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.

Selective and Controlled Grafting from PVDF-Based Materials by Oxygen-Tolerant Green-Light-Mediated ATRP.

Poly(vinylidene fluoride) (PVDF) shows excellent chemical and thermal resistance and displays high dielectric strength and unique piezoelectricity, which are enabling for applications in membranes, electric insulators, sensors, or power generators. However, its low polarity and lack of functional groups limit wider applications. While inert, PVDF has been modified by grafting polymer chains by atom transfer radical polymerization (ATRP), albeit via an unclear mechanism, given the strong C-F bonds. Herein, we applied eosin Y and green-light-mediated ATRP to modify PVDF-based materials. The method gave nearly quantitative (meth)acrylate monomer conversions within 2 h without deoxygenation and without the formation of unattached homopolymers, as confirmed by control experiments and DOSY NMR measurements. The gamma distribution model that accounts for broadly dispersed polymers in DOSY experiments was essential and serves as a powerful tool for the analysis of PVDF. The NMR analysis of poly(methyl acrylate) graft chain-ends on PVDF-CTFE (statistical copolymer with chlorotrifluoroethylene) was carried out successfully for the first time and showed up to 23 grafts per PVDF-CTFE chain. The grafting density was tunable depending on the solvent composition and light intensity during the grafting. The initiation proceeded either from the C-Cl sites of PVDF-CTFE or via unsaturations in the PVDF backbones. The dehydrofluorinated PVDF was 20 times more active than saturated PVDF during the grafting. The method was successfully applied to modify PVDF, PVDF-HFP, and Viton A401C. The obtained PVDF-CTFE-g-PnBMA materials were investigated in more detail. They featured slightly lower crystallinity than PVDF-CTFE (12-18 vs 24.3%) and had greatly improved mechanical performance: Young's moduli of up to 488 MPa, ductility of 316%, and toughness of 46 × 106 J/m3.

Tailored PVDF Graft Copolymers via ATRP as High-Performance NCM811 Cathode Binders.

High-nickel layered oxides, e.g., LiNi0.8Co0.1Mn0.1O2 (NCM811), are promising candidates for cathode materials in high-energy-density lithium-ion batteries (LIBs). Complementing the notable developments of modification of active materials, this study focused on the polymer binder materials, and a new synthetic route was developed to engineer PVDF binders by covalently grafting copolymers from poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) with multiple functionalities using atom transfer radical polymerization (ATRP). The grafted random copolymer binder provided excellent flexibility (319% elongation), adhesion strength (50 times higher than PVDF), transition metal chelation capability, and efficient ionic conductivity pathways. The NCM811 half-cells using the designed binders exhibited a remarkable rate capability of 143.4 mA h g-1 at 4C and cycling stability with 70.1% capacity retention after 230 cycles at 0.5 C, which is much higher than the 52.3% capacity retention of nonmodified PVDF. The well-retained structure of NCM811 with the designed binder was systematically studied and confirmed by post-mortem analysis.

Polymer-Brush-Templated Three-Dimensional Molybdenum Sulfide Catalyst for Hydrogen Evolution.

Earth-abundant hydrogen evolution catalysts are essential for high-efficiency solar-driven water splitting. Although a significant amount of studies have been dedicated to the development of new catalytic materials, the microscopic assembly of these materials has not been widely investigated. Here, we describe an approach to control the three-dimensional (3D) assembly of amorphous molybdenum sulfide using polymer brushes as a template. To this end, poly(dimethylaminoethyl methacrylate) brushes were grown from highly oriented pyrolytic graphite. These cationic polymer films bind anionic MoS42- through an anion-exchange reaction. In a final oxidation step, the polymer-bound MoS42- is converted into the amorphous MoSx catalyst. The flexibility of the assembly design allowed systematic optimization of the 3D catalyst. The best system exhibited turnover frequencies up to 1.3 and 4.9 s-1 at overpotentials of 200 and 250 mV, respectively. This turnover frequency stands out among various molybdenum sulfide catalysts. The work demonstrates a novel strategy to control the assembly of hydrogen evolution reaction catalysts.

Caspofungin on ARGET-ATRP grafted PHEMA polymers: Enhancement and selectivity of prevention of attachment of Candida albicans.

There is a need for coatings for biomedical devices and implants that can prevent the attachment of fungal pathogens while allowing human cells and tissue to appose without cytotoxicity. Here, the authors study whether a poly(2-hydroxyethylmethacrylate) (PHEMA) coating can suppress attachment and biofilm formation by Candida albicans and whether caspofungin terminally attached to surface-tethered polymeric linkers can provide additional benefits. The multistep coating scheme first involved the plasma polymerization of ethanol, followed by the attachment of α-bromoisobutyryl bromide (BiBB) onto surface hydroxyl groups of the plasma polymer layer. Polymer chains were grafted using surface initiated activators regenerated by electron transfer atom transfer radical polymerization with 2-hydroxyethylmethacrylate, yielding PHEMA layers with a dry thickness of up to 89 nm in 2 h. Hydroxyl groups of PHEMA were oxidized to aldehydes using the Albright-Goldman reaction, and caspofungin was covalently immobilized onto them using reductive amination. While the PHEMA layer by itself reduced the growth of C. albicans biofilms by log 1.4, the addition of caspofungin resulted in a marked further reduction by >4 log units to below the threshold of the test. The authors have confirmed that the predominant mechanism of action is caused by antifungal drug molecules that are covalently attached to the surface, rather than out-diffusing from the coating. The authors confirm the selectivity of surface-attached caspofungin in eliminating fungal, not mammalian cells by showing no measurable toxicity toward the myeloid leukaemia suspension cell line KG-1a.