition of small molecules, polymers, or nanoparticles is shown. This versatile strategy enables patterning of curved surfaces with direct access to the substrate and no need in changing polymer composition to realize different surface patterns.

Towards controlled polymer brushes via a self-assembly-assisted-grafting-to approach

Precise synthesis of polymer brushes to modify the surface of nanoparticles and nanodevices for targeted applications has been one of the major focuses in the community for decades. Here we report a self-assembly-assisted-grafting-to approach to synthesize polymer brushes on flat substrates. In this method, polymers are pre-assembled into two-dimensional polymer single crystals (PSCs) with functional groups on the surface. Chemically coupling the PSCs onto solid substrates leads to the formation of polymer brushes. Exquisite control of the chain folding in PSCs allows us to obtain polymer brushes with well-defined grafting density, tethering points and brush conformation. Extremely high grafting density (2.12 chains per nm(2)) has been achieved in the synthesized single-tethered polymer brushes. Moreover, polymer loop brushes have been successfully obtained using oddly folded PSCs from telechelic chains. Our approach combines some of the important advantages of conventional 'grafting-to' and 'grafting-from' methods, and is promising for tailored synthesis of polymer brushes.

Control of nanoparticle formation using the constrained dewetting of polymer brushes

We have used coarse-grained molecular dynamics simulations to investigate the use of pinned micelles formed by the constrained dewetting of polymer brushes to act as a template for nanoparticle formation. The evaporation of a thin film containing a dissolved solute from a polymer brush was modeled to study the effect of solubility, concentration, grafting density, and evaporation rate on the nucleation and growth of nanoparticles. Control over particle nucleation could be imposed when the solution was dilute enough such that particle nucleation occurred following the onset of constrained dewetting. We predict that nanoparticles with sizes on the order of 1 nm to 10 nm could be produced from a range of organic molecules under experimentally accessible conditions. This method could allow the functionality of organic materials to potentially be imparted onto surfaces without the need for synthetic modification of the functional molecule, and with control over particle size and aggregation, for application in the preparation of surfaces with useful optical, pharmaceutical, or electronic properties.

Tuning surface wettability by designing hairy structures

We present a molecular dynamics simulation study on the controlling factors that influence the wettability of a hairy surface. By adopting the hairs with appropriate grafting density, hair length, and hair rigidity, the hairy surface shows good performance on droplet repellency. When the droplet sits on the hairy surface, the flexible hairs can spontaneously bundle with the appropriate amount of neighboring hairs to enhance the surface hydrophobicity, thus providing a new possibility to control the surface wettability. The hairy surface with tunable grafting density and hair rigidity, bridges the gap between surfaces with soft polymer brushes and surfaces that are completely hard but porous.

Interfacial slip on rough, patterned and soft surfaces: a review of experiments and simulations

Advancements in the fabrication of microfluidic and nanofluidic devices and the study of liquids in confined geometries rely on understanding the boundary conditions for the flow of liquids at solid surfaces. Over the past ten years, a large number of research groups have turned to investigating flow boundary conditions, and the occurrence of interfacial slip has become increasingly well-accepted and understood. While the dependence of slip on surface wettability is fairly well understood, the effect of other surface modifications that affect surface roughness, structure and compliance, on interfacial slip is still under intense investigation. In this paper we review investigations published in the past ten years on boundary conditions for flow on complex surfaces, by which we mean rough and structured surfaces, surfaces decorated with chemical patterns, grafted with polymer layers, with adsorbed nanobubbles, and superhydrophobic surfaces. The review is divided in two interconnected parts, the first dedicated to physical experiments and the second to computational experiments on interfacial slip of simple (Newtonian) liquids on these complex surfaces. Our work is intended as an entry-level review for researchers moving into the field of interfacial slip, and as an indication of outstanding problems that need to be addressed for the field to reach full maturity.