POLYMER LETTERS VOL. 3, PP . 875-878 (1965)

DYNAMIC OSMOTIC PRESSURE MEASUREMENTS ON LOWMOLECULARWEIGHTPOLYMERS

The lower l imi t of number average molecular weight (Gn) obtained from osmotic pressure measurements is determined by the semiperme- ability of the membrane (1,2). Many membranes have been described (3-7) for low molecular weight polymers, but in most cases the low rate of attainment of the theoretical osmotic pressure is inconvenient, espe- cially when static osmometers are used. Because of these limitations, relatively few membranes provide a satisfactory overlap between osmom- etry and other techniques for measuring an.

With the aid of a dynamic osmometer* (8) and the ultrafein membranes described by Vaughan (6,7), it has been possible to accurately measure the un of several well-characterized, narrow molecular weight distribu- tion polystyrene samples a s well a s pure pentaerythritol tetrastearate (molecular weight = 1200). Vaughan found that the ultrafein membranes originally produced for use only in aqueous systems were suitable in certain organic solvents and had a high selectivity.

tioned to anhydrous toluene by successive immersion for periods of a t least 4 hr. in (1) absolute ethanol, (2) 50/50 absolute ethanol-toluene mixture, and (3) toluene. Measurements were carried out a t 37 ? O.O2OC., and the usual conventions of osmotic pressure determinations were fol- lowed. The an was calculated from the specific osmotic pressure at zero concentration. The results of these experiments are summarized in Table I.

The a, values determined by membrane osmometry (Table I) show the excellent agreement with the reported fln values of the various samples. The wide range of molecular weights measured (1200-78,000) indicates the usefulness of the membrane and osmometer a t least for these sys- t e m s . It should be noted that the total t ime required for each a,., meas- urement did not exceed 3 hr., and each individual osmotic pressure de- termination required approximately 15 min. The observed osmotic pres- sures were constant for a t least 5 rnin., and any decrease in osmotic pressure resulting from solute diffusion was apparent to only a small ex- tent with the samples of a,, < 5,000.

The un of some of the low molecular weight samples was also meas- ured on a vapor pressure osmometer (VP0)S in benzene at 37OC. The

* Mechrolab, Inc., model 502 membrane osmometer. t These membranes can be obtained from the Carl Schleicher and

Schuell Co., Keene, N.H., and are designated Bac-T-Flex, B-20 mem- brane filters.

The membranes, originally stored in 20% aqueous ethanol, were condi-

Mechrolab, Inc., model 301A vapor pressure osmometer.

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4

878 POLYMER LETTERS

results are shown in Table II. The values of w,, obtained with the VPO are in excellent agreement with those obtained with the membrane os-

mometer (Table I), and serve to point up the reliability of the two pro- cedures with these systems.

on narrow molecular weight distribution polystyrene can be made accu- rately with the use of highly selective semipermeable membranes and a dynamic osmometer. The t ime required for measurement is quite short, and the a,, calculated from the osmotic pressure is reliable over a mo- lecular weight range of 1200 to at least 75,000. Another advantage of the dynamic osmometer is the small quantity of solution and sample (< 0.1 g. for materials of a,, < 16,000) needed for the determination.

In conclusion, it has been shown that osmotic pressure measurements

The author wishes to thank Mrs. Carole F. Southerland for assistance in these experiments.

References

(1) Staverman, A. J., Rec. Trav. Chim., 344 (1951). (2) Staverman, A. J., Rec. Trav. Chim., 623 (1952). (3) Meyerhoff, G., 2. Elektrochem., 41, 325 (1957). (4) Carter, S. R., and B. R. Record, J . Chem. SOC., (m 664. (5) Hookway, H. T., and R. Townsend, J . Chem. SOC., (1952) 3190. (6) Vaughan, M. F., J. Polymer Sci., 21, 417 (1958). (7) Vaughan, M. F., Chem. Ind. (London), (1958) 555. (8) Burge, D. E., R. E. Steele, W. E. Walker, and H. C. Ehrmantraut,

“A New High-speed Membrane Osmometer, ” paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectro- scopy, March 1963.

W. C. Feist

Forest Products Laboratory* Forest Service U.S. Department of Agriculture Madison, Wisconsin

Received May 20, 1965

*Maintained at Madison, W i s . , in cooperation with the University of Wisconsin.