Stimuli-Responsive Semi-Fluorinated Polymer Brushes on Porous Silica Substrates

Transition Metal Mediated Living Radical Polymerisation

Living radical polymerisation has witnessed an unprecedented interest from polymer and materials scientists. Traditionally, polymers tended to replace natural materials such as wood, cotton and glass, and were used primarily for their structural features and performance and cost advantages. New functional polymers are essential for the manufacture of cell phones, lap-top computers, new cosmetics, and many pharmaceuticals. It is important to be able to control how monomers are put together within the macromolecule for the design at the molecular level for specific applications. Living polymerisation allows for end group control, polymer chain length and relatively narrow polydispersity polymers. In nature, the ability to control monomer distribution and chain length is obvious with approximately 20 amino acids being the monomers for polymers as diverse as hair, insulin and haemoglobin. Living radical polymerisation solves many of the problems in the use of monomers that contain heteroatoms and functional groups. These tend to be reactive towards strong nucleophiles and electrophiles which are required in ionic polymerisation. Protecting group chemistry as used in small molecule organic synthesis is not practical in polymer synthesis. Thus radicals that are inert to most functional groups and in particular protic species seem to be the answer. The mechanism of the transition metal mediate systems is extremely complicated with a range of organometallic species present in the reaction mixture. Solvents and coordinating monomers drastically affect the ideal reaction conditions and it is impossible to predict the optimum conditions for each synthesis without certain experiments being carried out. Nevertheless, catalyst systems are available which are acceptable and work well enough to be able to make a plethora of different macromolecules for a diverse range of applications /properties.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Polyelectrolyte brushes: theory, modelling, synthesis and applications

Polyelectrolyte (PE) brushes are a special class of polymer brushes (PBs) containing charges. Polymer chains attain "brush"-like configuration when they are grafted or get localized at an interface (solid-fluid or liquid-fluid) with sufficiently close proximity between two-adjacent grafted polymer chains - such a proximity triggers a particular nature of interaction between the adjacent polymer molecules forcing them to stretch orthogonally to the grafting interface, instead of random-coil arrangement. In this review, we discuss the theory, synthesis, and applications of PE brushes. The theoretical discussion starts with the standard scaling concepts for polymer and PE brushes; following that, we shed light on the state of the art in continuum modelling approaches for polymer and PE brushes directed towards analysis beyond the scaling calculations. A special emphasis is laid in pinpointing the cases for which the PE electrostatic effects can be de-coupled from the PE entropic and excluded volume effects; such de-coupling is necessary to appropriately probe the complicated electrostatic effects arising from pH-dependent charging of the PE brushes and the use of these effects for driving liquid and ion transport at the interfaces covered with PE brushes. We also discuss the atomistic simulation approaches for polymer and PE brushes. Next we provide a detailed review of the existing approaches for the synthesis of polymer and PE brushes on interfaces, nanoparticles, and nanochannels, including mixed brushes and patterned brushes. Finally, we discuss some of the possible applications and future developments of polymer and PE brushes grafted on a variety of interfaces.

Preparation and applications of novel fluoroalkyl end-capped oligomeric nanocomposites

Fluoroalkanoyl peroxides were applied to the preparation of cross-linked fluorinated oligomeric nanoparticles and fluorinated oligomer/guest molecule nanocomposites.

Protein adsorption and cell adhesion/detachment behavior on dual-responsive silicon surfaces modified with poly(N-isopropylacrylamide)-block-polystyrene copolymer

Diblock copolymer grafts covalently attached to surfaces have attracted considerable attention because of their special structure and novel properties. In this work, poly(N-isopropylacrylamide)-block-polystyrene (PNIPAAm-b-PS) brushes were prepared via surface-initiated consecutive atom-transfer radical polymerization on initiator-immobilized silicon. Because of the inherent thermosensitivity of PNIPAAm and the hydrophobicity difference between the two blocks, the modified surfaces were responsive to both temperature and solvent. Moreover, the diblock copolymer brushes exhibited both resistance to nonspecific protein adsorption and unique cell interaction properties. They showed strong protein resistance in both phosphate-buffered saline and blood plasma. In particular, fibrinogen adsorption from plasma at either room temperature or body temperature was less than 8 ng/cm(2), suggesting that the surfaces might possess good blood compatibility. In addition, the adhesion and detachment of L929 cells could be "tuned", and the ability to control the detachment of cells thermally was restored by block polymerization of hydrophobic, cell-adhesive PS onto a thicker PNIPAAm layer. In addition to providing a simple and effective design for advanced cell-culture surfaces, these results suggest new biomedical applications for PNIPAAm.

Facile method to prepare smooth and homogeneous polymer brush surfaces of varied brush thickness and grafting density

This Article describes a facile method to prepare smooth and homogeneous polymer brush surfaces of variable grafting density from a solid surface by combining Langmuir-Blodgett (LB) deposition with surface-initiated atom transfer radical polymerization (SI-ATRP). This method is successfully demonstrated by the preparation of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush surfaces on smooth silicon and quartz substrates. With the custom-synthesized inert diluent whose chemical structure, except end-functionality, is the same as that of the reactive initiator, smooth and chemically homogeneous mixed monolayers of initiators and inert diluents are immobilized on a solid surface by LB deposition, allowing the further variation of the grafting density of PNIPAM brushes grafted from the initiator monolayers of varied initiator coverage. With the optimized molar ratio of deactivator, Cu(II) in the Cu(I)-ligand catalyst complex, the brush thickness of PNIPAM brushes at varied grafting density is controlled to grow nearly linearly with reaction time while smoothness and chemical homogeneity of PNIPAM brushes are achieved. For the demonstrated PNIPAM brush surfaces, the thermoresponsive characteristics of PNIPAM brushes are also verified. This combined LB-ATRP method can be applied to graft a variety of polymer brushes, including polyelectrolytes and block copolymers, from different solid substrates.

Development of a directly patterned low-surface-energy polymer brush in supercritical carbon dioxide

Carbon dioxide (CO2) is a sustainable solvent because it is nonflammable, exhibits a relatively low toxicity, and is naturally abundant. As a selective, nonpolar solvent, supercritical CO2 (scCO2) is an ideal fit for the development of low-surface-energy polymers. The development of directly patterned poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) brushes in scCO2 was investigated. PTFEMA, in particular, was selected over other fluorinated polymers because of its very high electron-beam (e-beam) sensitivity. PTFEMA brushes were grown on silicon substrates via controlled surface-initiated atom-transfer radical polymerization of TFEMA. Surface analysis techniques including ellipsometry, contact-angle goniometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy were used to characterize the thickness, hydrophilicity, roughness, and chemical composition of the polymer brushes. PTFEMA brushes were directly patterned in a single step using e-beam lithography and were processed in an environmentally benign scCO2 solvent. Tapping-mode AFM imaging confirmed the successful e-beam patterning and development of these brushes. The sensitivity of PTFEMA brushes toward direct patterning with the e-beam, followed by scCO2 development, was studied and compared to development in tetrahydrofuran solvent. Using this direct-patterning method, followed by dry development in scCO2, highly resolved nanostructured polymer brush lines down to 78 nm could be prepared. This method can be generalized to prepare fluorinated low-surface-energy polymer brush surfaces in a single step for various applications.

Stimuli-responsive surfaces using polyampholyte polymer brushes prepared via atom transfer radical polymerization

The synthesis of AB diblock copolymer polyampholyte polymer brushes of the type Si/SiO2//poly(acrylic acid-b-vinyl pyridine) prepared using atom transfer radical polymerization is reported. Both 2- and 4-vinyl pyridine have been used. The diblock polyampholyte polymer brushes demonstrate stimuli-responsive behavior with respect to pH, showing both polyelectrolyte and polyampholyte effects. Furthermore, we have quaternized the 4-vinyl pyridine segments to form a mixed weak/strong, or annealed/quenched, polyelectrolyte system. The quaternized polymer brush exhibits different pH-responsive behavior, with decreasing film thickness being observed with increasing pH.

Stimuli-responsive polyelectrolyte polymer brushes prepared via atom-transfer radical polymerization

We present an account of our research into polyelectrolyte polymer brushes that are capable of acting as stimuli-responsive films. We first detail the synthesis of poly(acrylic acid) polymer brushes using ATRP in a "grafting from" strategy. Significantly, we employed a chemical-free deprotection step that should leave the anchoring ester groups intact. We have demonstrated how these polymer assemblies respond to stimuli such as pH and electrolyte concentration. We have used poly(acrylic acid) polymer brushes for the synthesis of metallic nanoparticles and review this work. We have used XPS, ATR-FTIR, and AFM spectroscopy to show the presence of silver and palladium nanoparticles within polymer brushes. Finally, we report the synthesis of AB diblock polyampholyte polymer brushes that represent an extension of polyelectrolyte polymer brushes.