D. A. Kritskaya

Synthesis and transport properties of proton-conducting membranes based on polyvinylidene fluoride films with introduced and sulfonated polystyrene

Thermal polymerization of styrene sorbed into a polyvinylidene fluoride (PVdF) film from a toluene solution followed by sulfonation of the resulting material was performed. The kinetics of polystyrene (PS) accumulation in the PVdF film during thermal polymerization was studied. Samples with 6–30 wt % PS and ∼100% PS sulfonation were obtained. Proton-exchange membranes wsith an ion-exchange capacity of up to 2 mg-eq/g and proton conductivity of up to 0.008 S/cm at 75% relative humidity were prepared. The permeability coefficients of water, methanol, and hydrogen and their dependences on the amount of introduced PS, ion-exchange capacity, and water uptake of membranes were measured. The synthesized materials proved similar to MF-4SK membranes in their basic transport characteristics and can be used as proton-exchange membranes in hydrogen-air and alcohol fuel cells.

Investigation of poly(vinylidene chloride) distribution in perfluorinated cation-exchange membranes MF-4SK upon UV- and γ-initiated graft polymerization

A new process for grafting poly(vinylidene chloride) (PVDC) to the membrane material MF-4SK by UV-initiated graft polymerization of the monomer from the gas phase has been developed. Modified membranes containing up to 20 wt % of UV-grafted PVDC have been obtained. Microphotographs of thin sections of the modified membranes have been investigated. It has been shown that the pretreatment of the membranes and variation of UV- or γ-grafting conditions make it possible to achieve an uniform distribution of grafted PVDC both along the thickness of the membrane and in a thin surface layer. The values of the parameters determining the character of the distribution have been estimated. Numerical simulation of the UV- and γ-initiated graft polymerization of VDC gave solutions for the grafted-PVDC distribution fitting with the experimental data.

Kinetics of nanocomposite formation by thermal polymerization of styrene in the polyvinylidene fluoride matrix

The kinetics of polystyrene (PS) accumulation in polyvinylidene fluoride (PVDF) films as a result of the thermal polymerization of styrene sorbed from a styrene-toluene-divinylbenzene solution at 90°C was studied. After accumulation of ∼65 wt % PS in the sample, the thermal polymerization rate increased by approximately an order of magnitude. This was explained by an increase in the volume of the nanosized PS phase saturated with the monomer from the surrounding solution. The thermal polymerization of styrene in the PVDF matrix proceeded in the formed nanoreactors at a constant monomer concentration in them. The rate of thermal polymerization of styrene in the PVDF film was an order of magnitude higher than that in the surrounding solution. The initiation of styrene polymerization at the interface between PVDF and monomer-saturated PS nanoinclusions was assumed to be heterogeneous.

Proton conductivity of perfluorinated and nanocomposite ion exchange membranes in aqueous and water-methanol solutions

The proton conductivity of commercial perfluorinated membranes Nafion-115, MF-4SC and synthesized nanocomposite membranes (polyvinylidenefluoride-sulfonated polystyrene, ultra high molecular weight polyethylene-sulfonated polystyrene, polypropylene-sulfonated polystyrene) is studied as a function of concentration of the water-methanol solution sorbed by membranes, by means of contact impedancemetry with the regulated pressure on the electrode-membrane contacts. The optimal experimental conditions are shown to correspond to a pack of several membranes pressed between gold electrodes with the strength of no less than 40 kg/cm2. It is found that the specific conductivity of synthesized membranes in water at 24°C is 50–120 mS/cm, which virtually coincides with the corresponding values for Nafion-115 and MF-4SC. As the concentration of the water-methanol solution increases from 0 to 60%, the specific conductivity of Nafion-115 and MF-4SC membranes decreases by 30%. The specific conductivity of synthesized nanocomposite membranes decreases by 40–55% in this concentration range.

An NMR study of a water-methanol solution sorbed in MF-4SK sulfonated cation-exchange membranes

Concentration dependences of self-diffusion coefficients (SDCs), self-diffusion activation energies for water and methanol, and chemical shifts of the protons of the hydroxyl groups δOH simultaneously in an external water-methanol solution and the solution sorbed in MF-4SK membranes have been studied by NMR. It has been revealed that the SDC of pure methanol and pure water sorbed in an MF-4SK membrane is 3–5 times lower than that outside the membrane. It has been found that, in the presence of a small amount of methanol, the SDC of water in the membrane is 1.5–2 times higher than the SDC of pure sorbed water. At a solution concentration of 0.1–0.5 mole fraction, the SDC values of water and methanol in the membrane vary only slightly and are about 6 × 10−6 and 4 × 10−6 cm2/s, respectively. It has been determined that the δOH value in the membrane is 100–200 Hz higher than that in the external solution. The observed increase in δOH and decrease in SDC in the membrane suggest that the state of the solution in the MF-4SK sulfonated cation-exchange membrane has significantly changed compared to the external solution. The effect of the implanted carbon phase (CP) on the SDC of water and methanol and δOH of the solution sorbed in the MF-4SK membranes containing the CP has been studied. It has been revealed that at a methanol mole fraction of up to 0.5, the introduction of 23 wt % CP decreases the SDC of the solution components by no more than 10–20%. At a methanol mole fraction of 0.25–0.5, the self-diffusion activation energies for methanol and water in the external and membrane solutions decrease by 5–7 kJ/mol.

An NMR study of sorption-diffusion properties of MF-4SK-carbon composite membranes in aqueous methanol solutions

The distribution of water and methanol and their self-diffusion coefficients (SDCs) in carbon-doped MF-4SK perfluorinated membranes have been studied by NMR. It has been shown that at a concentration of 12–35 wt % in the external solution, the methanol concentration inside the membrane is higher by a factor of 1.4–2 than that in the external solution and increases with increasing the carbon phase content. It has been found that SDC values for water and methanol in the membrane decrease with an increase in the C phase content. In a membrane with 23 wt % C, the SDC of methanol is reduced by a factor of 1.6. It has been shown that the SDC of methanol in these membranes is below that of water by a factor of 1.5. The activation energies of self-diffusion of water and methanol have been determined. It has been found that they remain almost unaffected by the C content and have close values of 19–20 kJ/mol, which are 2–4 kJ/mol above the activation energy of self-diffusion of pure water in these membranes.

Synthesis of polymer nanocomposite ion-exchange membranes from sulfonated polystyrene and study of their properties

Methods for the preparation of composite ion-exchange membranes from polymer (polyvinylidene fluoride (PVDF), ultrahigh molecular weight polyethylene (UHMWPE), and polypropylene (PP)) matrices were considered. Polystyrene (PS) was introduced in the matrices by thermal polymerization of the monomer followed by sulfonation of the implant. The fundamentals of membrane synthesis from industrial polytetrafluoroethylene (PTFE, Teflon F-4) films by thermal polymerization of styrene in a film stretched in a monomer solution followed by sulfonation of incorporated PS were described. The literature on radiation- chemical synthesis of composite ion-exchange membranes based on polymer matrices with embedded polystyrene and its subsequent sulfonation was analyzed. Some problems of the kinetics and mechanism of thermal implantation of PS into various polymer matrices under different conditions were discussed. The physicochemical characteristics, structure, and transport properties of the membranes synthesized by thermal implantation of PS were reported. The obtained membranes were tested in low-temperature fuel cells.

Restructuring of polytetrafluoroethylene films during crazing in liquid media as an effective sorption method

The ability of polytetrafluoroethylene (Teflon) films stretched in liquid styrene, toluene, and other organic liquids at room temperature to absorb these liquids is studied. It is found that, when stretched to 200% in air and liquids, the volume of the polytetrafluoroethylene (PTFE) film increases by 40%, whereas the amount of liquid sorbed reaches 34 vol %. Stretched PTFE–polystyrene nanocomposites were prepared by in situ thermal polymerization of styrene sorbed into a PTFE film during stretching in a styrene–toluene–initiator solution.

Carbonization of polytetrafluoroethylene surface by UV-induced grafting of vinylidene chloride

A method has been developed for modifying the surface layer of polytetrafluoroethylene by incorporation of poly(vinylidene chloride) via UV radiation-initiated graft polymerization of vinylidene chloride from the vapor phase using a PRK-4 mercury lamp. By the subsequent treatment of the composition with aqueous ammonia, dehydrochlorination (carbonization) of the grafted poly(vinylidene chloride) has been performed. The kinetics of UV grafting and the distribution of the carbonized phase in the polytetrafluoroethylene matrix have been studied. A material with the carbonized surface layer of a 10—30 μm thickness and a contact angle of about 57° remaining stable over time has been obtained.

Polytetrafluoroethylene modification by radiation grafting of vinylidene chloride and its dehydrochlorination

A process has been designed for manufacturing a new composite material consisting of a PTFE matrix with a carbon phase implanted in the surface layer of up to 30 μm thickness. The process involves radiation graft polymerization of vinilydene chloride (VDC) from the vapor phase onto the PTFE matrix followed by dehydrochlorination of implanted PVDC. The VDC grafting kinetics, the distribution of the grafted polymer and the carbon phase, and the mechanical characteristics of the composite material have been investigated. It has been shown that unlike initial PTFE, the composite possesses good adhesive bonding properties and a small contact angle.