JOURNAL OF POLYMER SCIENCE VOL. XII, PAGES 439-443 (1954)

New Types of Membranes for Osmotic Pressure Measurements

E. H. IMMERGUT, S. ROLLIN, A. SALKIND, and H. MARK, Institute of Polymer Research, Polytechnic Institute of Brooklyn, Brooklyn, New York

Cellulosic membranes are the most common ones employed a t present for osmotic pressure measurements. They sutTer, however, from several disadvantages: they have a short lifetime at high temperatures, so that for measurements on solutions of polyethylene, where one usually works at about 9OoC., they have to be replaced frequently. They swell to different extents in different solvents causing thereby changes in the permeability of the membrane toward solvent as well as polymer. They are readily attacked by strongly basic solvents such as cuprammonium hydroxide or cupriethylene diamine, which are generally used as cellulose solvents, and it is therefore impossible to use such membranes for direct osmotic pressure measurements on cellulose. In view of all these considerations and espe- cially to develop a method for carrying out direct measurements on cellu- lose, a study of different materials which might be suitable for osmotic membranes was begun. In the course of this investigation several promis- ing membranes were developed, of which we will discuss here the two best ones : a polyvinyl butyral membrane and a polychlorotrifluoroethylene membrane.

I . The polyvinyl butyral film was prepared by casting the film on a rotating cylinder, starting out with a 5% solution of Vinyl Butyral Resin XYHL (Union Carbide & Carbon Co.) in 95% ethanol. The film is cast a t room temperature and the cylinder is rotated a t a speed of about 14 to 20 r.p.m. Three coats are put on at five-minute intervals, and five minutes after the last cast, when the film reaches a gel stage, i.e., a nonflowing con- sistency, the whole cylinder is immersed in water for a period of time corre- sponding to the desired degree of leaching. The porosity of the resulting film will be proportional to the degree of leaching. After immersion in the leaching bath, the film is airdried.

Osmotic pressure measurements were carried out on solutions of cellu- lose in cupriethylene diamine using such a polyvinyl butyral membrane which had been left in the leaching bath for four minutes. Due to the fact that the membranes obtained were very thin, osmometers of the Zimm- Myerson type with a very small cell diameter were used in order to permit clamping the membranes rigidly, thus minimizing the danger of bulging of the membranes. The membranes were first tested with the solvent alone and it was found that they were quite rapid, i.e., they had a high per-

439

"l@ IMMERGUT, ROLLIN, SALKIND, AND MARK

meability constant. Although slower membranes could be prepared by immersing the film for a shorter period in the leaching bath, the rapid membranes were used in order to cut down degradation of the cellulose samples during the course of the osmotic pressure measurement. Two different cellulose samples, one a rayon, the other a wood pulp, were measured and the results are shown in Figures 1 and 2 and Table I.

In Table I we have the DP as calculated from the reduced osmotic pres- sure versus concentration plot (Fig. 2) and also the DP as calculated from

TABLE I %?N (obs.) -

h ~ c ~ n . -+ DP Cellulose-copper -+ MN DP (obn.) (calc.) complex cellulose (Oh.)

Sulfite Pulp 3.00 468 102,000 73,300 450 Rayon 2.06 166 39,900 28,700 177

the intrinsic viscosity, using the K and a constants of the relation [ v ] = KM" given by Immergut and Mark.' It is seen that the two DP values for each cellulose sample are in good agreement.

Although the membranes become somewhat less permeable after a num- ber of measurements, they can be restored by prolonged treatment with sodium hydroxide solution of about 1 N strength. Polyvinyl butyral mem- branes were not attacked a t all by the basic solvents to which they were exposed, even after periods of over one month.

The polytrijluorochloroethylene films were prepared by spray-coating a stainless steel or a chromium plate with a dispersion of Kel-F in xylene or with Fluorothene in Solvesso 150 (Esso Company solvent). The stand- ard Kel-F (M. W. Kellogg Company), Grade N-1, dispersion is thinned down with xylene to contain about 20'% solids. The Fluorothene (Carbide and Carbon Chemicals Division) dispersion, which seems to give somewhat smoother films, is thinned down either with Solvesso 150 or with xylene. Before spraying the dispersion onto the metal plate, the latter is coated with a mold release agent, e.g., Dow Corning Co.'s DC200 (100 centistokes), Silicone Mold Release, and this coating is baked on to the plate by heating a t 25OoC. for fifteen minutes. Any excess silicone must be wiped off with a piece of cloth to avoid sticking of the Kel-F film to the plate. The Kel-F dispersion is then sprayed onto the plate with a spray gun having a fine nozzle in such a manner that a uniform coat is produced. The dispersion should be sprayed on in a crisscross manner in order to avoid leaving parts of the coated area uncoated. After such a coat has been applied, the plate is baked at 250°C. for about ten minutes in order to partly fuse the polymeric material. Then the next coat is applied in the same manner and again left at 250°C. for ten minutes. Four or five such coats are usually required to give a film of sufficient thickness to provide a reasonably rigid osmotic membrane, because when the films are too thin, they not only bulge when pressure is applied, but they no longer act as leakproof gaskets for the osmotic cell. After the final coat has been sprayed on, the plate is left

2.

NEW MEMBRANES FOR OSMOTIC PRESSURE MEASUREMENTS 441

b& c-!

1.0

( 0

0

I ? - I &-

. - A I I I

I

Fig. 1. A h us. time. Dynamic osmotic pressure determination for a CED solution of suKte pulp (conc. = 0.417 x gm./ml.). Membrane: polyvinyl butyral.

P U L P

Fig.%. Osmotic pressure data for cellulose in CED. Solvent: cupriethylene diamine. Membrane: polyvinylbutyral. Temp. : 30 "C.

442 IMMERGUT, ROLLIN, SALKIND, AND MARK

a t 25OOC. for a period of time depending on the porosity required. The longer the film is baked, the more the polymer fuses or sinters and the smaller the permeability of the resulting film.

In order to test the permeability of the fused Kel-F films in a rapid manner, the resistance of the film in a cell containing electrolyte solution was measured on a Klett conductivity bridge. The greater the permeabil- ity of the membrane, the lower the resistance measured by the bridge. Once a few different membranes, of which the resistance, as well as the permeability to solvent, has been determined, have been prepared, a cali- bration curve is easily established.

It is qaite =cult to prepare Kel-F films without pinholes, but by very careful spraying and by putting on a sufficient number of coats, good films can be obtained. In order to test whether a membrane has pinholes or not, it is placed in the osmometer and after filling the cell with pure sol- vent, the solvent level in the capillary connected to the interior of the cell is raised several centimeters above the solvent level in the reference capil- lary. The fall of the meniscus with time is then observed and log (A/I,,/A~,) is plotted2 versus time, where A h is the initial height difference between the solvent levels in the measuring capillary and in the reference capillary, and Ah, is the height difference at a time t. If there are no pinholes in the membrane, that is, if the solvent diffuses out of the cell, this plot will result in a straight line (if the membrane is vertical rather than horizontal, a linear relation is found only as long as A h remains large), but if the solvent streams out through the holes, no straight line is obtained.

Due to the fact that the polytrifluorochloroethylene membranes are completely insoluble in the solvents generally used for osmotic pressure measurements of high polymers, they can be used as a sort of universal membrane. Thus we have carried out measurements not only on solutions of cellulose in cupriethylene diamine but also on a solution of polystyrene in benzene to show the applicability of the membrane to a purely organic system.

Another advantage of the Kel-F membrane is its inertness toward oxi- dation and this property increases its lifetime at elevated temperatures to almost indefinite lengths of time. The membrane is thus suitable for high-temperature osmotic pressure measurements.

Finally, the membranes do not have to be conditioned for any particular solvent, but can be stored dry and used directly upon immersion in the solvent.

References

1. Immergut, E. H., and Mark, H., lnd . h'ng. Chem., in press. 2. Breitenbach, J. W., and Forster, E. L., Mueromol. Chem., 8, 140 (1952).

Synopsis

Semipermeable membranes used for osmotic pressure measurements are usually made Such from collodion or from regenerated cellulose or from other cellulosic materials.

NEW MEMBRANES FOR OSMOTIC PRESSURE MEASUREMENTS 443

membranes are readily attacked by Trrosive solvents, they are easily oxidized, and they can be used for only a few hours at high temperatures. To overcome these dif6- culties new noncellulosic membranes have been developed. The preparation of a p l y - vinyl butyral membrane and of a Kel-F (polychlorotrifluoroethy1ene)membrane is de- scribed. The data for typical osmotic pressure measurements using the new membraaes are presented. The Kel-F membrane can be used for measurements in all common organic solvents as well as for solutions of cellulose in solvents such as cupriethylene di- amine and cuprammonium hydroxide. It is also suitable for high-temperature osmotic pressure measurements.

RBsum6

Les membranes s6mipermtkbles pour la determination de la pression osmotique sont gh6ralement faites de collodion ou de cellulose &gene& ou d’autres mat&es cellu- losiques. Ces membranes sont aiskment attaquQs par des dissolvants corrosifs, elles sont oxidQs vite et ne peuvent &re employ& que pendant quelques heures B haute temperature. Pour surmonter ces diEcultk, on a d6velopp6 de nouvelles membranes non-cellulosiques. La preparation de membranes de polyvinyl-butyral ou de Kel-F @olychlorotrifluoroethyl&ne) a 6tB dkrite. Les rbultats de d6terminations typiques de la pression osmotique B l’aide des membranes nouvelles sont pr&ntb . La membrane Kel-F peut &re appliqu6e B des d6terminations dans tous les dissolvants organiques usuels ainsi qu’ B des solutions de cellulose dam des dissolvants c o m e le diamine de cupriethylhe et l’hydroxide de cuprammonium. Elle est aussi propre B des determina- tions de pressions osmotiques B hautes temperatures.

Zusammenfassung

Semipermeable Membranen fiir Messung des osmotischen Druckes pflegt man aus Kollodium, regenerierter Cellulose oder anderen Cellulosematerialien herzustellen. Derartige Membranen werden schnell durch atzende Liisungsmittel angegriffen, oxy- dieren sich leicht und kijnnen bei hohen Temperaturen nur einige Stunden lang verwen- det werden. Um diese Schwierigkeiten zu iiberwinden sind neue Membranen aus an- derem Material als Cellulose ausgebildet worden. Es wird die Herstellung einer Mem- bran aus Polyvinylbutyral und einer aus Kel-F (Polychlorotrifluoroathylen) beschrieben. Die Daten typischer Messungen des osmotischen Druckes mittels der neuen Membranen werden angegeben. Die Kel-F-Membran eignet sich fiir Messungen in allen gebrzuch- lichen organischeo Liisungsmitteln, desgleichen fiir Liisungen von Cellulose in Lbungs- mitteln wie Cupriathylendiamin und Cuprammoniumhydroxyd; sie ist auch verwend- bar fiir Messung osmotischer Drucke bei hohen Temperaturen.

Received January 30, 1953