Counterpropagating Gradients of Antibacterial and Antifouling Polymer Brushes

We report on the formation of counterpropagating density gradients in poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) brushes featuring spatially varying quaternized and betainized units. Starting with PDMAEMA brushes with constant grafting density and degree of polymerization, we first generate a density gradient of quaternized units by directional vapor reaction involving methyl iodide. The unreacted DMAEMA units are then betainized through gaseous-phase betainization with 1,3-propanesultone. The gas reaction of PDMAEMA with 1,3-propanesultone eliminates the formation of byproducts present during the liquid-phase modification. We use the counterpropagating density gradients of quaternized and betainized PDMAEMA brushes in antibacterial and antifouling studies. Completely quaternized and betainized brushes exhibit antibacterial and antifouling behaviors. Samples containing 12% of quaternized and 85% of betainized units act simultaneously as antibacterial and antifouling surfaces.

Film Properties and Antimicrobial Efficacy of Quaternized PDMAEMA Brushes: Short vs Long Alkyl Chain Length

Quaternized poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes bearing quaternary ammonium groups of different alkyl chain lengths (ACLs) were prepared and assessed as biocidal coatings. For the synthesis of the antimicrobial brushes, first well-defined PDMAEMA chains were grown by surface-initiated atom transfer radical polymerization on glass and silicon substrates. Next, the tertiary amine groups of the polymer brushes were modified via a quaternization reaction, using alkyl halides, to obtain the cationic polymers. The polymer films were characterized by Fourier-transform infrared spectroscopy, ellipsometry, atomic force microscopy, and water contact angle measurements. The effect of the ACL of the quaternary ammonium groups on the physicochemical properties of the films as well as the contact killing efficiency of the surfaces against representative Gram-positive and Gram-negative bacteria was investigated. A hydrophilic to hydrophobic transition of the surfaces and a significant decrease of the degree of quaternization of the DMAEMA moieties was found upon increasing the ACL of the quaternization agent above six carbon atoms, allowing the wettability, the thickness, and the pH-response of the brushes to be tuned via a facile postpolymerization, quaternization reaction. At the same time, antimicrobial tests revealed that the hydrophilic polymer brushes exhibited enhanced bactericidal activity against Escherichia coli and Bacillus cereus, whereas the hydrophobic surfaces showed a significant deterioration of the in vitro bactericidal performance. Our results elucidate the antimicrobial action of quaternized polymer brushes, dictating the appropriate choice of the ACL of the quaternization agent for the development of coatings that effectively inhibit biofilm formation on surfaces.

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.

Synthesis and Postpolymerization Modification of Thermoresponsive Coatings Based on Pentaerythritol Monomethacrylate: Surface Analysis, Wettability, and Protein Adsorption

Properties of novel temperature-responsive hydroxyl-containing poly(pentaerythritol monomethacrylate) (PPM) coatings, polymerized from oligoperoxide grafted to glass surface premodified with (3-aminopropyl)triethoxysilane, are presented. Molecular composition, chemical state, thickness, and wettability are examined with time of flight-secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle measurements, respectively. Temperature-induced changes in hydrophobicity of grafted PPM brushes are revealed by water contact angle and ellipsometric measurements. Partial postpolymerization modification of hydroxyl groups (maximum a few percent), performed with acetyl chloride or pyromellitic acid chloride, is demonstrated to preserve thermal response of coatings. Adsorption of bovine serum albumin to PPM brushes, observed with fluorescence microscopy, is higher than on glass in contrast to similar hydroxyl-containing layers reported as nonfouling. Enhanced and temperature-controlled protein adsorption is obtained after postpolymerization modification with pyromellitic acid chloride.

Swelling of polyelectrolyte and polyzwitterion brushes by humid vapors

Swelling behavior of polyelectrolyte and polyzwitterion brushes derived from poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) in water vapor is investigated using a combination of neutron and X-ray reflectivity and spectroscopic ellipsometry over a wide range of relative humidity (RH) levels. The extent of swelling depends strongly on the nature of the side-chain chemistry. For parent PDMAEMA, there is an apparent enrichment of water vapor at the polymer/air interface. Despite extensive swelling at high humidity level, no evidence of charge repulsion is found in weak or strong polyelectrolyte brushes. Polyzwitterionic brushes swell to a greater extent than the quaternized brushes studied. However, for RH levels beyond 70%, the polyzwitterionic brushes take up less water molecules, leading to a decline in water volume fraction from the maximum of ~0.30 down to ~0.10. Using a gradient in polymer chain grafting density (σ), we provide evidence that this behavior stems from the formation of inter- and intramolecular zwitterionic complexes.

Synthesis of Fluorinated Amphiphilic Block Copolymers Based on PEGMA, HEMA, and MMA via ATRP and CuAAC Click Chemistry

Synthesis of fluorinated amphiphilic block copolymers via atom transfer radical polymerization (ATRP) and Cu(I) catalyzed Huisgen 1,3-dipolar cycloaddition (CuAAC) was demonstrated. First, a PEGMA and MMA based block copolymer carrying multiple side-chain acetylene moieties on the hydrophobic segment for postfunctionalization was carried out. This involves the synthesis of a series of P(HEMA-co-MMA) random copolymers to be employed as macroinitiators in the controlled synthesis of P(HEMA-co-MMA)-block-PPEGMA block copolymers by using ATRP, followed by a modification step on the hydroxyl side groups of HEMA via Steglich esterification to afford propargyl side-functional polymer, alkyne-P(HEMA-co-MMA)-block-PPEGMA. Finally, click coupling between side-chain acetylene functionalities and 2,3,4,5,6-pentafluorobenzyl azide yielded fluorinated amphiphilic block copolymers. The obtained polymers were structurally characterized by1H-NMR,19F-NMR, FT-IR, and GPC. Their thermal characterizations were performed using DSC and TGA.

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.

A Polyurethane Surface Modifier: Contrasting Amphiphilic and Contraphilic Surfaces Driven by block and random Soft Blocks having Trifluoroethoxymethyl and PEG Side Chains

A conventional MDI-BD-PTMO polyurethane was modified using 2 wt.% polyurethanes (U) having copolyoxetane soft blocks with hydrophobic 3F, CF₃CH₂OCH₂- and hydrophilic MEn, CH₃O(CH₂CH₂O)_nCH₂-, n = 3, 7) side chains. In contrast to neat 3F-co-MEn-U, 2 wt.% 3F-co-MEn-U compositions have physically stable morphologies and wetting behavior. Surface composition (XPS) and amphiphilic or contraphilic wetting are controlled by the 3F-co-MEn polyoxetane soft block architecture and MEn side chain length. Importantly, θ_rec can be tuned for 2 wt.% 3F-co-MEn-U compositions independent of swelling, which is controlled by the bulk polyurethane. AFM imaging led to a new morphological model whereby fluorous/PEG-hard block nano-aggregates combine to form near surface features culminating in micron scale texturing.

Advances in nonfouling materials: perspectives for the food industry

Fouling of complex food components onto food-processing materials affects food quality, food safety, and operating efficiency. Developments in nonfouling and fouling-release materials for biomedical and marine applications enable the potential for adaptation to food applications; however, challenges remain. The purpose of this review is to present different strategies to prevent fouling and/or facilitate foulant removal with a critical point of view for an application of such materials on food-processing surfaces. Nonfouling, self-cleaning, and amphiphilic materials are reviewed, including an explanation of the mechanism of action, as well as inherent limitations of each technology. Perspectives on future research directions for the design of food processing surfaces with antifouling and/or fouling release properties are provided.

Formation of polyampholyte brushes via controlled radical polymerization and their assembly in solution

We describe the formation of polyampholytic block copolymer brushes and their assembly in solution. Specifically, we employ "surface-initiated" activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) sequentially to form diblock copolymer grafts comprising blocks of poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) and poly(sodium methacrylate) (PNaMA) on flat impenetrable silica surfaces, i.e., SiO(x)/PNaMA-b-PDMAEMA and SiO(x)/PDMAEMA-b-PNaMA. Protonation of the PNaMA block results in formation of poly(methacrylic acid) (PMAA). We demonstrate that ARGET-ATRP of NaMA provides a convenient route to preparation of PMAA, which is an alternative method to the more traditional approach based on preparing PMAA by polymerizing tert-butyl methacrylate (tBMA) followed by cleavage of the tert-butyl group. We also discuss conformational changes of the individual polyelectrolyte blocks in solution as a function of solution pH by monitoring adsorption behavior of functionalized polystyrene spheres.

Stimuli responsive polymers for nanoengineering of biointerfaces

There is an increasing demand on the development of "smart" switchable interfaces since controlling surface topography and chemical functionality on a nanometer scale is crucial for numerous biomedical applications. Those surfaces, which are based on stimuli responsive polymers (SRPs), are able to modify their interactions with cells, biomolecules responding to different physical (e.g., temperature) or chemical (e.g., pH) stimuli. Such behavior may partially mimic complex dynamic properties of natural systems that are regulated by many biological stimuli. This paper reviews major studies and applications of SRPs as biointerfaces in a form of thin polymeric films (gels) and surface tethered polymers (brushes).

Biomimetic anchors for antifouling and antibacterial polymer brushes on stainless steel

Barnacle cement (BC) was beneficially applied on stainless steel (SS) to serve as the initiator anchor for surface-initiated polymerization. The amine and hydroxyl moieties of barnacle cement reacted with 2-bromoisobutyryl bromide to provide the alkyl halide initiator for the surface-initiated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate (HEMA). The hydroxyl groups of HEMA polymer (PHEMA) were then converted to carboxyl groups for coupling of chitosan (CS) to impart the SS surface with both antifouling and antibacterial properties. The surface-functionalized SS reduced bovine serum albumin adsorption, bacterial adhesion, and exhibited antibacterial efficacy against Escherichia coli (E. coli). The effectiveness of barnacle cement as an initiator anchor was compared to that of dopamine, a marine mussel inspired biomimetic anchor previously used in surface-initiated polymerization. The results indicate that the barnacle cement is a stable and effective anchor for functional surface coatings and polymer brushes.

Superhydrophobic surfaces by anomalous fluoroalkylsilane self-assembly on silica nanosphere arrays

We present the self-assembled formation of nanosized PFDTS (1H,1H,2H,2H-perfluorodecyltrichlorosilane) features on multilayered silica sphere arrays. We reveal the importance of residual water within the microsphere multilayers during PFDTS deposition and discuss a possible mechanism for the formation of the siloxane nanostructures. The multiscaled roughness induced by these superstructures is shown to lead to superhydrophobic behavior. The role of PFDTS is twofold: it (i) lowers the surface energy and (ii) provides the essential roughness to achieve superhydrophobicity. Moreover, the absence of PFDTS nanostructures on monolayers or in the absence of water leads to considerably smaller contact angles thereby indicating the relevance of multiscaled roughness for superhydrophobicity.

Controlling comonomer distribution in random copolymers by chemical coloring of surface-tethered homopolymers: an insight from discontinuous molecular dynamics simulation

Postpolymerization chemical modification ("coloring") of homopolymer brushes made of A units using B chemical moieties produces surface-anchored random copolymers (RCPs) A(1-x)B(x), where x is the degree of "coloring". We employ discontinuous molecular dynamics to study the "coloring" process in macromolecular tethers made of various lengths grafted at low and high densities on flat impenetrable surfaces. We demonstrate that the comonomer distribution in the A(1-x)B(x) RCPs depends on the interplay among (1) the length and the grafting density of the A-based homopolymer anchors, (2) the solubility of the parent homopolymer, and (3) the solubility of the B coloring units. Chemical modification of sparsely spaced chains on the surface leads to nearly random comonomer distribution in the A(1-x)B(x) RCPs regardless of the solubility of A and B. In contrast, the distribution of A and B units in A(1-x)B(x) RCPs prepared from homopolymers tethered at high grafting densities depends on the solubility of the parent homopolymer. Chemical modification of well-solvated A homopolymer grafts results in comonomer distributions that resemble those of diblock copolymers, comprising lightly modified blocks near the surface and heavily "colored" blocks at the top of the grafts. The relative lengths of the two blocks can be tuned by varying the solubility of B. Under poor solvent conditions, the distribution of A and B in the A(1-x)B(x) RCP is more complex; it is governed by the conformation of the parent A macromolecular anchors that form collapsed clusters before the coloring reaction.