Youyong Xu

Water-soluble organo-silica hybrid nanowires.

There has been growing interest in the past decade in one-dimensional (1D) nanostructures, such as nanowires, nanotubes or nanorods, owing to their size-dependent optical and electronic properties and their potential application as building blocks, interconnects and functional components for assembling nanodevices. Significant progress has been made; however, the strict control of the distinctive geometry at extremely small size for 1D structures remains a great challenge in this field. The anisotropic nature of cylindrical polymer brushes has been applied to template 1D nanostructured materials, such as metal, semiconductor or magnetic nanowires. Here, by constructing the cylindrical polymer brushes themselves with a precursor-containing monomer, we successfully synthesized hybrid nanowires with a silsesquioxane core and a shell made up from oligo(ethylene glycol) methacrylate units, which are soluble in water and many organic solvents. The length and diameter of these rigid wires are tunable by the degrees of polymerization of both the backbone and the side chain. They show lyotropic liquid-crystalline behaviour and can be pyrolysed to silica nanowires. This approach provides a route to the controlled fabrication of inorganic or hybrid silica nanostructures by living polymerization techniques.

Undulated Multicompartment Cylinders by the Controlled and Directed Stacking of Polymer Micelles with a Compartmentalized Corona

Like a bamboo rod: Long, bamboo-like undulated cylinders with distinct branch points and end groups (see picture) were obtained by the stacking of block terpolymer micelles that contain a fluorinated polybutadiene core and a compartmentalized corona of poly(4-tert-butoxystyrene) and poly(tert-butyl methacrylate). Stacking of the polymer micelles, which can be reversed, occurred when the solvent was changed from dioxane to ethanol.

Switching the morphologies of cylindrical polycation brushes by ionic and supramolecular inclusion complexes.

Cylindrical polycation brushes form an ionic complex with surfactant sodium dodecyl sulfate (SDS) in aqueous solution, which causes a worm-to-sphere collapse of the brush. Alpha- and beta-cyclodextrin (CD) returns the brush to the worm-like conformation by forming a supramolecular inclusion complex with SDS. When beta-CD was employed for the inclusion complex, addition of 1-adamantylammonium chloride releases SDS by forming a stronger inclusion complex, causing the recollapse of the brush to spheres.

Manipulating cylindrical polyelectrolyte brushes on the nanoscale by counterions: collapse transition to helical structures

We present a study of the spatial structure of cationic cylindrical polymer brushes in the presence of counterions of different valencies. Dynamic light scattering (DLS), atomic force microscopy (AFM) and cryogenic transmission electron microscope (cryo-TEM) studies demonstrate that the brushes undergo a transition from an extended to a collapsed state when the ionic strength is raised. In the presence of a di- or trivalent salt this collapse passes through an intermediate state in which the cylindrical polymer brushes assume a helical conformation. The helical pitch (25 nm) is in the range expected from theoretical considerations. Moreover, we demonstrate that the collapse can be reversed by a photochemical cleavage of the complex trivalent counterion into one monovalent and one divalent ion (photoaquation). All data demonstrate that the spatial structure of cylindrical polymer brushes can be manipulated on the nanoscale in a defined fashion.

Influence of Counterion Valency on the Conformational Behavior of Cylindrical Polyelectrolyte Brushes

Using a dissipative particle dynamics approach, we study the conformations and interactions of a cylindrical polyelectrolyte brush (CPB) with added salt. The effects of counterion valency on the conformational behaviors of the CPB are analyzed in detail by considering various parameters like the distribution of bond lengths, the mean distance between two grafting points, the order parameter of side chains, etc. The lyotropic behavior of the CPB is also investigated through examining the backbone persistence. Our simulations demonstrate that the presence of the multivalent counterions can induce the collapse of the CPB, leading to various conformations. We identify a horseshoe to helical to coil-like conformation transition with increasing counterion valency. An important factor for the collapse of the CPB is the fact that the strong condensation of counterions induced by the higher electrostatic correlations decreases the osmotic pressure inside the brush. It is found that the ratio of the backbone persistence to the diameter of the CPB, l(p)/d, can only be affected to a slight extent by changing the counterion valency and the side chain length. These results may provide a valuable guideline that can be used to tailor the microstructure of the systems and to yield desired macroscopic behaviors.

Interaction of cylindrical polymer brushes in dilute and semi-dilute solution

We present a systematic study of flexible cylindrical brush-shaped macromolecules in a good solvent by small-angle neutron scattering (SANS), static light scattering (SLS), and by dynamic light scattering (DLS) in dilute and semi-dilute solution. The SLS and SANS data extrapolated to infinite dilution lead to the shape of the polymer that can be modeled in terms of a worm-like chain with a contour length of 380 nm and a persistence length of 17.5 nm. SANS data taken at higher polymer concentration were evaluated by using the polymer reference interaction site model (PRISM). We find that the persistence length reduce from 17.5 nm at infinite dilution to 5.3 nm at the highest concentration (volume fraction 0.038). This is comparable with the decrease of the persistence length in semi-dilute concentration predicted theoretically for polyelectrolytes. This finding reveals a softening of stiffness of the polymer brushes caused by their mutual interaction.

Polyelectrolyte Stars and Cylindrical Brushes

This review summarizes recent work in the field of polyelectrolyte stars and cylindrical brushes. Synthesis of anionic and cationic polyelectrolyte stars and cylindrical brushes by different synthetic strategies is introduced. Solution properties such as titration, osmotic pressure, and inter-polyelectrolyte complexes (IPECs) of the stars and brushes are briefly discussed. We also highlight the use of core–shell polyelectrolyte brushes as the template for the preparation of superparamagnetic hybrid cylinders.

Nano-patterned structures in cylindrical polyelectrolyte brushes assembled with oppositely charged polyions

We demonstrate that the electrostatically driven association of cylindrical polyelectrolyte brushes (CPBs) with oppositely charged linear polyelectrolytes (PEs) in dilute aqueous solution gives rise to well-defined and colloidaly stable (not undergoing secondary aggregation with time) polymeric nano-assemblies representing novel water-soluble interpolyelectrolyte complexes (IPECs). Each complex particle comprises a single “host” CPB whose charge is undercompensated by the “guest” PE chains. Molecular dynamics (MD) simulations were used to probe the structural organization of these nano-assemblies. We find that they spontaneously adopt the shape of a necklace of complex coacervate pearls which comprise charged monomer units of CPB and those of the guest PEs in approximately stoichiometric (1:1) ratio. Each complex coacervate pearl is decorated by a star-like PE corona formed by the branches of the CPB not involved in the interpolyelectrolyte complexation. Repulsive interactions between these coronas stabilize the periodic intra-molecular structure and assure aggregative stability of the IPEC derived from the single CPB. AFM images of the complex particles deposited on mica unambiguously support their pearl-necklace structural organization. Our work theoretically predicts and experimentally confirms a possibility to tune the periodicity of one-dimensional intramolecular nano-patterned structures at will by a variation of the base-molar ratio between the oppositely charged macromolecular building blocks incorporated in the polymeric nano-assembly, that is, by a variation of the stoichiometry of the IPECs.

Direct Synthesis of Poly(potassium 3-sulfopropyl methacrylate) Cylindrical Polymer Brushes via ATRP Using a Supramolecular Complex With Crown Ether.

A supramolecular complex between an ionic monomer 3-sulfopropyl methacrylate (SPMAK) and crown ether 18-crown-6 (18C6) has been employed to prepare a strong anionic cylindrical polyelectrolyte brush poly(potassium 3-sulfopropyl methacrylate) (PSPMAK) by atom transfer radical polymerization (ATRP) in polar solvent dimethyl sulfoxide (DMSO). This strategy solved the problem of the solubilities of the incompatible hydrophobic poly-initiator and hydrophilic ionic monomer. The formation of the PSPMAK brush is well proven by (1) H NMR, aqueous gel permeation chromatography (GPC), dynamic light scattering (DLS), static light scattering (SLS), atomic force microscopy (AFM), and cryogenic transmission electron microscopy (cryo-TEM) measurements. Cleavage of the side chains and further analysis reveal that the initiating efficiency of the polymerization is as low as 0.35.

Polyelectrolyte Stars and Cylindrical Brushes Made by ATRP: New Building Blocks in Nanotechnology

Star polymers and cylindrical polymer brushes (CPBs), i.e. polymers possessing side groups densely grafted from a linear main chain, have attracted considerable experimental and theoretical interest over the past decade, owing to their peculiar solution and bulk properties. We have used the grafting-from approach via ATRP to synthesize well-defined star polymers and core—shell CPBs with homopolymer and block copolymer side chains. The diblock copolymer side chains may include combinations of soft-hard, hydrophilic-hydrophobic and crystalline-amorphous block segments. In particular, we have been interested in polyelectrolyte blocks; then the polymers resemble intramolecular spherical and cylindrical micelles, respectively. Star polymers of poly(acrylic acid) (PAA) and poly(N,N-dimethylaminoethyl methacrylate) (DMAEMA) were made using sugar- or silsesquioxane-based ATRP initiators. Their LCST and UCST phase behaviour depends on pH, counterion charge, temperature, and light. PDMAEMA CPBs react in a similar way, and on addition of trivalent counterions they even form helical structures. We have also synthesized hybrid nanowires of semiconducting CdS and CdSe or nanomagnets of γ-Fe2O3 inside the PAA core of CPBs. Here, we present novel water-soluble and biocompatible silica nanowires based on CPBs. They have a core consisting of a silsesquioxane network of crosslinked poly(3-acryloylpropyl trimethoxysilane) (PAPTS) and a shell of poly(oligoe-thyleneglycol methacrylate) (POEGMA). Sequential ATRP of APTS and OEGMA initiated by a polyinitiator backbone (DP = 3,200) was carried out in benzene. Due to the cylindrical shape of the brushes the functional TMS moieties were arranged into a 1D manner and then crosslinked via alkaline condensation, rendering the rigid core—shell hybrid CPBs. Finally, uniform silica nanowires were achieved by the simultaneous removal of the hybrid CPB template via pyrolysis. The length as well as the diameter of silica nanowires are well-defined.