A Small-Volume High-speed Osmometer D. B. BRUSS
and
Shell Development
F. H. STROSS Co., Emeryville,
Calif.
b An osmometer of less than 2-ml. cell volume, but utilizing 2-inch membranes, is described. The instrument is easy to assemble, convenient to handle, and has replaceable glass capillaries for filling the osmometer and measuring the pressure.
T
HE increasing use of omotics preqsure measurements for the determination of number-average molecular weights of polymeric materials has led to the development of new instruments in recent years, and justifies the continued search for improved equipment, -4niong the many types of osmometers described in the literature [see ( 1 ) and (5) for reviews]. the Zimni-Myerson (9) osmometer is perhaps the most n idely used because of its simplicity of design and ease of manipulation. Rapid nieasuring osmometers such as the ZimmMyerson modification designed by Stabin and Immergut (8) and the instrument described by Hellfritz (4) have made possible the static equilibration of the solution-solvent system in a much shorter period of time. For limited quantities of sample, an osmometer m t h as small a cell volume as practical is desirable. Harness (3) introduced a small amount of 'is-inch glass beads into a Pinner-Stabin (7) osmometer to reduce the cell volume and by special sample handling techniques was able to use very small amounts of polymer for an osmotic pressure determination. This paper describes an osmometer of considerably reduced cell volume which retains the rapid equilibration and ease of handling of the Stabin-Immergut osmometer. The instrument is of more rugged construction and has easily replaceable capillaries for filling and pressure measurement. Instruments of this type have been used satisfactorily in these laboratories for over a year.
tainment of equilibrium is proportional to the effective area of the membrane. The cell volume should be as small as is practical to reduce the sample requirements. However, if the ratio of membrane area to cell volume becomes too great, errors due to sorption of the polymer molecules in and on the membrane may decrease the effective concentration of the cell solution. If the cell volume is too small, solvent flow into the cell during equilibration will change the concentration of the cell solution excessively. The osmometer should be easily and rapidly assembled. Membranes must be rapidly placed and clamped in the osinometer to avoid exposure to the atmosphere. The osmometer should be easily filled and rinsed. The complete assembly should be enclosed in the solvent jar and immersed in the constant-temperature bath. This ensures that the capillaries are a t the same temperature as the solvent and cell and eliminates the effect of thermal expansion and contraction of the solution in the capillaries. It also places the capillaries in an atmosphere saturated Ivith solvent and precludes evaporation of solvent from them. The osmometer should be of sufFilling Capillary
ficiently rugged construction to avoid damage in use. DESIGN FEATlIRES OF SMALL-VOLUME OSMOMETER
The small-volume osmometer is shown in Figares 1 and 2. The cell body is machined from a solid block of stainless steel, Two-inch diameter membranes are rigidly supported on each side of the cell block affording a large effective membrane area for solvent and solution contact. A well is set into each side of the block and a series of concentric grooves is machined in the bottom of each well so that each groove just cuts through to the central solution chamber, affording access of the solution to the grooves. TKOend plates (Figure 2), machined from stainless steel and incorporating the same system of grooves and lands as the cell body, fit into the wells of the cell body and form the supports between which the 2-inch membranes are clamped. Vertical slots, cut through the end plates, allow access of the solvent to the membrane. The central solution chamber consists only of a 3/32-inchdiameter hole drilled vertically into the block and positioned to meet the hole of the filling capillary (Figure 1). Another narrow tube connects the measuring capillary to the central chamber. Four bolts clamp the assembly to-
Measuring Capillary
T o p \'iew
B Hex Head Bolts
%," D
I
1
Center Hole Y
B
lI@"
DESIGN CONSIDERATIONS
The following points were considered to be important in the design of a satisfactory osmometer: The membrane arL- should be as large as possible and rigidly supported in the osmometer. Osmometers with rigid membrane support may use membranes of larger diameter. This is a distinct advantage since the rate of at1456
ANALYTICAL CHEMISTRY
Grooves a r e . %,I' Deep yIs"W i d e
C r o s s Section AA
C r o s s Section B B
Figure 1.
End View Showing Grooves
Osmometer cell body
DISCUSSION
Machine to Fit in O s m o m e t e r Body and L a p Siirfaces
Outside F a c e
AA
Figure 2.
Inside F a c e
Osmometer end plate
Two a r e required
gether. The membranes act as their own gaskets t o seal the cell. Spring lock washers under the bolt heads supply even pressure during the clamping process. As in the Stabin design, the osmometer is rinsed and filled by inserting a hypodermic needle into the cell through the filling capillary. A metal push rod, which just fits into the filling capillary, positions the meniscus in the measuring capillary. A mercury seal is used around the top of the positioning rod which extends through a hole in the top of the solvent jar (Figure 3) and holds the osmometer in a vertical plane in the jar.
The Truebore capillaries are ground flat on the bottom and have flanges on which the hex head bolts press to seal the capillaries to the cell body. Teflon gaskets aid in sealing and prevent cracking the capillaries (Figure 1). The well on the filling capillary is fitted with a. standard taper joint so that the hex head bolt can be slipped on the capillary tube (Figure 3). The reference capillary is attached to the measuring capillary with a small metal clip after the hex head bolt is slipped on. Capillaries are thus readily replaced or interchanged with capillaries of different diameters.
Filling Capillary I
1
7
I n the original Zimm-Myerson design the membrane was supported on one side only by a grooved metal plate. Membranes supported in this fashion are free to move as the hydrostatic head is varied. This disadvantage does not occur in the instruments described by Stabin and Hellfritz and in the block osmometers of Fuoss (2) and of McIntyre (6). Osmometers of the Stabin, Zimm-hlyerson, and Hellfritz type, which are enclosed in solvent jars, eliminate the problem of evaporation of solvent from the periphery of the membrane or of contamination of the membrane if a liquid bath is used. Assembly and manipulation of these instruments are convenient and rapid. The membranes act as their own gaskets to seal the cell. I n the Stabin design, each membrane is supported between grooved metal plates and Teflon gaskets are used to seal the end plates to the glass cell body. The filling capillary of the Stabin osmometer is conveniently positioned on top of the cell. This feature allows the osmometer to remain in the solvent jar while being filled and rinsed with the aid of a hypodermic syringe. The small-volume osmometer retains the handling characteristics of the Stabin osmometer, but with a considerably reduced cell volume and more rugged construction. I n both the Zimm-Myerson and Stabin instruments, if one capillary is broken the entire cell body and capillary assembly must be replaced. The equilibration characteristics of the small-volume osmometer are essentially the same as those of the StabinImmergut osmometer. Equilibration curves for various concentrations of a
Ground Joint
is 5 . 5 cm.
Clamping Ring 19 c m . Reference Capillary
d
31 5 cm.
I
I
1 1
I
1.58-1.62 mm,ID True Bore 5 mm.OD 0.5 mm1D Bottom Ground 5 mm.OD Flat Bottom Ground Flat 6 . 5 cm. Solvent J a r
Figure 3.
C 100
Figure Solvent jar and capillaries
4.
200 300 Time, minutes
400
500
Equilibration curves for polystyrene
0 N e w osmometer Stabin-lmmergut Osmometer
VOL. 32, NO. 11, OCTOBER 1960
1457
Table I. Comparison of Results Obtained with Different Osmometers
Sample
SmallVolume
Stabin
ZimmMyerson
Polystyrene polymer 126,000 120,000 122,000 Fraction 1
Fraction 2
2 0 0
44,000
40,000
I
1
0 4
0 6 C , g i 1 0 0 mi.
0 2
46,000
0.8
1 0
Figure 5. Extrapolation curves for polystyrene
202,000 206,000
...
0 0
polystyrene are shown in Figure 4 and are compared with the curve obtained from a Stabin instrument a t the 1% level (slightly different concentration). The same membranes were used in each case. Extrapolations to infinite dilution are shown in Figure 5 . The cell volume of the iiistrument is somewhat less than 2 ml. or about one fifth of that of the Stabin instrument. The osmometer may be rinsed twice and filled on less than 10 ml. of solution. Approximately 80% of the cell solution is in the grooves stlid is thus in intimate contact with the membrane. Dilution errors can be calculated from the capillary dimensions assuming a certain hydrostatic head. Taking 15 cm. as the hydrostatic head, the error by dilution (initial pressure of zero) is less than 1%. One advantage of the Zimm-3Iyerson osmometer with the cylindrical glass cell body is that it allows visual inspection of t8he chamber for air bubbles.
However, no difficulties were experienced with the small-volume design since the internal passages are so small that any bubbles are swept out during the filling procedure. No apparent concentration errors due to adsorption of polymer on the membrane were observed in spite of the high ratio of membrane area to cell volume. Effective leak-tight seals were obtained without difficulty, providing the sealing surfaces around the perimeter of the concentric grooves were free from surface mars and scratches. Table I shows some comparative results of determinations made on polystyrene samples with different osmometers. The same gel-cellophane membranes vere used in each case. RIeasurements using the same membranes show that the repeatability of the determination using the small-volume instrument is approximately the same as that found for the other os-
N e w osmometer Stabin-lmmergut osmometer
mometers and is estimated normally t o be about 1 5 % . LITERATURE CITED
(1) Bonnar;: R. U., Dimbat, M.,Stross,
F. H., Sumber Average Molecular Weight,” p. 191, Interscience, Kew York, 1958. (2) FUOSS, R. M., Mead, D. J., J . Phys. Chem. 47, 59 (1943). (3) Harness, A. A,, J. Polymer Sci. 25, 498 (195i), (4) . . Hellfritz, H., Makromol. Chem. 7, 184 (1951). Hellfritz, H., Kramer, H., Kunststofle (5) H 46, 450 (1956). 46,450 (6) hlcIntyre, D., Doderer, G. C., O’Mara, J. H., J . Research N a t l . Bur. Standard. f i X I(1959). 1 OR41 Standards 157 62,63 ( 7 ) Pinnei (7) Pinner, H. S., Stabin, J. V., J . Polymer Sci. 9, 575 (1952). (8) Stabin, J. V., Immergut, E. H., Ibid., 14, 209 (1954). (9) Zimm, B. H., Myerson, J., J . Am. Chem. SOC.68, 911 (1946). \
,
RECEIVED for review April Accepted July 18, 1960.
25, 1960.
The Influence of Viscometer Design on Non-Newtonian Measurements RAYMOND McKENNELL Ferranti Lid., Manchester 7 0, England
b Uncertainty arising from viscometric data obtained from nowNewtonian fluids is minimized by observing two principal conditions: The shear rate should be uniform throughout the measured sample; and a consistent experimental procedure, including the amount and duration of shear, should be adopted. The effect of shear rate variation within the sample fluid is discussed for some viscometers in common industrial use. The methods available for eliminating variations in shear rate, and hence the tedium of applying corrections to flow curves for different types of non-Newtonian behavior, are outlined. A viscometer is 1458
ANALYTICAL CHEMISTRY
described which combines uniform shearing conditions with a flexible automatic flow-curve recorder giving a wide range of rheological test programs.
T
problem in designing a viscometer for determinations on non-Xewtonian fluids lies in the inability of such fluids to sustain shear without changing the very properties one is attempting to measure. Because of this, i t is important to provide known, well defined shearing conditions within the viscometer so that, even if the shear rate is not uniform, a knowledge of the limits of variation HE FUNDAMENTAL
permits a meaningful interpretation of the flow data. INFLUENCE OF SHEAR RATE VARIATIONS
Variations in shear rate in a nonNewtonian liquid floring within a viscometer can give raise to errors of considerable magnitude if a value is assigned to the shear rate calculated on the basis of a Newtonian fluid. Such errors can often be eliminated by applying a correction term to the flow data which takes into account both the geometrical configuration of the viscometer and the flow properties of the fluid. Hon-ever, the tedium of this procedure
