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

Comparative Aminolysis Kinetics of Different Active Ester Polymer Brush Platforms in Postpolymerization Modification with Primary and Aromatic Amines

The kinetics of aminolysis between two different active ester polymer brush platforms, poly­(4-pentafluorophenyl acrylate) (poly­(PFPA)) and poly­(<i>N</i>-hydroxysuccinimide-4-vinyl benzoate) (poly­(NHS4VB)), are compared using primary and aromatic amines with varying reactivity toward postpolymerization modification. UV–vis was used to monitor the aminolysis of both brush platforms with 1-aminomethylpyrene (AMP), 1-aminopyrene (AP), and Ru­(bpy)₂(phen-5-NH₂)­(PF₆) (Ru²⁺A). Using a pseudo-first-order kinetics model, the pseudo-first-order rate constant (<i>k</i>′) was calculated for each system. The <i>k</i>′ of poly­(PFPA) modified with AMP, AP, and Ru²⁺A were 2.46 × 10^–1, 5.11 × 10^–3, and 2.59 × 10^–3 s^–1, respectively, while poly­(NHS4VB) can only be functionalized with the alkyl amine, albeit at a slower rate constant, <i>k</i>′ of 3.49 × 10^–3 s^–1, compared to that of poly­(PFPA) with AMP. The kinetics of surface-initiated photopolymerization of PFPA from oxide surfaces was also investigated as an effective method to control grafting density and film thickness

Covalent Grafting of Antifouling Phosphorylcholine-Based Copolymers with Antimicrobial Nitric Oxide Releasing Polymers to Enhance Infection-Resistant Properties of Medical Device Coatings

Medical device coatings that resist protein adhesion and bacterial contamination are highly desirable in the healthcare industry. In this work, an antifouling zwitterionic terpolymer, 2-methacryloyloxyethyl phosphorylcholine-<i>co</i>-butyl methacrylate-<i>co</i>-benzophenone (BPMPC), is covalently grafted to a nitric oxide (NO) releasing antimicrobial biomedical grade copolymer of silicone-polycarbonate-urethane, CarboSil, to significantly enhance the biocompatibility, nonspecific protein repulsion and infection-resistant properties. The NO donor embedded into CarboSil is <i>S</i>-nitroso-<i>N</i>-acetylpenicillamine (SNAP) and covalent grafting of the BPMPC is achieved through rapid UV-cross-linking, providing a stable, hydrophilic coating that has excellent durability over a period of several weeks under physiological conditions. The protein adsorption test results indicate a significant reduction (∼84–93%) of protein adhesion on the test samples compared to the control samples. Bacteria tests were also performed using the common nosocomial pathogen, <i>Staphylococcus aureus</i>. Test samples containing both NO donor and BPMPC show a 99.91 ± 0.06% reduction of viable bacteria when compared to control samples. This work demonstrates a synergistic combination of both antimicrobial and antifouling properties in medical devices using NO donors and zwitterionic copolymers that can be covalently grafted to any polymer surface

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

Evidence for the Phospholipid Sponge Effect as the Biocidal Mechanism in Surface-Bound Polyquaternary Ammonium Coatings with Variable Cross-Linking Density

Poly quaternary “-oniums” derived from polyethylenimine (PEI), poly­(vinyl-<i>N</i>-alkylpyridinium), or chitosan belong to a class of cationic polymers that are efficient antimicrobial agents. When dissolved in solution, the positively charged polycations are able to displace the divalent cations of the cellular phospholipid bilayer and disrupt the ionic cross-links and structural integrity of the membrane. However, when immobilized to a surface where confinement limits diffusion, poly -oniums still show excellent antimicrobial activity, which implies a different biocidal mode of action. Recently, a proposed mechanism, named phospholipid sponge effect, suggested that surface-bound polycationic networks are capable of recruiting negatively charged phospholipids out of the bacterial cell membrane and sequestering them within the polymer matrix. However, there has been insufficient evidence to support this hypothesis. In this study, a surface-bound <i>N,N</i>-dodecyl methyl-<i>co</i>-<i>N,N</i>-methylbenzophenone methyl quaternary PEI (DMBQPEI) was prepared to verify the phospholipid sponge effect. By tuning the irradiation time, the cross-linking densities of surface-bound DMBQPEI films were mediated. The modulus of films was measured by PeakForce Quantitative Nanomechanical Mapping (QNM) to indicate the cross-linking density variation with increasing irradiation time. A negative correlation between the film cross-linking density and the absorption of a negatively charged phospholipid (DPhPG) was observed, but no such correlations were observed with a neutral phospholipid (DPhPC), which strongly supported the action of anionic phospholipid suction proposed in the lipid sponge effect. Moreover, the killing efficiency toward <i>S. aureus</i> and <i>E. coli</i> was inversely affected by the cross-linking density of the films, providing evidence for the phospholipid sponge effect. The relationship between killing efficiency and film cross-linking density is discussed

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 <i>o</i>-naphthoquinone methide (<i>o</i>NQM), 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 <i>o</i>NQM 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

Balancing Melt Solubility and Morphology in Epitaxial Nucleation: The Case of Nicotinic Acid and Poly(hydroxybutyrate-<i>co</i>-hydroxyhexanoate)

Nicotinic acid was evaluated as a melt-soluble nucleator for a poly(hydroxy­butyrate-co-hydroxy­hexanoate) (PHBHHx) copolymer for melt-processing applications. A series of nicotinic acid concentrations (1–5 wt %) were analyzed to determine the best concentration for overall nucleation performance. 2 w/w% nicotinic acid was found to be the optimal concentration, successfully crystallizing PHBHHx at a peak temperature of 73 °C under nonisothermal conditions. When extrusion is performed at 150 °C, 2 w/w% of nicotinic acid occupies an optimal concentration within the polymer where all the nicotinic acid is dissolved in the PHBHHx matrix, which is not the case at higher concentrations. 2 w/w% also recrystallizes rapidly to produce many fine, needle-like crystals that are highly active toward PHBHHx nucleation, which is not observed with other concentrations. Powder X-ray diffraction (PXRD) analysis of the differing nicotinic acid crystals determined that the (110) and (120) faces are likely responsible for nucleation. Gel permeation chromatography (GPC) analysis revealed a modest degradation of molecular weight, likely due to the E1cb degradation mechanism common in PHAs

Rapid Electrochemical Reduction of Ni(II) Generates Reactive Monolayers for Conjugated Polymer Brushes in One Step

This article reports the development of a robust, one-step electrochemical technique to generate surface-bound conjugated polymers. The electrochemical reduction of arene diazonium salts at the surface of a gold electrode is used to generate tethered bromobenzene monolayers quickly. The oxidative addition of reactive Ni(0) across the aryl halide bond is achieved in situ through a concerted electrochemical reduction of Ni­(dppp)­Cl₂. This technique limits the diffusion of Ni(0) species away from the surface and overcomes the need for solution deposition techniques which often require multiple steps that result in a loss of surface coverage. With this electrochemical technique, the formation of the reactive monolayer resulted in a surface coverage of 1.29 × 10¹⁴ molecules/cm², which is a 6-fold increase over previously reported results using solution deposition techniques

Thermal Conductance of Poly(3-methylthiophene) Brushes

A wide variety of recent work has demonstrated that the thermal conductivity of polymers can be improved dramatically through the alignment of polymer chains in the direction of heat transfer. Most of the polymeric samples exhibit high conductivity in either the axial direction of a fiber or in the in-plane direction of a thin film, while the most useful direction for thermal management is often the cross-plane direction of a film. Here we show poly­(3-methylthiophene) brushes grafted from phosphonic acid monolayers using surface initiated polymerization can exhibit through-plane thermal conductivity greater than 2 W/(m K), a 6-fold increase compared to spin-coated poly­(3-hexylthiophene) samples. The thickness of these films (10–40 nm) is somewhat less than that required in most applications, but the method demonstrates a route toward higher thermal conductivity in covalently grafted, aligned polymer films

On the Role of Disproportionation Energy in Kumada Catalyst-Transfer Polycondensation

Kumada catalyst-transfer polycondensation (KCTP) is an effective method for the controlled polymerization of conjugated polymers. Nevertheless, side reactions leading to early termination and unwanted chain coupling cause deviations from the target molecular weight, along with increasing polydispersity and end group variation. The departure from the KCTP cycle stems from a disproportionation reaction that leads to experimentally observed side products. The disproportionation energies for a series of nickel-based initiators containing bidentate phosphino attendant ligands were computed using density functional theory at the B3LYP/DZP level. The initiator was found to be less favorable toward disproportionation by 0.5 kcal mol^–1 when ligated by 1,3-bis­(diphenylphosphino)­propane (dppp) rather than 1,2-bis­(diphenylphosphino)­ethane (dppe). Trends in disproportionation energy (<i>E</i>_disp) with a variety of bidentate phosphine ligands match experimental observations of decreased polymerization control. Theoretical <i>E</i>_disp values can thus be used to predict the likelihood of disproportionation in cross-coupling reactions and, therefore, aid in catalyst design