SynopsisSorption phenomena of water and aqueous salt solutions by a perfluorinated polymer containing sulfonic acid groups (Nafion) were investigated. The temperature and concentration dependencies of the sorption by the membranes in the acid and salt forms were studied. The apparent activation energies for the diffusion of water in the H-form membrane and in the K-salt form were obtained as 4.9 and ca. 13.0 kcal/mole, respectively. The sorption kinetics during the neutralization of the membranes were observed in several aqueous solutions. A maximum in the sorption curve during the neutralization process was found and explained as resulting from the differences in the diffusion coefficients of water and of the cations and from the different number of water molecules absorbed by a SO:H+ (acid) site and a neutralized site. The diffusion coefficients D of several cations (K+, Cs+, Ba2+, and Ca2+) were determined and found to be considerably smaller than that of water. For the various cations, log D was related linearly to q/a, where q is the cation charge and a is the separation between centers of charge of the cation and anion. The dependence of water sorption upon the degree of neutralization of the membrane was also studied at room temperature. It was observed that for membranes of a low degree of neutralization a secondary sorption process existed, while no such secondary sorption could be found for the pure acid or the highly neutralized membranes. This secondary sorption was attributed to a structural rearrangement in the polymer. The apparent diffusion coefficient of water and the number of water molecules absorbed at equilibrium by an ionic site, n,, were obtained as a function of the degree of neutralization. The diffusion coefficient of water was dependent strongly on both the degree of neutralization and type of the salt, but no quantitative relation could be established. For all the salts studied in this paper, n, was linearly related to the degree of neutralization, x , supporting the assumption that the value of n, could be divided into those water molecules absorbed by an SO:H+ (acid) site, nh, and those absorbed by a neutralized site, n,. It was found that the value of q X n, had a strong correlation with a characteristic constant of the cations since a plot of q X n, versus log ( q / r ) yielded a straight line (r being the radius of the cation).
The densities and expansion coefficients of several Nafions in the acid and salt form were measured. For some samples, effects of variations in the equivalent weight, moisture content, and degree of uniaxial orientation were explored. The densities, which show considerable experimental scatter, seem to be independent of the equivalent weight but to be a strong function of the moisture content. Thin films of the as‐received, unbacked polymer are strongly uniaxially oriented. After annealing, reproducible values of the expansion coefficient can be obtained. Two distinct breaks in the linear expansion curve are seen; in the salts, the upper one correlates with the glass transition of the material, while the lower one is found in the vicinity of the mechanical β‐dispersion of the polymer. The large scatter of the experimental densities is consistent with the hypothesis that the sample is subject either to a microphase separation process which is strongly influenced by the sample history and which affects the density of the material very strongly, or to partial crystallization of the polymer, or both.
ABSTRA~T:Films of polyethylene, polytetrafluoroethylene and poly (vinylidene fluoride) were po!anzed to form electrets under a high static field at a high temperature. Electret films were cooled to near liquid nitrogen temperature and the depolarization current was measured as temperature increased. Several peaks of depolarization current were observed which indicated the temperatures at which the change of thermal molecular motion o; the phase transi~ion in polymer structure took place. A peak was also observed near the temperature at which the electret was formed. The results show that the measurement of depolarization current in electret polymers could become a new method for thermal analysis of polymer properties.KEY WORDS Electret / Depolarization Current / Polyethylene / Polytetrafluoroethylene / Poly (Vinylidene Fluoride) / Thermal Analysis / Molecular Motion / Films of many kinds of polymers can be formed into electrets by applying a strong d.c. electric field at a high temperature and subsequently cooling to room temperature under an applied electric field. 1 • 2 Accumulation of surface and volume charges as well as the orientation of dipoles in molecules bring about the formation of the electret state. If the electrodes on the surface of the film are connected to a sensitive e!ectrometer, forming a short circuit between the two electrodes, a depolarization current is induced. The integrated amount of the depolarization current during the heating of the film gives the total electrical charge stored in the electret. 3 In the present paper, many kinds of polymer films were transformed into electrets. The depolarization current of the electrets was measured in the course of increasing temperature from that of liquid nitrogen. Multiple peaks were then observed in the depolarization current vs. temperature curves, indicating a close connection between the temperature variation in the depolarization current and the thermal change in the polymer properties. The purpose of this paper is to describe quantitatively the experimental observations and to suggest some reasons for the existence of such multiple peaks in the depolarization current vs. temperature curve. EXPERIMENTAL METHODFilms of poly(vinylidene fluoride) with a thickness of 0.32 mm, low density polyethylene (Alkathene 7F4), high density polyethylene (Sholex 6000-50) with a thickness of 0.1-0.3 mm, and polytetrafluoroethylene (PTFE) with a thickness of 0.58 mm were used. Circular samples some 35 mm in diameter were cut out and areas about 25 mm across were painted centrally on both sides with silver paste, to serve as electrodes. A static high field between 0.5 and 2.5 kV was applied between the two electrodes at different temperatures ranging from room temperature to near melting point. The films were then cooled down to about -160°C with the strong electric field conditions sustained. Near the temperature of liquid nitrogen the films were short-circuited and the resultant depolarization current was detected by a Keithley 610B electrometer and record...
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