Multifunctional fluorine-containing nanofilms: current problems
Chemisorption of a monolayer
The interaction between electronegative elements of fluorine and surface cause forces that can vary in a wide range from weak van der Waals to strong covalent bonds, which are characteristic of stable compounds. Fluorine-containing surfactants, when reaching conditions of chemisorption of monomolecular layer, have low surface energy. The most significant factors for the modifying influence of surfactants on the surface are its microrelief and purity.
Molecules of fluorine deposited by thermal activation (heating) and the reaction of the chemisorption occurs with release of energy, the value of which, as well as the speed of the reaction, determined by the temperature of the environment and the object. The optimum temperature for the reaction and fixing of the coating is from 38 to 110°C.
When activating Epilam compositions, the radiation has a significant influence on the adsorption, the formation of a uniform monolayer and chemisorption. After IR- or UV-exposure may be significant changes in the phase state and physic-mechanical properties of the materials. The effect of radiation on thin and very thin structures of the polymer compositions can be analyzed by means of the DM-1 installation, which allows to measure the deflection of the films and membranes round shape .
One of the activation methods of fluoro-polymers – ultrasonication. In the works of V.Panin the use of ultrasound accelerates diffusion and adhesion processes in the coating of various materials. According to A.Lesnikova, ultrasound penetrates into the layer of the liquid phase of the coating, changing its structure, in particular, helping to reduce the grain size of alloys. According to the authors of , ultrasound activates a spatially periodic structure of the compositions, which are based on atoms or plastically deformed grains.
In some cases typical solubility of fluoropolymers are in the range of temperatures from 35 to 650 °C at a pressure of more than 50 MPa . To bring the polymer solution in supercritical CO2 out from the area of solubility, it is necessary to reduce the temperature of the hermetically closed tank or to reduce the pressure through the release of CO2 from the tank.
In ultrasonic generators, typically used emitters with piezocrystal or magnetostrictive transducer and resonant oscillatory rod system. The core of magnetostrictive material, placed in a conductive winding, may change its length depending on the waveform. In practice can be used eductors – mixing system that uses the Venturi effect (fig.2). The pumped flow involves in mixing a 4-fold amount of fluid from the bathtub volume. Thus, at ultrasonication, 80% of volume of the circulating fluid moves at almost zero hydraulic resistance, that allows significantly reduce pump power.
Character of the absorption spectrum of fluorine doesn't allow to directly determine the energy of dissociation. Though so far calculations are performed indirectly (using the level of surface energy, the energy of dissociation, the temperature of formation or the thermal conductivity of the porous surface, given its affinity to fluorine) practical results of application of nanofilms of fluorine-containing surfactant Epilam allow to systemize the studied parameters. Dissociation of nanofilms of fluoride has a direct dependence on the surface temperature of the material due to minor differences in the thermal capacity of atomic and molecular fluorine – in the range from 1000 to 0 ºK the absolute value of the heat of dissociation of mole of fluorine increases only by 2.44 kcal. This proves decrease in surface energy, including in a case of difficult surface profilogram and in systems of the type "porous surface – liquid (fluorine-containing surfactant)".
Self-organizing layers and evaluation of the quality of coatings
Several compositions of a series Epilam create on the surfaces molecular self-organizing layers, which are similar on the structure to the formations with so-called "fence Langmuir". This ensures reliable retention of the oil in friction pairs and prevents dry friction. The structure, obtained by epilaming of coverage is consistent with the theoretical concept of Langmuir-Blodgett films.
New ecocompositions of fluorine-containing surfactants Epilam are developed according to the Traube's rule (in any homologous range at low concentrations, the lengthening of the carbon chain on one CF or CH group increases the surface activity of 3 – 3.5 times). There are different methods of assessing the quality of the obtained coatings. For example, the authors of the method of plasma-initiated thermoluminescence offer to study of radiation aging of standard fluorine-containing coating quickly to irradiate it with glow discharge plasma in argon. Based on the analysis of glow curves it is assumed that the exposure standard coatings mainly causes breaks the molecules and increases the defect surface, while the impact of the plasma glow discharge in argon at room temperature leads to etching of the surface, removal of the surface defect layer and predominant formation of crosslinks. Based on the analysis of glow curves it is assumed that the exposure of standard coatings mainly causes breaks of the molecules and increases deficiency of a surface. The effect of plasma glow discharge in argon at room temperature leads to etching of the surface, removal of the surface defect layer and predominant formation of crosslinks.
Despite the instability of the phase state of the fluorine-containing surfactant in the coating process and at physico-mechanical influences (boiling, exposure to ultrasound, shaking of aerosol), they retain the ability to form a monomolecular film with a uniform distribution of molecules (fig.3). The impact of high-frequency ultrasonic waves and ultraviolet radiation with a high probability provide a stable geometry of nanostructures, which is especially valuable, for example, when processing composite materials that are used in a complex environment.
electrodeposition of nickel, zinc and chromium
Fluorine-containing polymer compositions can be used in the electrodeposition of nickel, zinc and chromium to improve the cathodic polarization, which occurs not only due to the complexing, but also with the introduction in the electrolyte of fluorine-containing surfactant. Adsorbed on the surface, surfactant block its areas or the entire surface .
Fig.4 illustrates obtaining of thin films of fluorine-containing polymers on various substrates. In the first case, the discharge of cations occurs on the free areas of the cathode. The sharp decrease of the cathode surface causes an increase in current density and polarization. In the second case, the discharge is carried out through a continuous film of surfactant, causing inhibition of the cathodic process, and, accordingly, the increase in polarization. According to the theory of complexing, colloidal compositions of fluorine-containing surfactant forms with metal cations modifying complexes – metal colloids. Thanks to the strong links between metal ions and surfactants these complexes increase the polarization of hydrophilic and hydrophobic ends of the molecules.
It can be assumed that the liquid polymer composition flowing in and out through the holes in surface of metal colloid, and the incoming stream may be diffusive, and the outflowing stream can possess the properties of the central spherical symmetry . If to analyze water-soluble lubricant composition, taking into account the energy of synthesis of molecules of hydrogen (436 kJ/mol), it is possible to calculate approximate amount of energy, that must be released in the electrolytic solution by analogy with the methods of production of one cubic meter of hydrogen under conditions when its free atoms are combined into molecules . Physical chemistry of these processes, most likely, is that the flow of fluorine-containing surfactant create on the boundary of its contact with the surface of the material conditions that are necessary for adsorption of monomolecular layer, and then – for her chemisorption to the surface with thermal activation. ■