Macromolecular anchoring layers for polymer grafting: comparative study

A Novel Bio-Adhesive Mesh System for Medical Implant Applications: In Vivo Assessment in a Rabbit Model

Injectable surgical sealants and adhesives, such as biologically derived fibrin gels and synthetic hydrogels, are widely used in medical products. While such products adequately adhere to blood proteins and tissue amines, they have poor adhesion with polymer biomaterials used in medical implants. To address these shortcomings, we developed a novel bio-adhesive mesh system utilizing the combined application of two patented technologies: a bifunctional poloxamine hydrogel adhesive and a surface modification technique that provides a poly-glycidyl methacrylate (PGMA) layer grafted with human serum albumin (HSA) to form a highly adhesive protein surface on polymer biomaterials. Our initial in vitro tests confirmed significantly improved adhesive strength for PGMA/HSA grafted polypropylene mesh fixed with the hydrogel adhesive compared to unmodified mesh. Toward the development of our bio-adhesive mesh system for abdominal hernia repair, we evaluated its surgical utility and in vivo performance in a rabbit model with retromuscular repair mimicking the totally extra-peritoneal surgical technique used in humans. We assessed mesh slippage/contraction using gross assessment and imaging, mesh fixation using tensile mechanical testing, and biocompatibility using histology. Compared to polypropylene mesh fixed with fibrin sealant, our bio-adhesive mesh system exhibited superior fixation without the gross bunching or distortion that was observed in the majority (80%) of the fibrin-fixed polypropylene mesh. This was evidenced by tissue integration within the bio-adhesive mesh pores after 42 days of implantation and adhesive strength sufficient to withstand the physiological forces expected in hernia repair applications. These results support the combined use of PGMA/HSA grafted polypropylene and bifunctional poloxamine hydrogel adhesive for medical implant applications.

Influence of the Atmosphere on the Wettability of Polymer Brushes

Polymer brushes, i.e., end-tethered polymer chains on substrates, are sensitive to adaptation, e.g., swelling, adsorption, and reorientation of the surface molecules. This adaptation can originate from a contacting liquid or atmosphere for partially wetted substrates. The macroscopic contact angle of the aqueous drop can depend on both adaptation mechanisms. We analyze how the atmosphere around an aqueous droplet determines the resulting contact angle of the wetting droplet on polymer brush surfaces. Poly(N-isopropylacrylamide) (PNiPAAm)-based brushes are used due to their exceptional sensitivity to solvation and liquid mixture composition. We develop a method that reliably measures wetting properties when the drop and the surrounding atmosphere are not in equilibrium, e.g., when evaporation and condensation tend to contaminate the liquid of the drop and the atmosphere. For this purpose, we use a coaxial needle in the droplet, which continuously exchanges the wetting liquid, and in addition, we constantly exchange the almost saturated atmosphere. Depending on the wetting history, PNiPAAm can be prepared in two states, state A with a large water contact angle (∼65°) and state B with a small water contact angle (∼25°). With the coaxial needle, we can demonstrate that the water contact angle of a sample in state B significantly increases by ∼30° when a water-free atmosphere is almost saturated with ethanol, compared to an ethanol-free atmosphere at 50% relative humidity. For a sample in state A, the relative humidity has little influence on the water contact angle.

Effect of the Composition of Copolymers Based on Glycidyl Methacrylate and Fluoroalkyl Methacrylates on the Free Energy and Lyophilic Properties of the Modified Surface

This study proposes to use reactive copolymers based on glycidyl methacrylate and fluoroalkyl methacrylates with a low fluorine content in the monomer unit as agents to reduce the surface free energy (SFE). This work reveals the effect of the structure and composition of copolymers on the SFE and water-repellent properties of these coatings. On a smooth surface, coatings based on copolymers of glycidyl methacrylate and fluoroalkyl methacrylates with fluorine atoms in the monomer unit ranging from three to seven are characterized by SFE values in the range from 25 to 13 mN/m, which is comparable to the values for polyhedral oligomeric silsesquioxanes and perfluoroalkyl acrylates. On textured aluminum surfaces, the obtained coatings provide time-stable superhydrophobic properties with contact angles up to 170° and sliding angles up to 2°. The possibility of using copolymers based on glycidyl methacrylate and fluoroalkyl methacrylates for the creation of self-cleaning polymer coatings is shown.

Memory effects in polymer brushes showing co-nonsolvency effects

Densely packed polymer chains grafted to a substrate, especially polymer brushes, have been studied intensively. Of special interest are systems that react to changes in external conditions or"remember" previous conditions. With this focus, we explore the properties of PNiPAAm brushes and relate published work to own results. The co-nonsolvency effect leads to a collapse of a PNiPAAm brush for a certain mixing ratio of ethanol in water. This also influences the wetting behavior of PNiPAAm brushes. We show that through prewetting of a brush with different liquids (water and ethanol), the contact angle of subsequent water drops changes significantly. To explain this change, the swelling of the brush was investigated with spectroscopic ellipsometry and the orientation of the molecules at the surface with sum-frequency generation (SFG). Only little change in swelling was found. The SFG measurements reveal in the ethanol prewetted case a well ordered hydrophobic methyl layer at the interface, which is consistent with the contact angle measurement.

FRET-Integrated Polymer Brushes for Spatially Resolved Sensing of Changes in Polymer Conformation

Polymer brush surfaces that alter their physical properties in response to chemical stimuli have the capacity to be used as new surface-based sensing materials. For such surfaces, detecting the polymer conformation is key to their sensing capabilities. Herein, we report on FRET-integrated ultrathin (<70 nm) polymer brush surfaces that exhibit stimuli-dependent FRET with changing brush conformation. Poly(N-isopropylacrylamide) polymers were chosen due their exceptional sensitivity to liquid mixture compositions and their ability to be assembled into well-defined polymer brushes. The brush transitions were used to optically sense changes in liquid mixture compositions with high spatial resolution (tens of micrometers), where the FRET coupling allowed for noninvasive observation of brush transitions around complex interfaces with real-time sensing of the liquid environment. Our methods have the potential to be leveraged towards greater surface-based sensing capabilities at intricate interfaces.

Methods for the Assembly and Characterization of Polyelectrolyte Multilayers as Microenvironments to Modulate Human Mesenchymal Stromal Cell Response

Thin films are of interest in materials design because they allow for the modification of surface properties of materials while the bulk properties of the material are largely unaffected. In this work, we outline methods for the assembly of thin films using a technique known as layer-by-layer (LbL). Furthermore, their interactions with human mesenchymal stromal cells (hMSCs) are discussed. hMSCs are a subject of growing interest because of their potential to treat or cure diseases, given their immunosuppressive properties, multipotent differentiation capabilities, and tissue regeneration capabilities. Numerous improvements and modifications have been suggested for the harvesting, treatment, and culture of hMSCs prior to their administration in human subjects. Here, we discuss methods to assess the interactions of hMSCs with thin LbL-assembled films of heparin and collagen. Three different methods are discussed. The first details the preparation of heparin/collagen multilayers on different surfaces and the seeding of cells on these multilayers. The second method details the characterization of multilayers, including techniques to assess the thickness, roughness, and surface charge of the multilayers, as well as in situ deposition of multilayers. The third method details the analysis of cell interactions with the multilayers, including techniques to assess proliferation, viability, real-time monitoring of hMSC behavior, analysis of hMSC-adhesive proteins on the multilayers, immunomodulatory factor expression of hMSCs, and cytokine expression on heparin/collagen multilayers. We propose that the methods described in this work will assist in the design and characterization of LbL-assembled thin films and the analysis of hMSCs cultured on these thin films.

Enzyme immobilization on protein-resistant PNIPAAm brushes: impact of biotin linker length on enzyme amount and catalytic activity

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes with terminal click functionality can be used to selectively immobilize enzymes. Exploiting their inherent protein-repellent and thus non-fouling properties, surfaces with specific bioactivity can be created in this way. This report describes the functionalization of alkyne-PNIPAAm brushes with two biotin linkers of different poly(ethylene glycol) (PEG) spacer length via click chemistry and the subsequent immobilization of streptavidin-conjugated horseradish peroxidase (SA-HRP) by the strong interaction between biotin and streptavidin. Spectroscopic ellipsometry is used to quantify the biotin and the SA-HRP amount on the PNIPAAm brushes. Enzyme activities are determined by UV‑vis spectroscopy. A better accessibility to the alkyne-functionalized chain ends for the short biotin-PEG₃ linker leads to a higher biotin amount on these PNIPAAm brushes, which in turn results in higher SA‑HRP amounts on biotin-PEG₃-modified brushes in the swollen (20 °C) and collapsed state (37 °C) compared to biotin-PEG₂₃-modified ones. For both linkers, an increased immobilization temperature leads to higher SA-HRP amounts due to an enhanced reaction kinetics and mobility. All immobilized SA-HRP amounts are in range of 81-98% monolayer coverage, except for SA-HRP immobilized on biotin-PEG₂₃-modified PNIPAAm brushes at 20 °C, exhibiting only 43% monolayer coverage. A high mobility of the biotin linker combined with a low surface loading of SA-HRP is found to be beneficial for the activity of SA-HRP. Hence, the highest specific activity as measured by HRP-catalyzed 3,3´,5,5´‑tetramethylbenzidine (TMB) oxidation is found for a low SA-HRP amount on biotin-PEG₂₃-modified brushes.

Cononsolvency Transition of Polymer Brushes: A Combined Experimental and Theoretical Study

In this study, the cononsolvency transition of poly(N-isopropylacrylamide) (PNiPAAm) brushes in aqueous ethanol mixtures was studied by using Vis-spectroscopic ellipsometry (SE) discussed in conjunction with the adsorption-attraction model. We proved that the cononsolvency transition of PNiPAAm brushes showed features of a volume phase transition, such as a sharp collapse, reaching a maximum decrease in thickness for a very narrow ethanol volume composition range of 15% to 17%. These observations are in agreement with the recently published preferential adsorption model of the cononsolvency effect.

Molecular Interactions and Hydration States of Ultrathin Functional Films at the Solid-Liquid Interface

We significantly improve the infrared analysis of ultrathin films in aqueous environments by employing in situ infrared ellipsometry. Combining it with rigorous optical modeling avoids otherwise typical misinterpretations of spectral features and enables the simultaneous quantification of chemical composition, hydration states, structure, and molecular interactions. We apply this approach to study covalently end-grafted, nanometer-thin brushes of poly(N-isopropylacrylamide), a thermoresponsive model polymer for proteins at solid-liquid interfaces. Quantitative analyses are based on a dielectric layer model that accounts for film swelling and deswelling, hydration of hydrophilic amide and hydrophobic isopropyl side groups, as well as molecular interactions of the polymer's amide moieties. We thereby quantify the hydration and structure dependence of intra- and intermolecular C═O···H-N and C═O···H₂O hydrogen bonds, elucidating their role in the brush's temperature-induced phase separation. The presented method is directly applicable to functional and biorelated films like polymer and polypeptide layers, which is of topical interest for interface studies, such as membrane processes and protein unfolding.

Gradient Polymer Nanofoams for Encrypted Recording of Chemical Events

We have fabricated gradient-grafted nanofoam films that are able to record the presence of volatile chemical compounds in an offline regime. In essence, the nanofoam film (100-300 nm thick) is anchored to a surface cross-linked polymer network in a metastable extended configuration that can relax back to a certain degree upon exposure to a chemical vapor. The level of the chain relaxation is associated with thermodynamic affinity between the polymer chains and the volatile compounds. In our design, the chemical composition of the nanofoam film is not uniform; therefore, the film possesses a gradually changing local affinity to a vapor along the surface. Upon vapor exposure, the nonuniform changes in local film morphology provide a permanent record or "fingerprint" for the chemical event of interest. This permanent modification in the film structure can be directly detected via changes not only in the film surface profile but also in the film optical characteristics. To this end, we demonstrated that sensing/recording nanofoam films can be prepared and interrogated on the surfaces of optical waveguides, microring optical resonators. It is important that the initial surface profile and structure of the nanofoam film are encrypted by the distinctive conditions that were used to fabricate the film and practically impossible to replicate without prior knowledge.

Monitoring the Switching of Single BSA-ATTO 488 Molecules Covalently End-Attached to a pH-Responsive PAA Brush

We describe a novel combination of a responsive polymer brush and a fluorescently labeled biomolecule, where the position of the biomolecule can be switched from inside to outside the brush and vice versa by a change in pH. For this, we grafted ultrathin, amino-terminated poly(acrylic acid) brushes to glass and silicon substrates. Individual bovine serum albumin (BSA) molecules labeled with fluorophore ATTO 488 were covalently end-attached to the polymers in this brush using a bis-N-succinimidyl-(pentaethylene glycol) linker. We investigated the dry layer properties of the brush-protein ensemble, and it is swelling behavior using spectroscopic ellipsometry. Total internal reflection fluorescence (TIRF) microscopy enabled us to study the distance-dependent switching of the fluorescently labeled protein molecules. The fluorescence emission from the labeled proteins ceased (out-state) when the polymer chains stretched away from the interface under basic pH conditions, and fluorescence recurred (in-state) when the chains collapsed under acidic conditions. Moreover, TIRF allowed us to study the fluorescence switching behavior of fluorescently labeled BSA molecules down to the single-molecule level, and we demonstrate that this switching is fast but that the exact intensity during the in-state is the result of a more random process. Control experiments verify that the switching behavior is directly correlated to the responsive behavior of the polymer brush. We propose this system as a platform for switchable sensor applications but also as a method to study the swelling and collapse of individual polymer chains in a responsive polymer brush.

Computing thermomechanical properties of dry homopolymers used as raw materials for formulation of biomedical hydrogels

Different static properties have been calculated with COMPASS force field for polyacrylamide, poly(2-hydroxyethylacrylate) (HEA), poly(2-hydroxyethylmethacrylate) (HEMA), poly(glycidylmethacrylate) (GMA), polyethylene glycol (PEG), and poly(2,2,2-trifluoroethylmethacrylate) (TFEM). For each polymers, the calculated values were averaged on five equilibrated configurations of amorphous cell composed of one atactic chain containing 100 repeat units. The ranking obtained from the densities calculated at 300 K is TFEM > HEA ≈ xpolycrylamide > HEMA ≈ GMA > PEG. Concerning the glass transition temperature we have obtained polyacrylamide > HEMA ≈ GMA ≈ HEA > PEG, and polyacrylamide > HEMA ≈ HEA > GMA ≈ PEG > TFEM for the bulk modulus. The calculated results, when available, have been compared with experimental data coming from literature.

Functional Reactive Polymer Electrospun Matrix

Synthetic multifunctional electrospun composites are a new class of hybrid materials with many potential applications. However, the lack of an efficient, reactive large-area substrate has been one of the major limitations in the development of these materials as advanced functional platforms. Herein, we demonstrate the utility of electrospun poly(glycidyl methacrylate) films as a highly versatile platform for the development of functional nanostructured materials anchored to a surface. The utility of this platform as a reactive substrate is demonstrated by grafting poly(N-isopropylacrylamide) to incorporate stimuli-responsive properties. Additionally, we demonstrate that functional nanocomposites can be fabricated using this platform with properties for sensing, fluorescence imaging, and magneto-responsiveness.

Tunable Hydrophilic or Amphiphilic Coatings: A "Reactive Layer Stack" Approach

Thin films with tunable properties are very interesting for potential applications as functional coatings with, for example, anti-icing or improved easy-to-clean properties. A novel "reactive layer stack" approach was developed to create covalently grafted mono- and multilayers of poly(glycidyl methacrylate)/poly(tert-butyl acrylate) diblock copolymers. Because these copolymers contain poly(glycidyl methacrylate) blocks they behave as self-cross-linking materials after creation of acrylic acid functionalities by splitting off the tert-butyl units. The ellipsometrically determined coating thickness of the resulting hydrophilic multilayers depended linearly on the number of applied layers. Amphiphilic films with tunable wettability were prepared using triblock terpolymers with an additional poly(methyl methacrylate) block. The mechanism of the formation of the (multi)layers was investigated in detail by studying the acidolysis of the surface-linked tert-butyl acrylate blocks by infrared reflection absorbance spectroscopy, accompanied by surface analysis using atomic force microscopy and contact angle measurements. In the case of the amphiphilic and switchable terpolymer layers this reaction was very sensitive to the used acidic reagent.

Ultrathin random copolymer-grafted layers for block copolymer self-assembly

Hydroxyl-terminated P(S-r-MMA) random copolymers (RCPs) with molecular weights (Mn) from 1700 to 69000 and a styrene unit fraction of approximately 61% were grafted onto a silicon oxide surface and subsequently used to study the orientation of nanodomains with respect to the substrate, in cylinder-forming PS-b-PMMA block copolymer (BCP) thin films. When the thickness (H) of the grafted layer is greater than 5-6 nm, a perpendicular orientation is always observed because of the efficient decoupling of the BCP film from the polar SiO2 surface. Conversely, if H is less than 5 nm, the critical thickness of the grafted layer, which allows the neutralization of the substrate and promotion of the perpendicular orientation of the nanodomains in the BCP film, is found to depend on the Mn of the RCP. In particular, when Mn = 1700, a 2.0 nm thick grafted layer is sufficient to promote the perpendicular orientation of the PMMA cylinders in the PS-b-PMMA BCP film. A proximity shielding mechanism of the BCP molecules from the polar substrate surface, driven by chain stretching of the grafted RCP molecules, is proposed.

Temperature controlled shape change of grafted nanofoams

We demonstrated that nanoscale-level actuation can be, in principle, achieved with grafted polymer nanofoams by forces associated with conformational changes of stretched macromolecular chains. The nanofoams are fabricated via a two-step procedure. First, the "grafting to" technique is used to obtain a 20-200 nm anchored and cross-linked poly(glycidyl methacrylate) film. Second, the film is swollen in solvent and freeze dried until the solvent is sublimated. The grafted nanofoam possesses the behavior of a shape-memory material, exhibiting gradual mechanical contraction at the nanometer scale as temperature is increased. Both the thickness and shape-recovery ratio of the nanofoam have a close to linear dependency on temperature. We also demonstrated that by modification of the poly(glycidyl methacrylate) nanofoam with grafting low molecular weight polymers, one can tune an absolute nanoscale mechanical response of the porous polymer film.

Mid-infrared materials and devices on a Si platform for optical sensing

In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiN x waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Synthesis of raspberry-like poly(styrene-glycidyl methacrylate) particles via a one-step soap-free emulsion polymerization process accompanied by phase separation

We herein report a facile method to prepare raspberry-like poly(styrene-glycidyl methacrylate) [P(S-GMA)] particles with controllable structure via a one-step soap-free emulsion polymerization process accompanied by phase separation. In this method, corona particles with a size of 10-20 nm were produced in situ in the later polymerization stage by the migrating of S-enriched polymers from GMA-enriched core particles. The size of the corona particles and the roughness of the raspberry-like particles can be easily controlled by adjusting the amount of styrene (S), glycidyl methacrylate (GMA), and divinylbenzene (DVB). The structure of raspberry-like P(S-GMA) particles was confirmed by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. A possible mechanism of the formation of raspberry-like particles was proposed.

Affinity separation of lectins using porous membranes immobilized with glycopolymer brushes containing mannose or N-acetyl-d-glucosamine

Porous membranes with glycopolymer brushes were prepared as biomaterials for affinity separation. Glycopolymer brushes contained acrylic acid and D-mannose or N-acetyl-D-glucosamine, and were formed on substrates by surface-initiated atom transfer radical polymerization. The presence of glycopolymer brush was confirmed by X-ray photoelectron spectroscopy, contact angle, and ellipsometry measurements. The interaction between lectin and the glycopolymer immobilized on glass slides was confirmed using fluorescent-labeled proteins. Glycopolymer-immobilized surfaces exhibited specific adsorption of the corresponding lectin, compared with bovine serum albumin. Lectins were continuously rejected by the glycopolymer-immobilized membranes. When the protein solution was permeated through the glycopolymer-immobilized membrane, bovine serum albumin was not adsorbed on the membrane surface. In contrast, concanavalin A and wheat germ agglutinin were rejected by membranes incorporating D-mannose or N-acetyl-D-glucosamine, respectively. The amounts of adsorbed concanavalin A and wheat germ agglutinin was increased five- and two-fold that of adsorbed bovine serum albumin, respectively.

Influence of the polymeric interphase design on the interfacial properties of (fiber-reinforced) composites

In fiber-reinforced composites, the interphase nanostructure (i.e., the extended region between two phases in contact) has a pronounced influence on their interfacial adhesion. This work aims at establishing a link between the interphase design of PS-based polymeric fiber coatings and their influence on the micromechanical performance of epoxy-based composite materials. Thiol-ene photochemistry was utilized to introduce a polymeric gradient on silica-like surfaces following a two-step approach without additional photoinitiator. Two complementary grafting-techniques were adapted to modify glass fibers: "Grafting-onto" deposition of PB-b-PS diblock copolymers for thin-film coatings (thickness<20 nm) at low grafting density (<0.1 chains/nm2)--and "grafting-from" polymerization for brush-like PS homopolymer coatings of higher thickness (up to 225 nm) and higher density. Polymer-coated glass fibers were characterized for polymer content using thermogravimetric analysis (TGA) and their nanostructural morphologies by scanning electron microscopy (SEM). Model substrates of flat glass and silicon were studied by atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). The change in interfacial shear strength (IFSS) due to fiber modification was determined by a single fiber pull-out experiment. Thick coatings (>40 nm) resulted in a 50% decrease in IFSS. Higher shear strength occurred for thinner coatings of homopolymer and for lower grafting densities of copolymer. Increased IFSS (10%) was found upon dilution of the surface chain density by mixing copolymers. We show that the interfacial shear strength can be increased by tailoring of the interphase design, even for systems with inherently poor adhesion. Perspectives of polymeric fiber coatings for tailored matrix-fiber compatibility and interfacial adhesion are discussed.

Temperature-sensitive swelling of poly(N-isopropylacrylamide) brushes with low molecular weight and grafting density

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) brushes with different molecular weights M(n) and grafting densities σ were prepared by the "grafting-to" method. Changes in their physicochemical properties according to temperature were investigated with the help of in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush criteria indicate a transition between a brush conformation below the lower critical solution temperature (LCST) and an intermediate to mushroom conformation above the LCST. By in situ ellipsometry distinct changes in the brush layer parameters (wet thickness, refractive index, buffer content) were observed. A broadening of the temperature region with maximum deswelling occurred with decreasing grafting density. The brush layer properties were independent of the grafting density below the LCST, but showed a virtually monotonic behavior above the LCST. The midtemperature ϑ(half) of the deswelling process increased with increasing grafting density. Thus grafting density-dependent design parameters for such functional films were presented. For the first time, ATR-FTIR spectroscopy was used to monitor segment density and hydrogen bonding changes of these very thin PNIPAAm brushes as a function of temperature based on significant variations of the methyl stretching, Amide I, as well as Amide II bands with respect to intensity and wavenumber position. No dependence on M(n) and σ in the wavenumber shift of these bands above the LCST was found. The temperature profile of these band intensities and thus segment density was found to be rather step-like, exceeding temperatures around the LCST, while the respective profile of their wavenumber positions suggested continuous structural and hydration processes. Remaining buffer amounts and residual intermolecular segment/water interaction in the collapsed brushes above the LCST could be confirmed by both in situ methods.

Mechanism of nanoparticle actuation by responsive polymer brushes: from reconfigurable composite surfaces to plasmonic effects

The mechanism of nanoparticle actuation by stimuli-responsive polymer brushes triggered by changes in the solution pH was discovered and investigated in detail in this study. The finding explains the high spectral sensitivity of the composite ultrathin film composed of a poly(2-vinylpyridine) (P2VP) brush that tunes the spacing between two kinds of nanoparticles-gold nanoislands immobilized on a transparent support and gold colloidal particles adsorbed on the brush. The optical response of the film relies on the phenomenon of localized surface plasmon resonances in the noble metal nanoparticles, giving rise to an extinction band in visible spectra, and a plasmon coupling between the particles and the islands that has a strong effect on the band position and intensity. Since the coupling is controlled by the interparticle spacing, the pH-triggered swelling-shrinking transition in the P2VP brush leads to pronounced changes in the transmission spectra of the hybrid film. It was not established in the previous publications how the actuation of gold nanoparticles within a 10-15 nm interparticle distance could result in the 50-60 nm shift in the absorbance maximum in contrast to the model experiments and theoretical estimations of several nanometer shifts. In this work, the extinction band was deconvoluted into four spectrally separated and overlapping contributions that were attributed to different modes of interactions between the particles and the islands. These modes came into existence due to variations in the thickness of the grafted polymeric layer on the profiled surface of the islands. In situ atomic force microscopy measurements allowed us to explore the behavior of the Au particles as the P2VP brush switched between the swollen and collapsed states. In particular, we identified an interesting, previously unanticipated regime when a particle position in a polymer brush was switched between two distinct states: the particle exposed to the surface of the collapsed layer and the particle engulfed by the swollen brush. On average, the characteristic distance between the particles and the islands increased upon the brush swelling. The observed behavior was a result of the anchoring of the particles to polymeric chains that limited the particles' vertical motion range. The experimental findings will be used to design highly sensitive optical nanosensors based on a polymer-brush-modulated interparticle plasmon coupling.

Towards universal enrichment nanocoating for IR-ATR waveguides

Polymer multilayered nanocoating capable of concentrating various chemical substances at IR-ATR waveguide surfaces is described. The coating affinity to an analyte played a pivotal role in sensitivity enhancement of the IR-ATR measurements, since the unmodified waveguide did not show any analyte detection.

Probing the collapse dynamics of poly(N-isopropylacrylamide) brushes by AFM: effects of co-nonsolvency and grafting densities

Collapse of poly(N-isopropylacrylamide) (PNIPAM) brushes in the mixed solvent system (water/methanol 50% v/v) is studied by in-situ atomic-force microscopy (AFM). PNIPAM brushes with three different grafting densities and similar chain lengths are synthesized via surface-initiated atom-transfer radical polymerization. By changing the solvent from water to a water/methanol (50% v/v) mixture, the polymer brushes switch from a swollen to collapsed state. AFM force measurements using a silica colloidal probe attached to the tip are employed to obtain the Young's moduli of the polymer brushes in different solvation states. The collapse dynamics of the brush is followed by monitoring the pull-off force (adherence) in situ. The modulus of the swollen high-density polymer brush is four times lower than that of the same brush in the collapsed state. It is shown that in the case of the high-density polymer brush with a thickness (t(in water) ) of 900 nm, the collapse takes place in a time scale of ~25 s, whereas the collapse occurs faster for the medium-density brush (t(in water) = 630 nm) and much more rapidly for the low-density brush (t(in water) = 80 nm). This difference in the response kinetics is primarily ascribed to the time needed for solvent exchange in the polymer brushes.

Thermo-responsive copolymer coatings for flow regulation on demand in glass microcapillaries

This study presents thermo-responsive on-demand regulation of water flow rate in glass microcapillaries with a recently developed water-stable, stimuli-responsive poly(methyl methacrylate/N-isopropyl acrylamide) [P(MMA/NIPAM)] copolymer grafted at the inner walls. It is shown that the grafted coatings are stable and can withstand significant tractions under temperature variation. Such microcapillaries allow flow regulation on demand by changing temperature across the lower critical solution temperature (LCST) of the copolymer layer, which makes it swell or shrink, thus changing the bore available for pressure-driven flow. The grafted copolymer layers were subjected to different pressure drops applied to the capillary open ends, as well as to periodic temperature variation across the copolymer LCST to determine the best grafting conditions for microfluidic operation. Then, by varying the temperature, the flow rate in the capillaries was changed periodically on demand due to the swelling/shrinkage of the grafted copolymer layer. It was also shown that the entrapped air bubbles are present in the coating which can result in an apparent slip.

In situ studies on the switching behavior of ultrathin poly(acrylic acid) polyelectrolyte brushes in different aqueous environments

The pH-dependent switching of a poly(acrylic acid) (PAA) polyelectrolyte brush was investigated in situ using infrared spectroscopic ellipsometry (IRSE). The brush was synthesized by a "grafting to" procedure on silicon substrate with a native oxide layer. The overall thickness of the PAA brush in the dry state was approximately 5 nm. Reversible switching of the polymer brush was studied at titration from pH 2 to 10 and back in steps of 1 pH unit. The switching process was observed by monitoring the characteristic vibrational bands of the carboxylic groups of the PAA molecules. Decreasing of the C=O vibrational band amplitude and arising of a COO(-) vibrational band proved the chemical changes in the molecular structure of the brushes due to changes of the pH value in the aqueous solution. Due to the strong absorption of these bands in the IR region, the switching process could be monitored clearly. Switching the brush in several cycles with increasing and decreasing pH value showed a hysteresis-like behavior. For the first time, such hysteresis is observed in titration experiments of polyelectrolyte brushes. This behavior is attributed to the complex mechanisms of the ion's mobility in the brush layer which is explained with a suggested simplifying model describing the influence of ions inside the brush layer. In addition to the IRSE measurements, X-ray standing waves (XSW), in situ visible ellipsometry, and contact angle measurements have been performed and were in good agreement with the results from IRSE. Repetition of the in situ measurement cycles proved the good reversibility of the switching process which is highly important for practical applications of polymer brushes.

Fluorescent nanoparticles stabilized by poly(ethylene glycol) containing shell for pH-triggered tunable aggregation in aqueous environment

Fluorescent silica nanoparticles decorated with a responsive shell, a mixed polymer brush, were synthesized. Specifically, a poly(2-vinylpyridine), P2VP, and poly(ethylene glycol), PEG, binary polymer brush was synthesized on silica nanoparticles via the "grafting to" technique. The selection of the components (PEG and P2VP) for the responsive brush shell was motivated by potential biomedical applications. Poly(glycidyl methacrylate), PGMA, labeled with Rhodamine B, RhB, was used to form a reactive and fluorescent shell on the nanoparticle surface. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change in pH. At lower pH, aqueous dispersions of the particles are stable, since PEG and P2VP are water-soluble, extended and contribute to the steric and electrostatic mechanisms of colloidal stability. An increase of pH to 6 causes a slow aggregation as a consequence of the hydrophobic attraction between the collapsed and almost nonprotonated P2VP macromolecules. The aggregation was well controlled and occurred within 90-120 min of the pH change. The aggregation was fully reversible after the decrease in pH. The pH variation did not quench the fluorescence of the colloidal suspensions. The pH-tunable aggregation of the fluorescent nanoparticles could find diverse applications for labeling and contrasting of cells and tissues when the size of the label and the intensity of the optical signals can be tuned by and related to the pH of the host environment.

Click chemistry grafting of poly(ethylene glycol) brushes to alkyne-functionalized pseudobrushes

A versatile method for the grafting of azide-terminated polymer chains to alkyne-functionalized pseudobrushes by the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition has been developed. First, poly[(propargyl methacrylate)-r-(glycidyl methacrylate)-r-(methyl methacrylate)] random copolymers with monomer ratios of respectively 27/27/46, 41/31/28, and 45/55/0 were synthesized by RAFT polymerization. Then, dense alkyne-functionalized pseudobrushes were grafted in melt by thermal ring-opening of the glycidyl groups by the silanols from the silicon substrate. Finally, the grafting of tailor-made alpha-methoxy-omega-azido-poly(ethylene glycol)s (M(w) approximately 5000, 20,000, and 50,000 g/mol) by Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition was performed in sealed reactors at 60 degrees C for 72 h using a polymer weight fraction of 10% in tetrahydrofuran and Cu(PPh(3))(3)Br/DIPEA as the catalytic system. Alkyne-functionalized pseudobrushes and poly(ethylene glycol) brushes were characterized by ellipsometry, scanning probe microscopy, and water contact angle measurements. This "grafting-to" approach represents a fast and versatile method to provide thick and homogeneous polymer brushes with a high surface coverage. A major benefit of this strategy is the tunable and versatile tethering of alkyne functionalities to silicon substrates using a straightforward spin-coating procedure.