High density scaffolding of functional polymer brushes: surface initiated atom transfer radical polymerization of active esters

Modification of nanocellulose via atom transfer radical polymerization and its reinforcing effect in waterborne UV-curable resin

Cellulose nanocrystals (CNCs) are green reinforcing materials, and their potential has been evaluated in the preparation of waterborne UV-curable resin composites with high-performance. Herein, we present a novel and scalable approach for preparing surface-modified CNCs with acrylic-based polymers to strengthen the compatibility and interaction between CNCs and UV-curable resins. Using tert-butyl acrylate as the monomer, the nanocellulose grafted copolymer CNC-g-PtBA was successfully synthesized via atom transfer radical polymerization (ATRP) in the presence of a macromolecular initiator. Then, the CNC-g-PtBA is blended into the acrylic resin as a nanofiller to prepare the UV-curable nanocomposite. The results indicated that the contact angle of the CNCs increased from 38.7° to approximately 74.8°, and their thermal stability was significantly improved after graft modification. This contributed to the effective alleviation of the agglomeration phenomenon of nanocomposites due to the high hydrophilicity of pure CNCs. Notably, not only was the UV curing efficiency of the nanocomposites greatly increased but the mechanical properties were also further enhanced. Specifically, with the addition of 0.5 wt% CNC-g-PtBA, the curing time of the nanocomposite was shortened from >30 mins down to approximately 6 mins, and the bending strength was increased from 10 MPa for the original resin and 5 MPa for the addition of pure CNCs to 14.3 MPa, and the bending modulus was also greatly increased (up to approximately 730 MPa). Compared to pure CNCs, they are compatible with the resin, exhibiting high mechanical strength and flexibility, and have virtually no effect on the light transmission of the nanocomposites. Additionally, dielectric analysis (DEA) was used to monitor the dielectric constant and conductivity of the UV-curable nanocomposites in real time to further characterize their curing kinetics. The permittivity of these nanocomposites increased by 125 % compared to pristine resin, which shows potential for applications in high dielectric composites or for improving electrical conductivity. This work provides a feasible method for preparing UV-curable nanocomposites with high curing efficiency and permittivity, realizing a wider application of this high-performance nanocomposite.

Surface Design of Antifouling Vascular Constructs Bearing Biofunctional Peptides for Tissue Regeneration Applications

Antifouling polymer layers containing extracellular matrix-derived peptide motifs offer promising new options for biomimetic surface engineering. In this contribution, we report the design of antifouling vascular grafts bearing biofunctional peptide motifs for tissue regeneration applications based on hierarchical polymer brushes. Hierarchical diblock poly(methyl ether oligo(ethylene glycol) methacrylate-block-glycidyl methacrylate) brushes bearing azide groups (poly(MeOEGMA-block-GMA-N₃)) were grown by surface-initiated atom transfer radical polymerization (SI-ATRP) and functionalized with biomimetic RGD peptide sequences. Varying the conditions of copper-catalyzed alkyne-azide “click” reaction allowed for the immobilization of RGD peptides in a wide surface concentration range. The synthesized hierarchical polymer brushes bearing peptide motifs were characterized in detail using various surface sensitive physicochemical methods. The hierarchical brushes presenting the RGD sequences provided excellent cell adhesion properties and at the same time remained resistant to fouling from blood plasma. The synthesis of anti-fouling hierarchical brushes bearing 1.2 × 10³ nmol/cm² RGD biomimetic sequences has been adapted for the surface modification of commercially available grafts of woven polyethylene terephthalate (PET) fibers. The fiber mesh was endowed with polymerization initiator groups via aminolysis and acylation reactions optimized for the material. The obtained bioactive antifouling vascular grafts promoted the specific adhesion and growth of endothelial cells, thus providing a potential avenue for endothelialization of artificial conduits.

Facile synthesis of optically active helical poly(phenyl isocyanide) brushes on a silicon surface and their chiral resolution ability

Optically active helical poly(phenyl isocyanide) brushes grafted on a silicon surface were prepared and their chiral resolution ability was investigated.

Versatile Methodology for Glycosurfaces: Direct Ligation of Nonderivatized Reducing Saccharides to Poly(pentafluorophenyl acrylate) Grafted Surfaces via Hydrazide Conjugation

In this work, we report a convenient and versatile strategy for surface-grafted glycopolymer constructs with the goal of surface modification that controls the chemical presentation and grafting density of carbohydrate side chains. This approach employs a difunctional hydrazine linker, chemically modified to an active ester containing poly(pentafluorophenyl acrylate) grafted scaffold, to conjugate a variety of saccharides through the reducing end. The successive conjugation steps are carried out under mild conditions and yield high surface densities of sugars, as high as 4.8 nmol·cm^-2, capable of multivalency, with an intact structure and retained bioactivity. We also demonstrate that this glycosylated surface can bind specific lectins according to the structure of its pendant carbohydrate. To demonstrate bioactivity, this surface platform is used to study the binding events of a human respiratory tract pathogen, Mycoplasma pneumoniae, on surfaces conjugated with sialylated sugars.

Diels-Alder "Clickable" Polymer Brushes: A Versatile Catalyst-Free Conjugation Platform

Polymeric brushes provide an attractive functional interface for a variety of applications in materials and biomedical sciences. Facile access to functionalized brushes can be realized through effective postpolymerization functionalization of reactive brushes. Over the past decade, efficient chemical transformations based on various "click" reactions have been employed for functionalization of polymeric brushes. This paper reports the first example of utilization of the Diels-Alder cycloaddition reaction based functionalization strategy that allows efficient conjugation of maleimide-containing molecules onto furan-containing polymer brushes under mild and reagent-free conditions. Polymers incorporating furan groups as side chains are "grafted from" silicon oxide surfaces and investigated toward their functionalization. Brushes are fabricated using atom transfer radical polymerization with varying amounts of furfuryl methacrylate to enable control over extent of functionalization, along with a poly(ethylene glycol) chain containing methacrylate as a comonomer to impart hydrophilic and antibiofouling characteristics. Functionalization of these reactive brushes were investigated through the immobilization of a model compound N-ethylmaleimide, a fluorescent dye BODIPY-maleimide, and a maleimide-containing biotin based ligand to direct the immobilization of streptavidin-coated quantum dots.

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.

Post-polymerization modification of styrene–maleic anhydride copolymer brushes

Amine-anhydride reactions on polymer brushes provide a modular post-modification strategy to functional surfaces.

Nanoscale Surface Creasing Induced by Post-polymerization Modification

Creasing in soft polymeric films is a result of substantial compressive stresses that trigger instability beyond a critical strain and have been directly related to failure mechanisms in different materials. However, it has been shown that programming these instabilities into soft materials can lead to new applications, such as particle sorting, deformable capillaries, and stimuli-responsive interfaces. In this work, we present a method for fabricating reproducible nanoscale surface instabilities using reactive microcontacting printing (μCP) on activated ester polymer brush layers of poly(pentafluorophenyl acrylate). The sizes and structures of the nanoscale creases can be modulated by varying the grafting density of the brush substrate and pressure applied during μCP. Stress is generated in the film under confinement due to the molecular weight increase of the side chains during post-polymerization modification, which results in substantial in-plane growth in the film and leads to the observed nanoscale creases.

SuFEx on the Surface: A Flexible Platform for Postpolymerization Modification of Polymer Brushes

Polymer brushes present a unique architecture for tailoring surface functionalities due to their distinctive physicochemical properties. However, the polymerization chemistries used to grow brushes place limitations on the monomers that can be grown directly from the surface. Several forms of click chemistry have previously been used to modify polymer brushes by postpolymerization modification with high efficiency, however, it is usually difficult to include the unprotected moieties in the original monomer. We present the use of a new form of click chemistry known as SuFEx (sulfur(VI) fluoride exchange), which allows a silyl ether to be rapidly and quantitatively clicked to a polymer brush grown by free-radical polymerization containing native -SO2F groups with rapid pseudo-first-order rates as high as 0.04 s(-1). Furthermore, we demonstrate the use of SuFEx to facilely add a variety of other chemical functional groups to brush substrates that have highly useful and orthogonal reactivity, including alkynes, thiols, and dienes.

Controlled decoration of the surface with macromolecules: polymerization on a self-assembled monolayer (SAM)

Polymer functionalized surfaces are important components of various sensors, solar cells and molecular electronic devices. In this context, the use of self-assembled monolayer (SAM) formation and subsequent reactions on the surface have attracted a lot of interest due to its stability, reliability and excellent control over orientation of functional groups. The chemical reactions to be employed on a SAM must ensure an effective functional group conversion while the reaction conditions must be mild enough to retain the structural integrity. This synthetic constraint has no universal solution; specific strategies such as "graft from", "graft to", "graft through" or "direct" immobilization approaches are employed depending on the nature of the substrate, polymer and its area of applications. We have reviewed current developments in the methodology of immobilization of a polymer in the first part of the article. Special emphasis has been given to the merits and demerits of certain methods. Another issue concerns the utility - demonstrated or perceived - of conjugated or non-conjugated macromolecules anchored on a functionally decorated SAM in the areas of material science and biotechnology. In the last part of the review article, we looked at the collective research efforts towards SAM-based polymer devices and identified major pointers of progress (236 references).

A dynamic duo: pairing click chemistry and postpolymerization modification to design complex surfaces

Advances in key 21st century technologies such as biosensors, biomedical implants, and organic light-emitting diodes rely heavily on our ability to imagine, design, and understand spatially complex interfaces. Polymer-based thin films provide many advantages in this regard, but the direct synthesis of polymers with incompatible functional groups is extremely difficult. Using postpolymerization modification in conjunction with click chemistry can circumvent this limitation and result in multicomponent surfaces that are otherwise unattainable. The two methods used to form polymer thin films include physisorption and chemisorption. Physisorbed polymers suffer from instability because of the weak intermolecular forces between the film and the substrate, which can lead to dewetting, delamination, desorption, or displacement. Covalent immobilization of polymers to surfaces through either a "grafting to" or "grafting from" approach provides thin films that are more robust and less prone to degradation. The grafting to technique consists of adsorbing a polymer containing at least one reactive group along the backbone to form a covalent bond with a complementary surface functionality. Grafting from involves polymerization directly from the surface, in which the polymer chains deviate from their native conformation in solution and stretch away from the surface because of the high density of chains. Postpolymerization modification (PPM) is a strategy used by our groups over the past several years to immobilize two or more different chemical functionalities onto substrates that contain covalently grafted polymer films. PPM exploits monomers with reactive pendant groups that are stable under the polymerization conditions but are readily modified via covalent attachment of the desired functionality. "Click-like" reactions are the most common type of reactions used for PPM because they are orthogonal, high-yielding, and rapid. Some of these reactions include thiol-based additions, activated ester coupling, azide-alkyne cycloadditions, some Diels-Alder reactions, and non-aldol carbonyl chemistry such as oxime, hydrazone, and amide formation. In this Account, we highlight our research combining PPM and click chemistry to generate complexity in polymer thin films. For the purpose of this Account, we define a complex coating as a polymer film grafted to a planar surface that acts as a template for the patterning of two or more discrete chemical functionalities using PPM. After a brief introduction to grafting, the rest of the review is arranged in terms of the sequence in which PPM is performed. First, we describe sequential functionalization using iterations of the same click-type reaction. Next, we discuss the use of two or more different click-like reactions performed consecutively, and we conclude with examples of self-sorting reactions involving orthogonal chemistries used for one-pot surface patterning.

High-density liquid-crystalline azobenzene polymer brush attained by surface-initiated ring-opening metathesis polymerization

High-density polynorbornene azobenzene liquid-crystalline brushes are synthesized on quartz substrates by surface-initiated ring-opening metathesis polymerization (SI-ROMP) using Grubbs third-generation catalyst. The grafting process is controlled over the thickness of the brush from a solid-supported substrate in a stoichiometric manner. A highly ordered liquid-crystal arrangement was formed for such brushes as revealed by spectroscopic measurements and grazing angle X-ray diffraction analysis. Marked features of this method in the structure and photoalignment behavior are unveiled by comparison with brushes made by surface-initiated atom-transfer radical polymerization (SI-ATRP).

Photo-click chemistry strategies for spatiotemporal control of metal-free ligation, labeling, and surface derivatization

Three photo-click ligation strategies described in this account provide scientists with efficient and selective tools for derivatization of various molecules, polymers, and surfaces. Fast photochemical reactions that are utilized in these techniques permit spatiotemporal control of the process. The absence of activating reagents and catalysts, as well as compatibility with aqueous media, makes photo-click ligations suitable for biomedical applications. The first of these approaches relies on the photochemical decarbonylation of cyclopropenones to produce cyclooctynes. The latter undergo rapid catalyst-free strain-promoted azide–alkyne cycloaddition (SPAAC) to azide-tagged substrates. The second method is based on a very fast (>104 M–1 s–1) light-triggered hetero-Diels–Alder reaction and permits efficient derivatization of substrates bearing vinyl ether moiety. An even faster reaction between photochemically generated naphthoquinone methides (oNQMs) and thiols (~2 × 105 M–1 s–1) serves as a basis for a third method. This thiol photo-click chemistry allows for the selective derivatization of thiol-functionalized substrates or labeling of free cysteine residues in proteins. The thioether linkage produced by the reaction of oNQMs and a thiol is stable under ambient conditions, but can be cleaved by UV irradiation, regenerating free thiol. This feature permits the removal or replacement of immobilized compounds, as well as traceless substrate release.

Self-sorting click reactions that generate spatially controlled chemical functionality on surfaces

This Article describes the generation of a patterned surface that can be postpolymerization modified to incorporate fragile macromolecules or delicate biomolecules without the need for special equipment. Two monomers that undergo different click reactions, pentafluorophenyl acrylate (PFPA) and 4-(trimethylsilyl) ethynylstyrene (TMSES), were sequentially polymerized from a silicon surface in the presence of a shadowmask with UV light, generating 12.5 and 62 μm pitch patterns. Two different dyes, 1-aminomethylpyrene (AMP) and 5-azidofluorescein (AF), were covalently attached to the polymer brushes through aminolysis and dual desilylation/copper(I)-catalyzed alkyne/azide cycloaddition (CuAAC) in one pot. Unlike most CuAAC reactions, the terminal alkyne of TMSES was not deprotected prior to functionalization. Although a 2 nm thickness increase was observed for poly(PFPA) brushes after polymerization of TMSES, cross-contamination was not visible through fluorescence microscopy after functionalization.

Redox active polymer brushes with phenothiazine moieties

We have investigated two different concepts to synthesize redox active polymer brushes using surface initiated atomic transfer radical polymerization (SI-ATRP). This polymerization technique allows the synthesis of well-defined grafted polymer brushes. In the initial step the surface was functionalized with a self-assembling monolayer of the SI-ATRP starter. Then, polymer brushes carrying phenothiazine moieties were grafted from the surface via SI-ATRP. The first concept consists of polymerizing monomers with phenothiazine pendant moieties to directly incorporate the redox functionality as side group in the growing polymer brush. The second concept consists of using grafted activated ester brushes which are functionalized with phenothiazine redox moieties in a successive reaction step. The electrochemical properties of the grafted redox active brushes were examined by cyclic voltammetry. Furthermore, the surface morphology and the chemical composition of the polymer brushes were characterized using scanning force microscopy (SFM), X-ray techniques, and UV/vis spectroscopy. Apart from their redox behavior, the synthesized brushes revealed increased mechanical stability on the nanoscale.

Rate determination of azide click reactions onto alkyne polymer brush scaffolds: a comparison of conventional and catalyst-free cycloadditions for tunable surface modification

The postpolymerization functionalization of poly(N-hydroxysuccinimide 4-vinylbenzoate) brushes with reactive alkynes that differ in relative rates of activity of alkyne-azide cycloaddition reactions is described. The alkyne-derived polymer brushes undergo "click"-type cycloadditions with azido-containing compounds by two mechanisms: a strain-promoted alkyne-azide cycloaddition (SPAAC) with dibenzocyclooctyne (DIBO) and azadibenzocyclooctyne (ADIBO) or a copper-catalyzed alkyne-azide cycloaddition (CuAAC) to a propargyl group (PPG). Using a pseudo-first-order limited rate equation, rate constants for DIBO, ADIBO, and PPG-derivatized polymer brushes functionalized with an azide-functionalized dye were calculated as 7.7 × 10(-4), 4.4 × 10(-3), and 2.0 × 10(-2) s(-1), respectively. The SPAAC click reactions of the surface bound layers were determined to be slower than the equivalent reactions in solution, but the relative ratio of the reaction rates for the DIBO and ADIBO SPAAC reactions was consistent between solution and the polymer layer. The rate of functionalization was not influenced by the diffusion of azide into the polymer scaffold as long as the concentration of azide in solution was sufficiently high. The PPG functionalization by CuAAC had an extremely fast rate, which was comparable to other surface click reaction rates. Preliminary studies of dilute solution azide functionalization indicate that the diffusion-limited regime of brush functionalization impacts a 50 nm polymer brush layer and decreases the pseudo-first-order rate by a constant diffusion-limited factor of 0.233.

Polymer Brushes as Functional, Patterned Surfaces for Nanobiotechnology

Polymer brushes have many desirable characteristics such as the ability to tether molecules to a substrate or change the properties of a surface. Patterning of polymer films has been an area of great interest due to the broad range of applications including bio-related and medicinal research. Consequently, we have investigated patterning techniques for polymer brushes which allow for two different functionalities on the same surface. This method has been applied to a biosensor device which requires both polymer brushes and a photosensitizer to be polymerized on a patterned gold substrate. Additionally, the nature of patterned polymer brushes as removable thin films was explored. An etching process has enabled us to lift off very thin membranes for further characterization with the potential of using them as Janus membranes for biological applications.

Photoreactive polymer brushes for high-density patterned surface derivatization using a Diels-Alder photoclick reaction

Reactive polymer brushes grown on silicon oxide surfaces were derivatized with photoreactive 3-(hydroxymethyl)naphthalene-2-ol (NQMP) moieties. Upon 300 or 350 nm irradiation, NQMP efficiently produces o-naphthoquinone methide (oNQM), which in turn undergoes very rapid Diels-Alder addition to vinyl ether groups attached to a substrate, resulting in the covalent immobilization of the latter. Any unreacted oNQM groups rapidly add water to regenerate NQMP. High-resolution surface patterning is achieved by irradiating NQMP-derivatized surfaces using photolithographic methods. The Diels-Alder photoclick reaction is orthogonal to azide-alkyne click chemistry, enabling sequential photoclick/azide-click derivatizations to generate complex surface functionalities.

Synthesis and morphological study of thick benzyl methacrylate-styrene diblock copolymer brushes

We demonstrate, for the first time, the synthesis of model poly(benzyl methacrylate) [P(BnMA)] brushes of very high thickness (>300 nm) on silicon wafer. P(BnMA) brush is also synthesized from the surface of silica nanoparticles, from a covalently anchored initiator monolayer, using ambient temperature ATRP. The kinetic studies and block copolymerization from the surface anchored P(BnMA)-Br macroinitiator showed that the polymerization was controlled in nature. AFM, ellipsometry, and water contact angle were used for the characterization of the polymer brush. The grafting density of the P(BnMA) brush, formed by immersion in a dilute monomer solution, was relatively less (∼11% less) in comparison to that obtained by immersion in neat monomer under similar conditions. The P(BnMA)-Br macroinitiator brushes were used to synthesize P(BnMA-b-S) diblock copolymer brushes by the ATRP of styrene at 95 °C. The P(BnMA-b-S) brushes showed stimulus response to a selective solvent and various nanopatterns were observed according to the composition of the block copolymer.

Versatility of alkyne-modified poly(glycidyl methacrylate) layers for click reactions

Functional soft interfaces are of interest for a variety of technologies. We describe three methods for preparing substrates with alkyne groups, which show versatility for "click" chemistry reactions. Two of the methods have the same root: formation of thin, covalently attached, reactive interfacial layers of poly(glycidyl methacrylate) (PGMA) via spin coating onto silicon wafers followed by reactive modification with either propargylamine or 5-hexynoic acid. The amine or the carboxylic acid moieties react with the epoxy groups of PGMA, creating interfacial polymer layers decorated with alkyne groups. The third method consists of using copolymers comprising glycidyl methacrylate and propargyl methacrylate (pGP). The pGP copolymers are spin coated and covalently attached on silicon wafers. For each method, we investigate the factors that control film thickness and content of alkyne groups using ellipsometry, and study the nanophase structure of the films using neutron reflectometry. Azide-terminated polymers of methacrylic acid and 2-vinyl-4,4-dimethylazlactone synthesized via reversible addition-fragmentation chain transfer polymerization were attached to the alkyne-modified substrates using "click" chemistry, and grafting densities in the range of 0.007-0.95 chains nm(-2) were attained. The maximum density of alkyne groups attained by functionalization of PGMA with propargylamine or 5-hexynoic acid was approximately 2 alkynes nm(-3). The alkyne content obtained by the three decorating approaches was sufficiently high that it was not the limiting factor for the click reaction of azide-capped polymers.

Sensitive protein microarray synergistically amplified by polymer brush-enhanced immobilizations of both probe and reporter

Great challenge remains to continuously improve sensitivity of protein microarrays for broad applications. A copolymer brush is in situ synthesized on both substrate and silica nanoparticle (SNP) surface to efficiently immobilize probe and reporter protein respectively for synergistic amplification of protein microarray signals. As a demonstration, sandwich immunoassay for a cancer biomarker carcinoembryonic antigen (CEA) detection is performed on microarray platform, showing a limit of detection (LOD) of 10 pg/ml and dynamic range of 10 pg/ml to 100 ng/ml. Two orders improvement of LOD is achieved in comparison to the small crosslinker-activated substrate. The improved sensitivity is attributed to not only the high immobilization amount of both probe and reporter but also the favorite protein binding orientations offered by the flexible brushes. This work provides a universal approach to inexpensively and significantly improve protein microarray sensitivity.

Anti-fouling bioactive surfaces

Bioactive surfaces refer to surfaces with immobilized bioactive molecules aimed specifically at promoting or supporting particular interactions. Such surfaces are of great importance for various biomedical and biomaterials applications. In the past few years, considerable effort has been made to create bioactive surfaces by forming specific biomolecule-modified surfaces on a non-biofouling "base" or "background". Hydrophilic and bioinert polymers have been widely used as anti-fouling layers that resist non-specific protein interactions. They can also serve as "spacers" to effectively move the immobilized biomolecule away from the surface, thus enhancing its bioactivity. In this review we summarize several successful approaches for the design and preparation of bioactive surfaces based on different types of anti-fouling/spacer materials. Some perspectives on future research in this area are also presented.