Abstract—The thermal polymerization of styrene sorbed from the gas-phase into polymer films of polyvinylidene fluoride (PVDF) is carried out at 110°С. By this method, the “matrix‑polystyrene” composites containing up to 70 wt % polystyrene (PS), which serve as precursors of ion-exchange membranes, are synthesized. Sulfonation of grafted PS produces ion-exchange membranes with the exchange capacitance of 1–2.7 mmol/g and the protonic conductivity reaching 20–200 mS/cm when saturated with water at 25°С. The conductivity values indicate that the nonuniformity of PS distribution over film-matrix cross-section usually encountered when monomer sorbed from the gas phase is polymerized does not exert any noticeable effect on the conduction properties of sulfonated composites. The developed method of preparing composites “polymer matrix‑grafted polystyrene” substantially simplifies the synthesis of the precursor of ion-exchange membranes, decreases the necessary amount of reagents, and considerably enhances the safety of synthesis.

Dependences of the transport rate of liquid water and saturated water vapor across commercial membranes (Nafion, MF-4SK) and proton-exchange membranes synthesized by the authors (PVDF, PP, UHMWPE, PTFE films modified with sulfonated polystyrene) on the membrane thickness have been studied. It has been found that at room temperature (17–25°C), the transport rate of liquid water and saturated water vapor across the membranes into an air stream hardly depends on the membrane type and thickness (60–240 μm), with the transport rate of saturated vapor being almost an order of magnitude below that of liquid water contacting one of the membrane surfaces. The fact that the flux of water and water vapor across the membrane does not depend on membrane thickness under conditions of maximum moistening suggests that the flow resistance is determined by the resistance at the feed and permeate interfaces. If one of the membrane surfaces is in contact with liquid water, the transport rate is equal to the rate of water removal from the permeate surface of the membrane; in the case of contact with saturated vapor, the transport rate is determined by the rate of water sorption from the vapor phase by the membrane. The results can be used to optimize the operation of fuel cells based on polymer proton-exchange membranes.

Methanol vapor permeability and pore formation features in stretched polytetrafluoroethylene (PTFE) films used as a precursor of composite cation-exchange membranes have been studied. Porous structures of the precursor have been formed via stretching PTFE films in air, toluene, isopropyl alcohol, and CCl4. Permeability has been determined according to the evaporation of a liquid through a porous film; porosity, according to the increase in the film volume during stretching; pore formation features, according to optical microscopy images of porous films and their transverse microsections. It has been found that, with an increase in the stretch ratio, the porosity of PTFE films increases almost linearly, while the methanol vapor permeability increases exponentially. The permeability of the films stretched in liquids is 20 times higher than the permeability of the films stretched in air at comparable stretch ratio and porosity values. The considerably higher permeability of the films stretched in liquids and the observed differences in their porous structure suggest that the liquids are actively involved in the formation of through pores in the direction connecting the film surfaces, i.e., in the direction that determines the transport and conductive properties of composite membranes based on stretched PTFE films.

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.

Polytetrafluoroethylene–polystyrene composite films are prepared by polymerizing styrene in a stretched polytetrafluoroethylene (PTFE) matrix immersed into a styrene monomer solution at 90°C. The kinetics of the thermal polymerization of styrene sorbed into the pores formed in the matrix is studied. A model of the polymerization process is proposed capable of describing the kinetics of polystyrene (PS) accumulation in the film, which, in particular suggests that the polymerization occurs in the bulk of growing PS inclusions at a constant concentration of monomer in them and that the formation of active sites most efficiently proceeds at their interface with the matrix. Stretched PTFE–PS composites containing up to 70 wt % PS in the matrix are prepared.

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.

Исследована протонная проводимость коммерческих перфторированных мембран Нафион-115, МФ-4СК и синтезированных нанокомпозитных мембран (поливинилиденфторид-сульфированный полистирол, сверхвысокомолекулярный полиэтилен – сульфированный полистирол, полипропилен – сульфированный полистирол) в зависимости от концентрации сорбированного в них водно-метанольного раствора с использованием контактного метода импедансометрии с регулируемым давлением на контактах “электрод–мембрана”. Установлено, что оптимальным условием измерения является использование пакета из нескольких мембран, зажимаемых между золотыми электродами с усилием не менее 40 кг/см2. Установлено, что удельная проводимость синтезированных мембран в воде при 24°C составляет 50–120 мСм/см, что практически совпадает с аналогичными значениями для мембран Нафион-115 и МФ-4СК. При увеличении концентрации водно-метанольного раствора от 0 до 60% удельная проводимость мембран Нафион-115 и МФ-4СК снижается на 30%. Снижение удельной проводимости синтезированных нанокомпозитных мембран в этой области концентраций составляет 40–55%.

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.

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.

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.

Разработан метод модифицирования поверхностного слоя политетрафторэтилена путем внедрения в него поливинилиденхлорида прививочной полимеризацией винилиденхлорида из газовой фазы, инициированной УФ-облучением ртутной лампы ПРК-4. Последующей обработкой композиции водным раствором аммиака осуществлено дегидрохлорирование (карбонизация) привитого поливинилиденхлорида. Исследована кинетика УФ-прививки и распределение карбонизованной фазы в матрице политетрафторэтилена. Получен материал с карбонизованным поверхностным слоем толщиной 10–30 мкм и устойчивым во времени углом смачивания около 57°.

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.

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.