Stuart C. Thickett

Controlled/Living Radical Polymerization in Dispersed Systems: An Update.

This review is an extensive update to the comprehensive review on controlled/living radical polymerization (CLRP) in dispersed systems published in 2008.

Lanthanide-containing polymer microspheres by multiple-stage dispersion polymerization for highly multiplexed bioassays

We describe the synthesis and characterization of metal-encoded polystyrene microspheres by multiple-stage dispersion polymerization with diameters on the order of 2 mum and a very narrow size distribution. Different lanthanides were loaded into these microspheres through the addition of a mixture of lanthanide salts (LnCl(3)) and excess acrylic acid (AA) or acetoacetylethyl methacrylate (AAEM) dissolved in ethanol to the reaction after about 10% conversion of styrene, that is, well after the particle nucleation stage was complete. Individual microspheres contain ca. 10(6)-10(8) chelated lanthanide ions, of either a single element or a mixture of elements. These microspheres were characterized one-by-one utilizing a novel mass cytometer with an inductively coupled plasma (ICP) ionization source and time-of-flight (TOF) mass spectrometry detection. Microspheres containing a range of different metals at different levels of concentration were synthesized to meet the requirements of binary encoding and enumeration encoding protocols. With four different metals at five levels of concentration, we could achieve a variability of 624, and the strategy we report should allow one to obtain much larger variability. To demonstrate the usefulness of element-encoded beads for highly multiplexed immunoassays, we carried out a proof-of-principle model bioassay involving conjugation of mouse IgG to the surface of La and Tm containing particles and its detection by an antimouse IgG bearing a metal-chelating polymer with Pr.

Graphene oxide (GO) nanosheets as oil-in-water emulsion stabilizers: influence of oil phase polarity

HYPOTHESIS Two-dimensional nanoparticles such as graphene oxide (GO) can serve as emulsion stabilizers due their ability to adsorb at oil-water (o/w) interfaces with high atom efficiency. The ability for GO to act as a surfactant is hypothesized to be highly dependent on the nature (i.e. polarity) of the oil phase, which has not considered previously. MODELLING AND EXPERIMENTS The stabilization energy associated with adsorption of GO sheets at an o/w interface was modelled as a function of the polarity of the oil phase using surface tension contributions terms and Hansen solubility parameters (HSPs). Oil-in-water (o/w) miniemulsions were prepared via ultrasonication in the presence of GO for a variety of different oil phases, and were studied using dynamic light scattering (DLS). FINDINGS The stabilization energy associated with GO adsorption was greater for non-polar oil phases compared to more polar oils. This behaviour is driven by the significant reduction in the oil-water interfacial tension as the polarity of the oil increases, to the point where GO adsorption is no longer thermodynamically favourable. This was verified by DLS measurements experiments, as GO-stabilized emulsion were successfully prepared for hydrophobic and aromatic oil phases (e.g. styrene), but not for polar oil phases such as methyl methacrylate.

Interplay between Dewetting and Layer Inversion in Poly(4-vinylpyridine)/Polystyrene Bilayers

We investigated the morphology and dynamics of the dewetting of metastable poly(4-vinylpyridine) (P4VP) thin films situated on top of polystyrene (PS) thin films as a function of the molecular weight and thickness of both films. We focused on the competition between the dewetting process, occurring as a result of unfavorable intermolecular interactions at the P4VP/PS interface, and layer inversion due to the lower surface energy of PS. By means of optical and atomic force microscopy (AFM), we observed how both the dynamics of the instability and the morphology of the emerging patterns depend on the ratio of the molecular weights of the polymer films. When the bottom PS layer was less viscous than the top P4VP layer (liquid-liquid dewetting), nucleated holes in the P4VP film typically stopped growing at long annealing times because of a combination of viscous dissipation in the bottom layer and partial layer inversion. Full layer inversion was achieved when the viscosity of the top P4VP layer was significantly greater (>10⁴) than the viscosity of the PS layer underneath, which is attributed to strongly different mobilities of the two layers. The density of holes produced by nucleation dewetting was observed for the first time to depend on the thickness of the top film as well as the polymer molecular weight. The final (completely dewetted) morphology of isolated droplets could be achieved only if the time frame of layer inversion was significantly slower than that of dewetting, which was characteristic of high-viscosity PS underlayers that allowed dewetting to fall into a liquid-solid regime. Assuming a simple reptation model for layer inversion occurring at the dewetting front, the observed surface morphologies could be predicted on the basis of the relative rates of dewetting and layer inversion.

Functionalization of graphene oxide for the production of novel graphene-based polymeric and colloidal materials

Graphene oxide (GO) has been long-considered the most convenient route towards the large scale production of graphene. Additionally, the functional groups present within GO permit both covalent and non-covalent chemical functionalization, in particular with polymeric materials. The functionalization of GO therefore enables the development of graphene-based composite materials which possess the properties of both the matrix and the remarkable electrical, thermal and mechanical properties of both GO and graphene. In this review, we discuss the functionalization of GO in two broad settings: macromolecular functionalization of GO, and the use of GO in dispersed/colloidal systems. We review numerous methods for the functionalization of GO with initiators or chain transfer agents to permit controlled/living radical polymerization (CLRP) to take place from the surface of GO; the use of GO as a polymerization initiator, as well as non-covalent polymeric modification is discussed. For applications of GO in dispersed systems, we discuss the incorporation of GO into miniemulsions, emulsions and other dispersed phase polymerization systems, as well as techniques such as layer-by-layer assembly and colloidal templating. The use of GO as a ‘colloidal surfactant’ is also reviewed. These functionalization methods are discussed within the framework of creating materials with enhanced properties for specific applications including electrodes, capsules, polymer particles and composite films.

Bio-Functional, Lanthanide-Labeled Polymer Particles by Seeded Emulsion Polymerization and their Characterization by Novel ICP-MS Detection

We present the synthesis and characterization of monodisperse, sub-micron poly(styrene) (PS) particles loaded with up to and including 10(7) lanthanide (Ln) ions per particle. These particles have been synthesized by seeded emulsion polymerization with a mixture of monomer and a pre-formed Ln complex, and analyzed on a particle-by-particle basis by a unique inductively coupled plasma mass cytometer. Seed particles were prepared by surfactant-free emulsion polymerization (SFEP) to obtain large particle sizes in aqueous media. Extensive surface acid functionality was introduced using the acid-functional initiator ACVA, either during seed latex synthesis or in the second stage of polymerization. The loading of particles with three different Ln ions (Eu, Tb, and Ho) has proven to be close to 100 % efficient on an individual and combined basis. Covalent attachment of metal-tagged peptides and proteins such as Neutravidin to the particle surface was shown to be successful and the number of bound species can be readily determined. We believe these particles can serve as precursors for multiplexed, bead-based bio-assays utilizing mass cytometric detection.

Micropatterned surfaces for atmospheric water condensation via controlled radical polymerization and thin film dewetting

Inspired by an example found in nature, the design of patterned surfaces with chemical and topographical contrast for the collection of water from the atmosphere has been of intense interest in recent years. Herein we report the synthesis of such materials via a combination of macromolecular design and polymer thin film dewetting to yield surfaces consisting of raised hydrophilic bumps on a hydrophobic background. RAFT polymerization was used to synthesize poly(2-hydroxypropyl methacrylate) (PHPMA) of targeted molecular weight and low dispersity; spin-coating of PHPMA onto polystyrene films produced stable polymer bilayers under appropriate conditions. Thermal annealing of these bilayers above the glass transition temperature of the PHPMA layer led to complete dewetting of the top layer and the formation of isolated PHPMA domains atop the PS film. Due to the vastly different rates of water nucleation on the two phases, preferential dropwise nucleation of water occurred on the PHPMA domains, as demonstrated by optical microscopy. The simplicity of the preparation method and ability to target polymers of specific molecular weight demonstrate the value of these materials with respect to large-scale water collection devices or other materials science applications where patterning is required.

Early and Intermediate Stages of Guided Dewetting in Polystyrene Thin Films

We investigated the early and intermediate stages of the guided dewetting of polystyrene (PS) thin films on chemically patterned silicon, achieved by micro-contact printing of non-wettable self-assembling monolayers of an alkylsilane. Two different types of ordered patterns could be achieved depending on the annealing temperature and time. Study of the dynamics of hole growth revealed a deviation of the growth profile from the trend on homogeneous substrates, attributed to the pinning of the PS rims on the borders of the hydrophobic regions. The ordered patterns produced could be useful in applications that require spatially localized features of controlled surface chemistry, such as studies in proteomics, single cell studies, and biosensors.

Micropatterned substrates made by polymer bilayer dewetting and collagen nanoscale assembly support endothelial cell adhesion

The ability to control protein and cell positioning on a microscopic scale is crucial in many biomedical applications, such as tissue engineering and the development of biosensors. We demonstrate here that the assembly of collagen on patterned surfaces produced by the dewetting of metastable poly(N-vinylpyrrolidone) (PNVP) films on top of polystyrene films supports the adhesion and survival of a biologically relevant cell type, human endothelial cells. Micropatterning of Type 1 collagen was achieved on such substrates by exploiting the different protein affinity of the two polymers, the effect of treatment with an air plasma, and the control over the nanoscale assembly of collagen using different adsorption conditions. The simplicity of the dewetting approach, coupled with the ability to coat and pattern non-planar substrates, gives rise to possible applications in the coating of biological implants such as arterial stents.

Preparation of inverse polymerized high internal phase emulsions using an amphiphilic macro-RAFT agent as sole stabilizer

Oil-in-water (‘inverse’) High Internal Phase Emulsions (HIPEs) have been prepared using an amphiphilic macro-RAFT agent with toluene as the internal dispersed phase (∼80 vol%) and an aqueous monomer solution as the continuous phase. The water phase consisted of the monomers acrylamide (AM) and N,N′-methylenebisacrylamide (MBAM), an initiator as well as the amphiphilic macro-RAFT agent, that is 2-(butylthiocarbonothioylthio)-2-poly(n-butyl acrylate)-b-poly(acrylic acid), which was used as an anionic polymeric surfactant. The presence of these amphiphilic species allowed the successful preparation of a polyHIPE upon polymerization. The effect of concentration of macro-RAFT agent, pH, initiator, hexadecane as an organic modifier and the polymerization temperature on the morphology of the resulting porous materials was investigated. Varying the lengths of the hydrophilic and hydrophobic blocks of the macro-RAFT agent resulted in polyHIPEs with different porous structures. The presence of RAFT functionality in the polyHIPE was confirmed by elemental analysis, EDX-SEM, Raman and FT-IR spectroscopies. Raman mapping revealed full coverage of the void walls with dithiocarbamate groups.