Synthesis of hydrophilic polymer-grafted ultrafine inorganic oxide particles in protic media at ambient temperature via atom transfer radical polymerization: use of an electrostatically adsorbed polyelectrolytic macroinitiator

Fabrication of topologically anisotropic microparticles and their surface modification with pH responsive polymer brush

This paper describes the fabrication of topologically anisotropic cup shaped polylactide (PLA)/poly[methyl methacrylate‑co‑2‑(2‑bromopropionyloxy) ethyl methacrylate] (poly(MMA-co-BEMA)) (75/25) composite particles of ~6 μm size using electrojetting technique. An attempt was made to understand the mechanism of cup shape formation from the miscible blend by electrojetting. Both the solution parameters and the processing conditions affected the particles' shape which can be varied from cup shaped to discoid type. Surface initiated atom transfer radical polymerization (ATRP) of stimuli responsive DMAEMA (2‑dimethylamino ethyl methacrylate) was subsequently carried out for 1 h onto the surface of cup shaped particles to observe pH responsiveness of the modified anisotropic particles. Interestingly, morphology of the cup shaped particles was changed to elongated cup which did show significant swelling under acidic pH (swelling ratio:~1.6) and enhanced dye adsorption at specific pH as observed by optical microscope and confocal laser scanning microscope implying that DMAEMA polymerization happened onto the surface of the composite microparticles. The Raman microscopy and FTIR spectra obtained from the particles after polymerization further confirmed the immobilization of pH responsive poly(DMAEMA) brushes onto the cup shaped particles which may potentially function as triggered/targeted drug delivery vehicles. Moreover, the brush modified cup shaped particles were found to be two times more efficient in adsorbing dye compared to disc shaped one indicating a clear advantage of using cup shaped particles over other shapes for immobilizing/adsorbing charged species e.g. sensitive biomolecules.

Molecular Engineering of the Cellulose-Poly(Caprolactone) Bio-Nanocomposite Interface by Reactive Amphiphilic Copolymer Nanoparticles

A molecularly engineered water-borne reactive compatibilizer is designed for tuning of the interface in melt-processed thermoplastic poly(caprolactone) (PCL)-cellulose nanocomposites. The mechanical properties of the nanocomposites are studied by tensile testing and dynamic mechanical analysis. The reactive compatibilizer is a statistical copolymer of 2-(dimethylamino)ethyl methacrylate and 2-hydroxy methacrylate, which is subsequently esterified and quaternized. Quaternized ammonium groups in the reactive compatibilizer electrostatically match the negative surface charge of cellulose nanofibrils (CNFs). This results in core-shell CNFs with a thin uniform coating of the compatibilizer. This promotes the dispersion of CNFs in the PCL matrix, as concluded from high-resolution scanning electron microscopy and atomic force microscopy. Moreover, the compatibilizer "shell" has methacrylate functionalities, which allow for radical reactions during processing and links covalently with PCL. Compared to the bio-nanocomposite reference, the reactive compatibilizer (<4 wt %) increased Young's modulus by about 80% and work to fracture 10 times. Doubling the amount of peroxide caused further improved mechanical properties, in support of effects from higher cross-link density at the interface. Further studies of interfacial design in specific nanocellulose-based composite materials are warranted since the detrimental effects from CNFs agglomeration may have been underestimated.

Core-independent approach for polymer brush-functionalised nanomaterials with a fluorescent tag for RNA delivery

A core-independent approach for the design of polymer brush-functionalised nanomaterials with a fluorescent tag for RNA delivery.

Poly(2-methacryloyloxyethyl phosphorylcholine)-based biodegradable nanogels for controlled drug release

We developed reduction degradable PMPC nanogels for controlled drug releaseviaprecipitation polymerization using a disulfide-containing crosslinker.

Core (Polystyrene)-Shell [Poly(glycerol monomethacrylate)] Particles

A set of water-swollen core-shell particles was synthesized by emulsion polymerization of a 1,3-dioxolane functional monomer in water. After removal of the 1,3-dioxolane group, the particles' shells were shown to swell in aqueous media. Upon hydrolysis, the particles increased in size from around 70 to 100-130 nm. A bicinchoninic acid assay and ζ-potential measurements were used to investigate the adsorption of lysozyme, albumin, or fibrinogen. Each of the core-shell particles adsorbed significantly less protein than the noncoated core (polystyrene) particles. Differences were observed as both the amount of difunctional, cross-linking monomer and the amount of shell monomer in the feed were changed. The core-shell particles were shown to be resistant to protein adsorption, and the degree to which the three proteins adsorbed was dependent on the formulation of the shell.

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.

Hydrophobic matrix-free graphene-oxide composites with isotropic and nematic states

We demonstrate a novel route to synthesise hydrophobic matrix-free composites of polymer-grafted graphene oxide (GO) showing isotropic or nematic alignment and shape-memory effects. For the first time, a cationic macroinitiator (MI) has been immobilised on anionic GO and subsequently grafted with hydrophobic polymer grafts. Dense grafts of PBA, PBMA and PMMA with a wide range of average graft lengths (MW: 1-440 kDa) were polymerised by surface-initiated controlled radical precipitation polymerisation from the statistical MI. The surface modification is designed similarly to bimodal graft systems, where the cationic MI generates nanoparticle repulsion, similar to dense short grafts, while the long grafts offer miscibility in non-polar environments and cohesion. The state-of-the-art dispersions of grafted GO were in the isotropic state. Transparent and translucent matrix-free GO-composites could be melt-processed directly using only grafted GO. After processing, birefringence due to nematic alignment of grafted GO was observed as a single giant Maltese cross, 3.4 cm across. Permeability models for composites containing aligned 2D-fillers were developed, which were compared with the experimental oxygen permeability data and found to be consistent with isotropic or nematic states. The storage modulus of the matrix-free GO-composites increased with GO content (50% increase at 0.67 wt%), while the significant increases in the thermal stability (up to 130 °C) and the glass transition temperature (up to 17 °C) were dependent on graft length. The tuneable matrix-free GO-composites with rapid thermo-responsive shape-memory effects are promising candidates for a vast range of applications, especially selective membranes and sensors.

Nanopatterned polymer brushes: conformation, fabrication and applications

Surfaces with end-grafted, nanopatterned polymer brushes that exhibit well-defined feature dimensions and controlled chemical and physical properties provide versatile platforms not only for investigation of nanoscale phenomena at biointerfaces, but also for the development of advanced devices relevant to biotechnology and electronics applications. In this review, we first give a brief introduction of scaling behavior of nanopatterned polymer brushes and then summarize recent progress in fabrication and application of nanopatterned polymer brushes. Specifically, we highlight applications of nanopatterned stimuli-responsive polymer brushes in the areas of biomedicine and biotechnology.

Synthesis of multi-hollow clay-armored latexes by surfactant-free emulsion polymerization of styrene mediated by poly(ethylene oxide)-based macroRAFT/Laponite complexes

We report the RAFT-mediated synthesis of Laponite-armored latexes by surfactant-free emulsion polymerization.

Effect of colloidal substrate curvature on pH-responsive polyelectrolyte brush growth

Coatings consisting of polymer brushes are an effective way to modify solid interfaces. Polymer brush-modified hybrid particles have been prepared by surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP) of 2-(diethylamino)ethyl methacrylate (DEA) on silica particles. We have optimized the synthesis with respect to changing the reducing agent, temperature, and reaction solvent from an aqueous ethanol mixture to an aqueous methanol mixture. Our flexible electrostatically adsorbed macroinitiator approach allows for the modification of a variety of surfaces. Polybasic brushes have been grown on silica particles of different sizes, from 120 to 840 nm in diameter, as well as on wafers, and a comparison of the products has allowed the effect of surface curvature to be elucidated. An examination of the thickness of the dry brush and the aqueous hydrodynamic brush at both pH 7 and at 4 demonstrated that growth increased substantially with substrate curvature for particles with a diameter below 450 nm. This is attributed to the increasing separation between active chain ends, reducing the rate of termination. This is believed to be the first time that this effect has been demonstrated experimentally. Furthermore, we have seen that polymer brush growth on planar wafers was significantly reduced when the reaction mixture was stirred.

pH-Responsive Brush-Modified Silica Hybrids Synthesized by Surface-Initiated ARGET ATRP

Brush-modified silica hybrids have been synthesized by growing poly(2-(diethylamino)ethyl methacrylate) (poly(DEA)) brushes on 120 nm diameter silica particles by surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP). This is the first report of using SI-ARGET ATRP to synthesize poly(DEA) brushes. The kinetics of poly(DEA) brush growth in 4:1 v/v ethanol/water was monitored. The hydrodynamic diameter of the resulting brush-modified particles was dependent on the solution pH due to the weak polybasic nature of the brushes. Below the pK_a of poly(DEA), the hydrodynamic diameter of the brush-modified particles increased with decreasing pH as a consequence of brush protonation, rearrangement and solvent uptake. This pH-response of the brushes was reversible and the hybrid particles exhibited significant hydrodynamic volume changes of up to 200% when the solution pH was cycled from pH 7 to pH 4.

Self-assembly of catecholic macroinitiator on various substrates and surface-initiated polymerization

A catechol-containing macroinitiator has been designed for the surface-initiated atom transfer radical polymerization (SI-ATRP) from various substrates at ambient temperature. Temperature-sensitive poly(N-isopropyl acrylamide) (PNIPAM) brushes were successfully grafted from a range of substrates surfaces, including metals and polyimides, via SI-ATRP using the resulting macroinitiator, which were characterized by X-ray photoelectron spectroscopy (XPS), water contact angle measurements, and atomic force microscopy (AFM). Effects of the temperature response behavior of PNIPAM brushes on the water contact angles and the impedance of the modified surfaces were also exhibited. The self-assembled film of macroinitiator and the resulting polymer brushes were both stable to soaking of basic solvents, and the brushes did not show any exfoliation or delamination even after 2 h of ultrasonic test. The advantages of the macroinitiator in strong interactions with surfaces and high stability and convenience make it possible to modify the native materials with polymer brushes in a convenient and nondestructive way. Importantly, the macroinitiator is compatible with microcontact printing, and patterned polymer brushes on Ti plate were demonstrated by microcontact printing of BrDOPAMA and the following SI-ATRP.

A substrate-independent method for surface grafting polymer layers by atom transfer radical polymerization: reduction of protein adsorption

A general method for producing low-fouling biomaterials on any surface by surface-initiated grafting of polymer brushes is presented. Our procedure uses radiofrequency glow discharge thin film deposition followed by macro-initiator coupling and then surface-initiated atom transfer radical polymerization (SI-ATRP) to prepare neutral polymer brushes on planar substrates. Coatings were produced on substrates with variable interfacial composition and mechanical properties such as hard inorganic/metal substrates (silicon and gold) or flexible (perfluorinated poly(ethylene-co-propylene) film) and rigid (microtitre plates) polymeric materials. First, surfaces were functionalized via deposition of an allylamine plasma polymer thin film followed by covalent coupling of a macro-initiator composed partly of ATRP initiator groups. Successful grafting of a hydrophilic polymer layer was achieved by SI-ATRP of N,N'-dimethylacrylamide in aqueous media at room temperature. We exemplified how this method could be used to create surface coatings with significantly reduced protein adsorption on different material substrates. Protein binding experiments using labelled human serum albumin on grafted materials resulted in quantitative evidence for low-fouling compared to control surfaces.

Thermally triggered assembly of cationic graft copolymers containing 2-(2-methoxyethoxy)ethyl methacrylate side chains

Thermoresponsive copolymers continue to attract a great deal of interest in the literature. In particular, those based on ethylene oxide-containing methacrylates have excellent potential for biomaterial applications. Recently, some of us reported a study of thermoresponsive cationic graft copolymers containing poly(N-isopropylacrylamide), PNIPAm, (Liu et al., Langmuir, 24, 7099). Here, we report an improved version of this new family of copolymers. In the present study, we replaced the PNIPAm side chains with poly(2-(2-methyoxyethoxy)ethylmethacrylate), PMeO(2)MA. These new, nonacrylamide containing, cationic graft copolymers were prepared using atom transfer radical polymerization (ATRP) and a macroinitiator. They contained poly(trimethylamonium)-aminoethyl methacrylate and PMeO(2)MA, i.e., PTMA(+)(x)-g-(PMeO(2)MA(n))(y). They were investigated using variable-temperature turbidity, photon correlation spectroscopy (PCS), electrophoretic mobility, and (1)H NMR measurements. For one system, four critical temperatures were measured and used to propose a mechanism for the thermally triggered changes that occur in solution. All of the copolymers existed as unimolecular micelles at 20 °C. They underwent reversible aggregation with heating. The extent of aggregation was controlled by the length of the side chains. TEM showed evidence of micellar aggregates. The thermally responsive behaviors of our new copolymers are compared to those for the cationic PNIPAm graft copolymers reported by Liu et al. Our new cationic copolymers retained their positive charge at all temperatures studied, have high zeta potentials at 37 °C, and are good candidates for conferring thermoresponsiveness to negatively charged biomaterial surfaces.

Co-nonsolvency effects for surface-initiated poly(2-(methacryloyloxy)ethyl phosphorylcholine) brushes in alcohol/water mixtures

Surface-initiated atom transfer radical polymerization (SI-ATRP) has been used to grow brushes of poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) from silicon wafers using a polyelectrolytic macroinitiator on planar silicon wafers. Film thicknesses of up to 450 nm were possible within 21 h, and the effect of adding activator and deactivator species on the brush growth rate was studied. The solvation of PMPC brushes in mixed alcohol/water solvents was investigated using in situ ellipsometry. Co-nonsolvency (a re-entrant swelling transition) behavior was observed in water/ethanol binary mixtures; that is, the PMPC brushes were highly swollen in either pure ethanol or water but became deswollen at specific ethanol-rich solvent compositions. A similar effect was obtained with water/2-propanol mixtures, except that in this case pure 2-propanol was not a particularly good solvent for the PMPC chains. However, co-nonsolvency was not observed for water/methanol binary mixtures, since the brushes remained well swollen at all solvent compositions. This is consistent with prior reports of co-nonsolvency effects in both PMPC gels and linear PMPC chains. However, this is the first report of this phenomenon for PMPC brushes and one of the first examples of co-nonsolvency observed for any polymer brush system. A direct comparison of brush and gel swelling reveals an approximate power-law relationship between the equilibrium volumes of these two systems at various solvent compositions, which is interpreted by treating the brush layer as a surface-attached gel. We believe this to be the first quantitative comparison of brush and gel swelling using the same polymer under the same conditions. The kinetics of the PMPC brush response to adjustment of the alcohol/water composition is relatively fast, with the brush volume change occurring on time scales of less than 1 min as judged by in situ ellipsometry.

End-Grafted Polymer Chains onto Inorganic Nano-Objects

Organic/inorganic nanohybrid materials have attracted particular scientific and technological interest because they combine the properties of the organic and the inorganic component. Inorganic nanoparticles exhibit interesting electrical, optical, magnetic and/or catalytic properties, which are related with their nano-scale dimensions. However, their high surface-to-volume ratio often induces agglomeration and leads to the loss of their attractive properties. Surface modification of the inorganic nano-objects with physically or chemically end-tethered polymer chains has been employed to overcome this problem. Covalent tethered polymer chains are realized by three different approaches: the “grafting to”, the “grafting from” and the “grafting through” method. This article reviews the synthesis of end-grafted polymer chains onto inorganic nanoparticles using “controlled/living” polymerization techniques, which allow control over the polymer characteristics and the grafting density of the end-tethered polymer chains.

Polymer-functionalized nanoparticles: from stealth viruses to biocompatible quantum dots

In this article, we focus on nanoparticles that have been functionalized by polymers. We draw our examples from nanoparticle systems that have found biomedical and therapeutic applications. Our aim is to highlight the physical principles that might explain why these systems have been found to be successful in biomedical applications and to highlight other physical properties that might lead to new applications. We consider viruses, gold nanoparticles, magnetic nanoparticles and quantum dots, focussing attention on the ways in which functionalization by polymers has been used to alter the physical characteristics of the particular nanoparticle to improve its function as a possible therapy. In the case of viral vectors, polymer functionalization tunes the biocompatibility, suppressing the binding of antibodies and conferring the nanoparticle with stealth properties. By contrast, the inorganic nanoparticles comprise materials in a form that is not normally encountered in the human body, and polymer functionalization is necessary to ensure biocompatibility.

Synthesis of biocompatible sterically-stabilized poly(2-(methacryloyloxy)ethyl phosphorylcholine) latexes via dispersion polymerization in alcohol/water mixtures

Poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) is soluble in either 2-propanol or water but becomes insoluble in certain alcohol-rich 2-propanol/water mixtures. We have exploited this unusual cononsolvency behavior in order to prepare new biocompatible sterically stabilized PMPC latexes via nonaqueous dispersion polymerization in 2-propanol/water mixtures. All polymerizations were conducted in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) as a reactive stabilizer, with some formulations including ethylene glycol dimethacrylate (EGDMA) as a cross-linker. Under optimized conditions, unimodal size distributions could be obtained with a mean latex diameter of approximately 1 microm, as judged by laser diffraction and DLS. The mean latex diameter depended on both the PEGMA and initiator concentration but was almost independent of the cross-linking density. Smaller PMPC latexes were obtained by increasing the alcohol content of the dispersion medium. On dilution with water, these latexes acquired microgel character. The microgel solution viscosity was insensitive to added salt due to the so-called "antipolyelectrolyte" effect, which is characteristic of polyzwitterions. Finally, copolymerization of MPC with a fluorescein-based methacrylic comonomer produced fluorescently labeled PMPC latexes, which may have potential biomedical applications.

Tuning the thickness of polymer brushes grafted from nonlinearly growing multilayer assemblies

A new versatile method for tuning the thickness of surface-tethered polymer brushes is introduced. It is based on the combination of polyelectrolyte multilayer deposition and surface-initiated atom transfer radical polymerization. To control the thickness of the brushes, the nonlinear growth of certain polyelectrolyte multilayer systems is exploited. The method is demonstrated to work with different polyelectrolytes and different monomers. The relevance for applications is demonstrated by cell adhesion experiments on grafted thermoresponsive polymer layers with varying thickness.

Applications of polymer brushes in protein analysis and purification

This review examines the application of polymer brush-modified flat surfaces, membranes, and beads for protein immobilization and isolation. Modification of porous substrates with brushes yields membranes that selectively bind tagged proteins to give 99% pure protein at capacities as high as 100 mg of protein per cubic centimeter of membrane. Moreover, enrichment of phosphopeptides on brush-modified matrix-assisted laser desorption/ionization (MALDI) plates allows detection and characterization of femtomole levels of phosphopeptides by MALDI mass spectrometry. Because swollen hydrophilic brushes can resist nonspecific protein adsorption while immobilizing a high density of proteins, they are attractive as substrates for protein microarrays. This review highlights the advantages of polymer brush-modified surfaces over self-assembled monolayers and identifies some research needs in this area.