POLYMER LETTERS VOL. 4, PP. 899-903 (1966)

OSMOTIC MEMBRANES FROM POLYSTYRENE-GRAFTED CELLULOSE FILMS

The techniques of grafting polystyrene onto cellulose have now be- come well developed (1,2) and have been applied mainly to cellulose in the form of textile fibers (3,4). It appears interesting to extend the grafting process to cellulose membranes. Such membranes are avail- able with pores of very different diameters (9, from less than 50 to 104 A. They show a cavity system whose effective average pore dia- meter can be decreased in a definite way by means of grafted polysty- rene.

A polystyrene gel layer is then obtained on a wide-pored cellulose substrate which shows semipermeability towards dissolved polymers. This thin gel layer on the cellulose carrier could be used a s an osmotic membrane. A polystyrene gel membrane has been described (6), but to our knowledge no applications were reported. Membranes of pure gel type, which consist of strongly crosslinked polymers, must be very thick but are still very sensitive mechanically (7). By grafting onto the cellulose matrix, we hoped to obtain a strong, stable fi lm and at the same time a thin gel layer. Only with a sufficiently thin layer is it pos- sible to obtain the required permeability of the solvent necessary for osmosis.

Schuell Co.) were chosen, which are identical with Cellafilter “fein” of Membranfiltergesellschaft Gottingen. Their pore diameter of 1000- 3000 A. allows polymer molecules of very high molecular weight, M, > lo6 , to p a s s the membrane.

Onto these cellulose f i lms, polystyrene was grafted using cobalt-60 irradiation according to the following method: cellulose membranes with a 9-cm. outer diameter were carefully solvent-exchanged from the origi- nal 20% aqueous ethanol solution to a solution,consisting of 33% sty- rene, 66% dioxane, and 1% water. They were then deareated in tubes by bubbling nitrogen through for several hours. The tubes were then sealed and irradiated at room temperature in a cobalt-60 source at a dose rate of 0.05 Mrads/hr. to a total dose of 5.6 Mrads. creased in weight by 43% after this treatment. They were then solvent- exchanged back to 20% aqueous ethanol again for storage before use.

The polystyrene layer on the cellulose carrier reduced the permeabil- ity P to water only very little.

A s a starting material, Grade 03 cellulose membranes (Schleicher and

The membranes in-

- = - P ( h - h m ) - dh F dt f

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5 10 _c_ c.10-3 [g/mJ

Fig. 1. Reduced osmotic pressure of polystyrene 111 in toluene for four different polystyrene-grafted cellulose membranes (about 30% styrene content).

where h is the decrease with t ime t of the height difference in the osmo- meter, and where F and f are the areas of the membrane and the capil- lary; values for P determined for water were between 5000 and 17,000 x

hr.-I. A change of the membrane to toluene resulted in a drop of this value to 4-35 x lo-' hr.-'. In water, these grafted polystyrene molecules take on a very small volume and decrease the pore diameter only slightly. Whereas in toluene, which is a good solvent for polysty- rene, the grafted polymer molecules expand strongly and decrease the pore diameter considerably.

In order to determine whether such a polystyrene cellulose membrane is suitable for osmotic purposes, it is best to determine the permeabil- ity limit for polymers. The permeability limit, also called the limiting molecular weight Mlimit , indicates the limiting molecular site up to which the membrane is semipermeable. This size depends of course to some extent on the solvent and on the type of polymer. W e determined it for polystyrene in toluene, using an osmometer previously described (8).

For this purpose, an atactic polystyrene with very broad molecular weight distribution (polystyrene 111 of BASF Ludwigshafen) was used. For this polymer, the distribution has been determined very precisely, so that the relationship between the measured apparent osmotic molecu- lar weight and the limit molecular weight could be established (9 ) . In Figure 1 are shown the reduced osmotic pressures a s obtained by the four different membranes for polystyrene 111. Corresponding to the very broad molecular weight distribution, one obtains measured osmotic

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Fig. 2. Reduced osmotic pressure of fractionated polymethacrylates in toluene with differently polystyrene-grafted cellulose membranes (about 30% styrene content).

molecular weights of M = 80,000-125,000. The higher the measured molecular weight, the higher the permeability limit.

atactic polymethacrylate) of Figure 2, the measured osmotic molecular weights differ only slightly. The three different membranes measured showed molecular weights of 254,000 2 3.5%, thus remaining inside the limit of experimental error which would also be shown by an ideal semi- permeable membrane.

In Table I are compiled the determined permeability limits obtained through further measurements by the above-mentioned method (9). In the second column are given the results of measurements which were obtained immediately after the change of the membrane from 20% aque- ous ethanol to the experimental solvent.

These measurements were repeated after the membranes were stored for 2 months in an osmometer containing toluene. All our measurements were carried out a temperature of 2OoC.

lies between 14,000 and 20,000. The lower limit is a little below that of gel cellulose 600; the upper value corresponds approximately to that of gel cellulose 400. The permeability l imit changes with membranes of low Mlimi t very little with time or not at all. With higher Mlimit, on the other hand, it increases. Accordingly, the stability of the mem- brane seems to increase with the amount of grafted polystyrene.

W e will discuss the influence of the solvent type elsewhere. Further researches are planned on the grafting of cellulose membranes with ini- tially narrower pore systems. Such investigations s e e m to be worth- while in the light of the first results described here.

If, on the other hand, a fractionated product is measured (such a s the

It is shown that all membranes have a limiting molecular weight which

References

(1) H. A. Krassig and V. Stannett, Advan. Polymer Sci., 4, 111 (1965). (2) E. Schwab, V. Stannett, and J . J . Hermans, TAPPI, 44, 251(1961).

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(3) R. Y. M. Huang and W. H. Rapson, in Fourth Cellulose Confer- ence (J. Polymer Sci. C, a, R. H. Marchessault, Ed., Interscience, New York, 1963, p. 169.

(4) Y. Kobayashi, J . Polymer Sci., 51, 359 (1961). (5) Supplied by Schleicher and Schuell Co., Keene, New Hampshire. (6) G. Kanig, D.B.P. 1023611 (1958). (7) G. Meyerhoff, unpublished results. (8) G. Meyerhoff, 2. Naturfrsch., u, 302 (1956). (9) G. Meyerhoff, Z. Elektrochem., a, 325 (1957).

V. Stannett

Camille Dreyfus Laboratory Research Triangle Institute Durham, North Carolina

G. Meyerhoff

Institut fur physikalische Chemie Universitat Mainz Germany

Received July 1, 1966