DIELECTRIC
THE DIELECTRIC
CONSTANT OF POLYSTYRENE
SOLUTIONS
235
CONSTANT OF POLYSTYRENE SOLUTIONS1 P. DEBYE and
F. BUECHE
Department of Chemistry, Cornell University, Ithaca, New York
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Received February 6, 1950
An attempt has been made to repeat the experiments previously reported by A. Voet (2) on the relation between dielectric constant and concentration of polystyrene-toluene solutions. Voet reported a very peculiar phenomenon.2 He found that as the concentration was increased, the dielectric constant of the solution remained essentially the same as that of the solvent until a critical concentration was reached. At concentrations higher than this critical value the dielectric constant reportedly rose rapidly in a presumed linear fashion with a slope which, if continued, would give a much too high dielectric constant for the solid polymer. We have observed no such effect for pure polystyrene-toluene solutions. However, with methanol added as an impurity to these solutions, an effect is observed which, if not discussed carefully, might be interpreted as giving something resembling a “critical” concentration. The present investigation was originally started using the heterodyne beat frequency method of measuring the capacitance at a frequency of 1.5 Me. Several samples of polystyrene were used. Two of the samples were fractions* having weight-average molecular weights of 150,000 and 65,000. All of these samples gave a linear dependence of the dielectric constant upon concentration. The measurements were made to concentrations as high as 20 g./100 ml. with no indication of a discontinuity of any kind. These straight lines extrapolated to a value of about 2.60 for the dielectric constant of the solid polymer. The relative values of the dielectric constant were obtained to an accuracy of 0.04 per cent. Since these measurements had been made using frequencies much higher than those used by Voet, it was decided to repeat them with an audiofrequency bridge. The central unit of the bridge was a Leeds & Northrup No. 1202 farad bridge. However, since this unit was not provided with sufficiently fine adjustment for the ratio arms, a specially constructed external condenser was used in one arm. This was essentially a good multiple-plate air condenser in parallel with a fine adjustment consisting of two flat brass half-circles which could be rotated with respect to each other.
The driving voltage was furnished by a variable-frequency Hewlett-Packard oscillator; although other audiofrequencies gave the same results, the values This work was carried out in connection with the Synthetic Rubber Program of the United States Government under contract with the Office of Rubber Reserve, Reconstruction Finance Corporation. ! Voet’s data are plotted as though his solvent were benzene, although he states that toluene was the solvent used. ! We are indebted to Dr. T. G Fox of this laboratory for these samples. 1
236
P. DEBYE
AND F. BUECHE
given here are for 2000 cycles. An audioamplifier and audioearphones were used for the detector. The cell was one which had been used previously for some years for determining dipole moments. It was constructed similarly to the one described by Smyth (1) and had a capacitance when filled with air of 51 cm. This apparatus gave relative values of dielectric constants to an accuracy of 0.1 per cent in the range of these measurements. Calibration was made at 30.0°C., using the values of 2.2660 and 2.3630 for the dielectric constants of benzene and toluene, respec-
tively. Low-frequency measurements were made on four fractions of polystyrene and unfractionated polystyrene. One fraction was from a batch of polystyrene made by thermal polymerization without catalyst in diethylbenzene. The others on one
Fio. 1. The dielectric constant of polystyrene solutions in toluene as a function of the concentration of polystyrene. Fig. 2. The effect of methanol upon the dielectric constant of solutions of polystyrene in toluene.
from different batches made by polymerization in benzene with benzoyl peroxide as catalyst. These samples were dissolved in benzene and precipitated in methanol. They were dried under vacuum for about 24 hr. at 40°C. before being redissolved in toluene. The results obtained for the dielectric constants of these samples in toluene coincide with one another within experimental error. They can all be represented by the same straight line. A typical set of data is shown in figure 1. Data for the other samples are equally good and, save for one sample, cover the same range of concentrations. Extrapolation leads to a dielectric constant of 2.61 for the solid polystyrene. This agrees well with the data in the literature. We believe that the peculiar results obtained by Voet are spurious. Whether his dielectric constant data are the result of instrumental difficulties or impure samples is not known. However, the data shown in figure 2 lend some credence were
DIELECTRIC CONSTANT OF POLYSTYRENE
SOLUTIONS
237
to the latter possibility. These data were taken using the same sample as was used in figure 1, except that here the polystyrene had been slightly moistened with commercial methanol, the common precipitant for polystyrene. It will be observed that the curve representing these data has such a form that a less accurate instrument might easily lead one to accept it as showing a break in the curve. Other impurities or varying amounts of this particular one would probably give many modifications of this curve. It is perhaps some such effect as this which Voet has observed. The cause of the large curvature in the methanol-toluene-polystyrene curve has been investigated further. The curvature is largely the result of evaporation of the methanol during the process of making the solutions and during the usual manipulation involved in filling the cell. This is shown quite clearly in figure 3. 2.75
DIELECTRIC CONSTANT
/°
/ ·
/
2.55
2.35
CONCENTRATION
in
CCVIOO CC.
Fig. 3. The effect of evaporation of methanol from toluene solutions of polystyrene having methanol as an impurity.
In figure 3 the open circles represent data taken using four separately prepared solutions of toluene-methanol-polystyrene. For each solution the ratio of methanol to polystyrene was 2.44 ml. of methanol to 10 g. of polystyrene. Since the methanol was added last, the loss of methanol during the filling of the cell was the important factor. On the other hand, if the most concentrated solution used above is removed from the cell, diluted with toluene, and remeasured, the points represented by solid circles are obtained. The data shown in figure 3 indicate that even when some care is exercised to prevent evaporation, the loss of some of the highly volatile methanol seriously affects the results obtained. It is clear that under ordinary handling the data will show an even greater curvature. With such solutions having methanol as an impurity it therefore appears that the shape of the curve obtained is a sensitive function of the care taken to eliminate evaporation.
238
RICHARD B. BERNSTEIN
AND DANIEL
CUBICCIOTTI
CONCLUSION
from our results that no discontinuity exists in the dielectric constant vs. concentration curve for polystyrene in toluene. The dielectric constant varies linearly with concentration in such a way as to extrapolate to the dielectric constant of the solid polymer. Impurities and the influence of handling of the samples have been shown to lead to experimental results which, if not interpreted carefully, might suggest the existence of a break in the curve.
It would
seem
REFERENCES (1) Smyth, C. P.: Dielectric Constant and Molecular Structure, p. 60. The Chemical Catalog Company, Inc., New York (1931). (2) Voet, A.: J. Phye. & Colloid Chem. 63, 597 (1949).
THE PERMEABILITY
OF ZIRCONIUM TO HYDROGEN1
RICHARD B. BERNSTEIN and DANIEL CUBICCIOTTI Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois Received February 6, 1950
Zirconium is known to dissolve hydrogen appreciably over a wide range of temperatures (3). Hall, Martin, and Rees (8) have made a careful study of the solubility of hydrogen in the metal. Their results indicated that the absorption and desorption of hydrogen occurred reversibly for clean zirconium. The present study reports measurements of the rate of permeation of hydrogen through zirconium at elevated temperatures. Zirconium reacts with many gases at high temperatures (6); above 500°C. it is readily oxidized by most oxygen-bearing gases, forming a black oxide which, under certain conditions, may diffuse and dissolve in the metal phase (4). The oxygen-rich metal dissolves less hydrogen under given conditions than the pure metal (8). The reaction with impure nitrogen forms a gold-yellow phase which is also soluble in the metal (6). Guldner and Wooten (7) have studied the low-pressure gas reactions of zirconium at elevated temperatures. One of the major experimental problems in this study of the hydrogen permeability of zirconium was the need for exclusion of foreign gases from the zirconium specimens. Smith (9) has reviewed the literature on the zirconium-hydrogen system, indicating some of the experimental difficulties associated with studies involving this active metal. EXPERIMENTAL
is a schematic diagram of the apparatus used in
Figure The system consists of 1
a
hydrogen purification train,
a
this investigation.
stainless-steel holder
This research was done under the sponsorship of the Office of Naval Research, United States Navy Department. 1
