Grafting well-defined polymers onto unsaturated PVDF using thiol-ene reactions

Poly(vinylidene fluoride) (PVDF) is commonly used in membranes, lithium-ion battery binders, and coatings due to its thermal and chemical robustness. Nevertheless, PVDF-based copolymers can broaden the application scope and performance capabilities of pristine PVDF. PVDF has been modified via grafting-from reactions. However, grafting density and graft length, two important properties of graft copolymers, cannot be accurately determined. Herein, we used grafting-onto thiol-ene reactions as a method to modify PVDF. The molar mass of pre-synthesized, thiol-terminated polymers were accurately determined, and grafting densities were calculated. Unsaturated sites were generated through dehydrofluorination and dehydrochlorination in PVDF and P(VDF-co-chlorotrifluoroethylene) (PVDF-CTFE). Various conditions were studied, including the molar mass and chemical structure of grafts, the degree of thiol substitution, and thiol-ene reaction mechanisms. Base-catalyzed Michael addition with secondary thiols performed best, with the highest grafting density calculated to be about 4 chains per PVDF chain. Despite the low grafting density, changes in material properties between the product and starting materials were observed, validating this controlled method for PVDF modification.National Science Foundation, NSF, (DMR 2202747); US-UK Fulbright Commission, (2022-21-1, RP/CPS/2022/007); Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, SNF, (194385); Ministerstvo Školství, Mládeže a Tělovýchovy, MŠMT, (CHE-0130903, CHE-1039870, CHE-1726525

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

Controlling topology, dynamics and function of polymer brushes via post-polymerization modification

Polymer brushes are a class of thin coatings, where each of chains is tethered to the underlying substrate via one chain end. Densely grafted polymer brushes feature a stretched chain conformation, which in a unique way enhances their lubrication and non-biofouling properties. Polymer brushes also present a high density of functional groups, whose exposure in solvated brushes is useful for catalysis or in medical applications, including diagnostics. Advances in surface-initiated controlled radical polymerizations (SI-CRP) have facilitated the synthesis of high grafting density polymer brushes with a great control over film thickness / polymer molecular weight and composition. These techniques enable access to plethora of polymer architectures. Due to the fact that SI-CRP strategies allow the use of a wide range of monomers, multiple routes for the post-polymerization modification of polymer brushes are possible. Combination of SI-CRP with post-polymerization modification provides possibilities to systematically study architecture- and composition-property relationships of polymer brushes. This Thesis aims to manipulate the topology and crosslinking dynamics of polymer brushes, whose effect on properties of the coatings will help to understand these relationships. Additionally, functions of polymer brushes can be extended by their modification. As an example, incorporation of nanoparticles is a way to provide catalytic functions. Swelling of the nanoparticle â polymer brush assemblies can expose the catalytically active sites and potentially maximize its activities. Chapter 1 provides an overview of manipulation strategies over the composition and topologies of polymer brushes, routes toward their crosslinking and methods for assembly of nanoparticles within polymer brush matrices. Chapter 2 demonstrates a two-step post-polymerization synthetic approach to generate loop polymer brushes with potentially improved non-biofouling and lubrication properties. In the first step, olefin groups are installed at the polymer chain-ends and then metathesis is used to induce loop closure. Chapter 3 presents a synthetic strategy to install different crosslinks onto polymer brush platforms. It comprises a two step-process: surface-initiated atom transfer polymerization (SI-ATRP) to generate a copolymer platform bearing azide groups, and copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) using propargyl-modified thiol precursors. Multiple crosslinking and decrosslinking cycles are studied under oxidative and reductive conditions. Chapter 4 explores the catalytic properties of 3-dimensional (3D) assemblies of amorphous molybdenum sulfide in polymer brushes as a template. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes are grown from highly-oriented pyrolytic graphite (HOPG) and used to bind anionic tetrathiomolybdate through an anion exchange reaction. In a final oxidation step, the polymer-bound tetrathiomolybdate is converted into the amorphous MoSx catalyst. The performance of polymer brush-catalyst system during hydrogen evolution reaction (HER) is studied

Reversibly Cross-Linking Polymer Brushes Using Interchain Disulfide Bonds

The introduction of interchain cross-links in surface-grafted polymer brushes increases the robustness and mechanical properties of these thin polymer films. In most cases, cross-linked polymer brushes contain permanent interchain cross-links. The use of reversible interchain cross-links, in contrast, provides opportunities to dynamically modulate the cross-link density and properties of surface-grafted polymer brushes. This study explores the use of disulfide bonds to reversibly cross-link poly(2-(dimethylamino)ethyl methacrylate) copolymer brushes. These brushes were prepared via surface-initiated atom transfer radical copolymerization of 2-(dimethylamino)ethyl methacrylate and an azide-containing comonomer, 3-azido-2-hydroxypropyl methacrylate, followed by copper(I)-catalyzed azide-alkyne cycloaddition of S-propargyl thioacetate and subsequent deprotection of the thioacetate moieties. Heating the polymer brushes to 60 degrees C under air for 2 h resulted in the formation of disulfide cross-links, which could be reduced to generate the corresponding free thiol groups upon brief exposure to tris(2-carboxyethyl)phosphine hydrochloride. The formation and cleavage of the interchain disulfide cross-links has a profound influence on the swelling and viscoelastic properties of the brushes. Cross-linking leads to a decrease in swelling ratio and a concomitant dehydration and loss in dissipative properties of the brush film. These changes were observed using ellipsometry and quartz crystal microbalance with dissipation monitoring experiments by exposing the polymer brushes to a sequence of successive cross-linking and uncross-linking steps. These experiments indicated that while cross-linking and uncross-linking were fully reversible during the first few cycles, the response of the brush films became less pronounced upon prolonged oxidation/reduction, which was attributed to the oxidation of thiol side-chain functional groups and a concomitant reduction in the cross-link density of the polymer brushes. The results presented in this study show that the incorporation of disulfide interchain cross-links allows access to polymer brush films that can be reversibly cross-linked and uncross-linked over many cycles

Synthesis of Loop Poly(Methyl Methacrylate) Brushes via Chain-End Postpolymerization Modification

Polymer brushes are typically densely grafted assemblies of polymer chains that are tethered via one end group to a solid substrate. Anchoring linear polymer chains via both end groups to a surface results in loop-type polymer brushes. Although loop polymer brushes have been shown to be able to outperform their linear, single-chain-end tethered analogues, for example, with respect to the prevention of biofouling or reducing friction, this brush architecture has received only relatively limited attention. Loop-type polymer brushes are mostly prepared following grafting-onto approaches using alpha,omega-heterobifunctional polymers. Grafting-from strategies, so far, have been rarely explored, but could further expand the range of accessible polymer molecular weights and brush grafting densities and allow the preparation of surface-attached polymer loops from a wider scope of monomers. This manuscript reports an alternative grafting-from strategy for the preparation of loop-type poly(methyl methacrylate) (PMMA) brushes. The strategy presented here starts with the preparation of linear polymer grafts using conventional surface-initiated atom transfer radical polymerization. The free chain-ends of the linear PMMA grafts are modified with an allyl group and subsequently subjected to a metathesis reaction to induce loop closure. The formation of the loop PMMA brushes was monitored by gel permeation chromatography analysis after cleavage of the polymer from the silica nanoparticles

Elevated transition temperature in Ge doped VO2 thin films

Thermochromic GexV1-xO2+y thin films have been deposited on Si (100) substrates by means of reactive magnetron sputtering. The films were then characterized by Rutherford backscattering spectrometry (RBS), four-point probe electrical resistivity measurements, X-ray diffraction, and atomic force microscopy. From the temperature dependent resistivity measurements, the effect of Ge doping on the semiconductor-to-metal phase transition in vanadium oxide thin films was investigated. The transition temperature was shown to increase significantly upon Ge doping (similar to 95 degrees C), while the hysteresis width and resistivity contrast gradually decreased. The precise Ge concentration and the film thickness have been determined by RBS. The crystallinity of phase-pure VO2 monoclinic films was confirmed by XRD. These findings make the use of vanadium dioxide thin films in solar and electronic device applications-where higher critical temperatures than 68 degrees C of pristine VO2 are needed-a viable and promising solution. Published by AIP Publishing

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 MoS₄^2– through an anion-exchange reaction. In a final oxidation step, the polymer-bound MoS₄^2– is converted into the amorphous MoS_<i>x</i> 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