Modification of surface properties of carbon-based and polymeric materials through fluorination routes: From fundamental research to industrial applications

Fluorination of PVC medical devices to prevent plasticizers migration

Medical devices (MD) are often made of plasticized polyvinylchloride (PVC). However, plasticizers may leach out into infused solutions and expose the patients to a toxic risk. The aim of the present work is to fluorinate plasticized PVC tubular MDs to create a barrier layer on their internal surface, and to study the impact of such a chemical treatment on the migration of the plasticizers. Following fluorination by pure molecular fluorine, the physico-chemical characterization of these modified MDs was carried out using various spectroscopic and microscopic techniques or tensile tests, evidencing the formation of covalent C-F bonds on the surface of the treated samples without modification of their mechanical and optical properties. The migration of plasticizers from fluorinated MDs was assessed using gas chromatography coupled with mass spectrometry and was found considerably decreased in comparison with the pristine MDs. After 24 h, the amount of tri-octyltrimellitate plasticizer (TOTM) detected in migrates from fluorinated MDs was even lower than the limit of quantification. Complementary cytotoxicity assays were performed according to the ISO EN 10993-5 standard, showing that the new fluorinated material does not cause a cytotoxic effect on L929 cells.

Effect of Low-Temperature Plasma Treatment on Starch-Based Biochar and Its Reinforcement for Three-Dimensional Printed Polypropylene Biocomposites

Uniform dispersion and high interfacial adhesion are two of the most difficult components of creating an ideally reinforced polymer composite. One of the solutions could be surface engineering of reinforcing filler materials utilizing innovative technologies. Low-temperature plasma treatments in the presence of sulfur hexafluoride (SF6) gas are proposed as a sustainable alternative to modify the surface properties of biochar carbon synthesized from sustainable starch-based packaging waste via a high-temperature/pressure pyrolysis reaction in the current study. X-ray photoelectron spectroscopy tests revealed that plasma treatments were effective in the fluorination of biochar carbon like wet chemical methods. By delivering fluorine-related functionalities only on the surface of the carbon, plasma treatments were efficient in changing the surface properties of biochar carbon while keeping the carbon's beneficial bulk properties intact, which is unique to this method. The modified biochar was effectively utilized to reinforce polypropylene. Mechanical properties like tensile strength improved by 91% when compared to neat polymers and 31% when compared to untreated biochar-reinforced polymers at 0.75 wt % loadings. Elongation at break increased from 12.7 to 38.78, showing an impressive 216% increase due to effective reinforcement by plasma functionalization. The decomposition onset temperature and maximum rate of decomposition temperature increased by 60 and 49 °C, respectively, when compared to neat polymers. Plasma-modified biochar-reinforced three-dimensional printed samples have shown promise to be utilized for the development of composite parts using additive manufacturing methods.

Generation of Elemental Fluorine through the Electrolysis of Copper Difluoride at Room Temperature

The safe generation of F₂ gas at room temperature by using simple cell configurations has been the "holy grail" of fluorine research for centuries. Thus, to address this issue, we report generation of F₂ gas through the electrolysis of CuF₂ in a CsF-2.45HF molten salt without the evolution of H₂ gas. The CuF₂ is selected through a series of thermodynamic and kinetic assessments of possible metal fluorides. Anode assessments on graphite and glass-like carbon demonstrate the effect of the absence of the anode during generation of F₂ gas owing to stabilized operations at room temperature. Although the Ni anode dissolves during electrolysis in the conventional medium-temperature cell, herein, it facilitates stable electrolysis over 100 h, achieving an F₂ gas purity of over 99 % with the potential to operate using one-compartment electrolysis. This work presents a safe and propitious method for the generation of high-purity F₂ gas for small-scale lab and industrial applications.

Direct Fluorination as Method of Improvement of Operational Properties of Polymeric Materials

Direct fluorination of polymers is a widely utilized technique for chemical modification. Such introduction of fluorine into the chemical structure of polymeric materials leads to laminates with highly fluorinated surface layer. The physicochemical properties of this layer are similar to those of perfluorinated polymers that differ by a unique combination of chemical resistance, weak adhesion, low cohesion, and permittivity, often barrier properties, etc. Surface modification by elemental fluorine allows one to avoid laborious synthesis of perfluoropolymers and impart such properties to industrial polymeric materials. The current review is devoted to a detailed consideration of wetting by water, energy characteristics of surfaces, adhesion, mechanical and electrical properties of the polymers, and composites after the direct fluorination.

Gas-Phase Fluorination on PLA Improves Cell Adhesion and Spreading

For the regeneration or creation of functional tissues, biodegradable biomaterials including polylactic acid (PLA) are widely preferred. Modifications of the material surface are quite common to improve cell-material interactions and thereby support the biological outcome. Typical approaches include a wet chemical treatment with mostly hazardous substances or a functionalization with plasma. In the present study, gas-phase fluorination was applied to functionalize the PLA surfaces in a simple and one-step process. The biological response including biocompatibility, cell adhesion, cell spreading, and proliferation was analyzed in cell culture experiments with fibroblasts L929 and correlated with changes in the surface properties. Surface characterization methods including surface energy and isoelectric point measurements, X-ray photoelectron spectroscopy, and atomic force microscopy were applied to identify the effects of fluorination on PLA. Gas-phase fluorination causes the formation of C-F bonds in the PLA backbone, which induce a shift to a more hydrophilic and polar surface. The slightly negatively charged surface dramatically improves cell adhesion and spreading of cells on the PLA even with low fluorine content. The results indicate that this improved biological response is protein- but not integrin-dependent. Gas-phase fluorination is therefore an efficient technique to improve cellular response to biomaterial surfaces without losing cytocompatibility.

Adhesion of Single-Walled Carbon Nanotube Thin Films with Different Materials

Single-walled carbon nanotubes (SWCNTs) possess extraordinary physical and chemical properties. Thin films of randomly oriented SWCNTs have great potential in many opto-electro-mechanical applications. However, good adhesion of SWCNT films with a substrate material is pivotal for their practical use. Here, for the first time, we systematically investigate the adhesion properties of SWCNT thin films with commonly used substrates such as glass (SiO₂), indium tin oxide (ITO), crystalline silicon (C-Si), amorphous silicon (a-Si:H), zirconium oxide (ZrO₂), platinum (Pt), polydimethylsiloxane (PDMS), and SWCNTs for self-adhesion using atomic force microscopy. By comparing the results obtained in air and inert Ar atmospheres, we observed that the surface state of the materials greatly contributes to their adhesion properties. We found that the SWCNT thin films have stronger adhesion in an inert atmosphere. The adhesion in the air can be greatly improved by a fluorination process. Experimental and theoretical analyses suggest that adhesion depends on the atmospheric conditions and surface functionalization.

Effects of Packing Density and Chain Length on the Surface Hydrophobicity of Thin Films Composed of Perfluoroalkyl Acrylate Chains: A Molecular Dynamics Study

A good understanding of the surface hydrophobicity of fluorinated materials is useful for their application as coating materials. The present study investigates the surface hydrophobicity of perfluoroalkyl acrylate (PFA) thin films using molecular dynamics simulations. Surface hydrophobicity is characterized by examining the contact angle of a water droplet on PFA surfaces and the cavity formation free energy in the vicinity of the surface. It is found that the calculated microscopic contact angles are in good agreement with the experimental results and partially capture the difference in the hydrophobicity of the surface arising from the variation of packing density and side chain length of PFA. The variations of cavity formation free energy in the vicinity of the surface elucidate that the surface hydrophobicity is mainly governed by the packing density rather than the chain length of PFA. The hydrophobicity generally increases with decreasing the packing density to some extent and then turns to decrease as further reducing the packing density. At higher packing density, the surface hydrophobicity slightly decreases with increasing the chain length, while at the lower packing density, the surface hydrophobicity is increased when chain length of PFA is longer than six carbons. Furthermore, we found that the influence of packing density on the surface hydrophobicity is directly related to the variation of the surface roughness and chain flexibility, that is, the surface hydrophobicity increases with increase in the surface roughness, while the chain flexibility plays a secondary role in the enhancement by affecting the stability of water staying near the interface. The study provides a significant insight into the local hydrophobicity and microscopic structure of the PFA surfaces, which would be useful for the application of surface modification.

Charge Injection Characteristics of Semi-Conductive Composites with Carbon Black-Polymer for HVDC Cable

Semi-conductive composites composed of carbon black-polymer play an important role in uniform electric field in high voltage direct current (HVDC) cable. They also affect space charge behaviors in the insulation material. However, the charge injection characteristics of semi-conductive composites are not detailed. In this work, the electrode structure of ‘Semi-conductive composites- Insulation material- Metal bottom’ (S-I-M) is proposed, and the currents formed by injected charges from semi-conductive composites are characterized by the thermally stimulated depolarization current (TSDC) method. Further, the experimental results based on the structure of S-I-M are compared with the traditional electrode structure of M-I-M (Metal upper electrode- Insulation material- Metal bottom electrode) and the simplified cable electrode structure of MS-I-M (Metal upper electrode-Semi-conductive electrode- Insulation material- Metal bottom electrode), respectively. The experimental results show that the semi-conductive composite plays an important role in the charge injection process and it presents a different tendency under different compound modes of temperature and electric field. For the low electric field (E ≤ 5 kV/mm) and the low temperature (T ≤ 50 °C), the current caused by the accumulated charges follows the rule, I_S > I_MS > I_M. For the low electric field and high temperature (T > 50 °C), the current caused by the injected charges follows the rule, I_MS > I_M > I_S. This phenomenon is closely related to the interface characterization and contact barrier.

Fluorine in the structure of polymers: influence on the gas separation properties

Results of studies on the separation of gases and vapours using fluorine-containing polymers are integrated and analyzed. Methods for the synthesis of these polymers are considered, including direct gas-phase fluorination, plasma polymerization of fluorine-containing precursors and modification of organic polymers, as well as diverse syntheses of monomers containing C−F and C−CF3 bonds and their subsequent polymerization. Structure – property relationships for these polymers are elucidated and their actual and potential applications as membrane materials are discussed.

The bibliography includes 165 references.

Surface Treatment of Carbon Fibers by Oxy-Fluorination

In this paper, the oxy-fluorination process and the influence of different concentrations of fluorine and oxygen in the gas phase on the physicochemical properties of polyacrylonitrile(PAN)-based carbon fibers are described. The properties of the treated carbon structures are determined by zeta potential and tensiometry measurements. In addition, changes in surface composition and morphology are investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Adhesion properties are characterized by the single fiber pull-out (SFPO) test. Furthermore, changes in intrinsic properties are described by means of tensile and density measurements. After a primary desizing effect by oxy-fluorination, an increased number of oxygen-containing surface functional groups could be detected, which led to more debonding work in SFPOs with an epoxy-based matrix. It was also shown that the polar surface energy grows with rising fluorine concentration in the reaction gas mixture. In addition, a minor increase of ~10% in the maximum strength of PAN-based carbon fibers is detected by single fiber tensile measurements after oxy-fluorination with a fluorine content of 5% in the reaction mixture.

Facile and green preparation of hemicellulose-based film with elevated hydrophobicity via cross-linking with citric acid

Hemicellulose has shown great potential in food packaging due to its excellent biodegradability and low oxygen permeability. However, its strong hydrophilicity leads to poor moisture resistance and hinders its wide application. To address this issue, herein a ternary carboxylic acid, citric acid (CA), was incorporated into hemicellulose as esterifying agent to form a crosslinking structure via the esterification reaction. The CA-modified hemicellulose films showed an increased contact angle of 87.5° (vs. 40.5° for unmodified film), demonstrating that the hydrophobicity of hemicellulose had been improved significantly. In addition, the esterification/cross-linking modification enhanced oxygen barrier performance with oxygen permeability decreasing from 1053 (cm3 μm) (m2 d kPa)-1 to 1.8 (cm3 μm) (m2 d kPa)-1. Moreover, the tensile strength rose to a peak value and then fell back at higher CA content. Effect of CA addition on elongation at break exhibited an opposite trend. The modified hemicellulose films with 20% CA addition possessed the highest tensile strength and the lowest elongation at break. Morphology observation with scanning electron microscopy indicated that at CA content exceeding 20%, the modified films were dense with a smooth surface, illustrating the improvement of phase compatibility. A possible mechanism for esterification/cross-linking was proposed to elucidate the connection between CA addition and film performances.

Enduring and Stable Surface Dielectric Barrier Discharge (SDBD) Plasma Using Fluorinated Multi-Layered Polyimide

In this work, multi-layered polyimide (PI) films were surface fluorinated at 328 K and 0.05 MPa using F₂/N₂ mixture with 20% F₂ by volume, for a fluorination time of 0, 30 and 60 min, respectively. Then, they were subjected to discharge plasma as barrier dielectrics of surface dielectric barrier discharge (SDBD) at ambient atmospheric air. The dielectric lifetime of SDBD greatly extends after 60 min surface fluorination. In addition, optical emission spectroscopy (OES) results indicate that during the plasma processing, SDBD with fluorinated PI can obtain more stable plasma parameters, including gas temperature and electron temperature. Dielectric surface properties were further evaluated by infrared thermography, scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). It is considered that both physical and chemical effects lead to the extension of dielectric lifetime. The physical effect is reflected in low surface temperature and increased surface roughness, while the chemical effect is reflected in the graft of fluorine groups.

Rapid and selective surface functionalization of the membrane for high efficiency oil-water separation via an atmospheric pressure plasma process

Oil-water separation is a worldwide challenge because of the increasing production of industrial oily wastewater and frequent oil spills. The growing environmental and economic demands emphasize the need to develop effective solutions to separate oil and water. Recently, oil-water separation methods were developed by tuning the wettability of membranes via surface functionalization. However, the industrialization of such methods remains challenging due to the easy-fouling, high cost and complex fabrication. Herein, a simple and rapid pathway to separate oil from oil-water mixtures is reported using plasma surface functionalization in an open-air environment. The fine tuning and study of the plasma process parameters enables the selective functionalization of each side of the membranes which led respectively to a superhydrophobic-superoleophilic and superhydrophobic-oleophobic sides. The successful separation, without any external force, of a 50 mL oil-water solution in 6 minutes was achieved. This work paves the way for an efficient, low cost and easily upscalable method for oil-water separation due to the high versatility of the atmospheric pressure plasma processes.

A Review of Water-Resistant Hemicellulose-Based Materials: Processing and Applications

Hemicelluloses, due to their hydrophilic nature, may tend to be overlooked as a component in water-resistant product applications. However, their domains of use can be greatly expanded by chemical derivatization. Research in which hydrophobic derivatives of hemicelluloses or combinations of hemicelluloses with hydrophobic materials are used with to prepare films and composites is considered herein. Isolation methods that have been used to separate hemicellulose from biomass are also reviewed. Finally, the most useful pathways to change the hydrophilic character of hemicelluloses to hydrophobic are reviewed. In this way, the water resistance can be increased and applications of targeted water-resistant hemicellulose developed. Several applications of these materials are discussed.

Probing plasma fluorinated graphene via spectromicroscopy

The graphene fluorination using CF₄ and SF₆ plasma is investigated by combining spectroscopy and microscopy techniques.

Influence of fluorination on barrier properties of polymers: Insights from Monte Carlo simulations of eicosanes + methane

Fluorination is widely used to improve the resistance and physical properties of polymers that are cheap to manufacture. This process improves the resistance properties of unfluorinated materials. This study examines the effects of varying the degree of fluorination on the clustering and absorption behaviour of methane n-eicosane. Monte Carlo simulations were performed for several different pressure values, at ambient temperature, to determine the uptake of methane into the eicosanes. The density of the pure eicosanes, simulated at ambient conditions, compared favourably with experimental data for the relevant polymers. The spatial configurations resulting from the absorption simulations were analysed to determine the clustering behaviour of absorbed methane. Both the prevalence of cluster formation in general, and the occurrence of specific cluster topologies of various sizes were considered. Cyclic clusters had a tendency to become more prevalent in unfluorinated eicosanes as the gas pressure was increased, while the presence of fluorine atoms on the eicosane backbone appeared to inhibit the formation of such clusters.

Effect of fibrous separators on the performance of lithium–sulfur batteries

In this paper we systematically investigated effect of separator morphology on the performance of Li–S batteries.

Fluorination-dependent molecular orbital occupancy in ring-shaped perfluorocarbons

Perfluorocarbons are a family of molecules consisting mainly of carbon and fluorine atoms. They have interesting chemical properties and have diverse applications in biomedicine, physical chemistry and polymer science. In this work, carbon K-edge absorption and emission spectra of liquid decalin are presented and compared to perfluorodecalin. A comprehensive picture of the electronic structure of decalin is provided based on soft X-ray absorption and emission spectroscopies. Experimental data are compared to theoretical time-dependent density functional theory for the hydrocarbon, the perfluorocarbon and the stepwise fluorinated derivatives. We observed a molecular orbital change from unoccupied to occupied orbitals for perfluorodecalin, which was induced through the fluorination process.

Selected physicochemical aspects of poly- and perfluoroalkylated substances relevant to performance, environment and sustainability-part one

The elemental characteristics of the fluorine atom tell us that replacing an alkyl chain by a perfluoroalkyl or polyfluorinated chain in a molecule or polymer is consequential. A brief reminder about perfluoroalkyl chains, fluorocarbons and fluorosurfactants is provided. The outstanding, otherwise unattainable physicochemical properties and combinations thereof of poly and perfluoroalkyl substances (PFASs) are outlined, including extreme hydrophobic and lipophobic character; thermal and chemical stability in extreme conditions; remarkable aptitude to self-assemble into sturdy thin repellent protecting films; unique spreading, dispersing, emulsifying, anti-adhesive and levelling, dielectric, piezoelectric and optical properties, leading to numerous industrial and technical uses and consumer products. It was eventually discovered, however, that PFASs with seven or more carbon-long perfluoroalkyl chains had disseminated in air, water, soil and biota worldwide, are persistent in the environment and bioaccumulative in animals and humans, raising serious health and environmental concerns. Further use of long-chain PFASs is environmentally not sustainable. Most leading manufacturers have turned to shorter four to six carbon perfluoroalkyl chain products that are not considered bioaccumulative. However, many of the key performances of PFASs decrease sharply when fluorinated chains become shorter. Fluorosurfactants become less effective and less efficient, provide lesser barrier film stability, etc. On the other hand, they remain as persistent in the environment as their longer chain homologues. Surprisingly little data (with considerable discrepancies) is accessible on the physicochemical properties of the PFASs under examination, a situation that requires consideration and rectification. Such data are needed for understanding the environmental and in vivo behaviour of PFASs. They should help determine which, for which uses, and to what extent, PFASs are environmentally sustainable.

Organic memory effect from donor–acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole

A novel D–A polymer is designed for resistance memory devices with a large off ratio, good endurance, and long retention time.

Improvement of polypropylene nonwoven fabric antibacterial properties by the direct fluorination

Fluorination of polypropylene nonwoven fabric totally suppresses Staphylococcus aureus and Candida albicans microfungus reproduction and partially – Escherichia coli reproduction.

Fluorination of graphene: a spectroscopic and microscopic study

Since the advent of graphene, there has been intense interest in exploring the possibility of incorporating fluorinated graphene (FG), an ultrathin insulator, into graphene electronics as barriers, gate dielectrics, and optoelectronic elements. Here we report on the synthesis of FG from single-layer graphene sheets grown by chemical vapor deposition (CVD) using CF4 plasma. We examine its properties systematically via microscopic and spectroscopic probes. Our studies show that, by controlling the conditions of the plasma, FG of varying fluorine coverage can be produced; however, the resulting material contains a mixture of CFx (x = 1-3) bonds. Existing grain boundaries and lattice defects of CVD graphene play an important role in controlling its rate of fluorination and the damage of the sheet. Combining topography and current mapping, we demonstrate that the spatial distribution of fluorine on CVD graphene is highly inhomogeneous, where multilayer islands and structural features such as folds, wrinkles, and ripples are less fluorinated and consequently form a conductive network through which charge transport occurs. It is the properties of this network that manifest in the electrical transport of FG sheets. Our experiments reveal the many challenges of deriving electronics-quality FG from current CVD graphene while at the same time point to the possible solutions and potential of FG in graphene electronics and optoelectronics.

Perfluorinated carboxylic acids in directly fluorinated high-density polyethylene material

Perfluorinated carboxylic acids (PFCAs) are ubiquitous in the environment and have been detected in human blood worldwide. One potential route is direct exposure to PFCAs through contact with polymers that have been fluorinated through a process referred to as direct fluorination. PFCAs are hypothesized to be reaction byproducts of direct fluorination when trace amounts of oxygen are present. The objective of this research was to investigate whether PFCAs could be measured in directly fluorinated high-density polyethylene (HDPE) bottles. PFCAs were quantified using Soxhlet extraction with methanol, followed by LC-MS/MS analysis. Total concentrations of PFCAs ranged from 8.5 ± 0.53 to 113 ± 2.5 ng/bottle (1 L), with the short-chain PFCAs, perfluoropropanoic, perfluorobutanoic, perfluoropentanoic, and perfluorohexanoic acids, being the dominant congeners observed. Relative PFCA concentrations varied depending on fluorination level. Structural isomers were detected using (19)F NMR and are hypothesized to have formed during the fluorination process; NMR data revealed the linear isomer typically comprised 55% of the examined sample. Internally branched, isopropyl branched, and t-butyl PFCA isomers of varying chain length were also identified. Electrochemical fluorination was previously thought to be the only source of branched PFCA isomers. The observation here of branched isomers suggests direct fluorination may be an additional source of exposure to these chemicals. The purpose of this study was to measure PFCAs in directly fluorinated material, serving as a previously unidentified source contributing to the environmental load of PFCAs, with potential for human exposure.