Chemistry Reference
In-Depth Information
3.1.5
Membrane Osmometry
The practical range of molecular weights that can be measured with this method
is approximately 30,000 to 1 million. The upper limit is set by the smallest
osmotic pressure that can be measured at the concentrations that can be used with
polymer solutions. The lower limit depends on the permeability of the membrane
toward low-molecular-weight polymers. The rule that “like dissolves like” is gen-
erally true for macromolecular solutes, and so the structure of solvents can be
similar to that of oligomeric species of the polymer solute. Thus low-molecular-
weight polystyrenes will permeate through a membrane that passes a solvent like
toluene. The net result of this less than ideal semipermeability of real membranes
is a tendency for the observed osmotic pressure to be lower than that which would
be read if all the solute were held back. From
Eq. (3-17)
, the molecular weight
calculated from the zero concentration intercept will then be too high. Membrane
osmometry is normally not used with lower molecular weight
pol
ymers for which
vapor phase osmometry (
Section 3.1.2
) is more suitable for
M
n
measurements.
Th
e membrane leakage error is usually not serious with synthetic polymers with
M
n
.
~
:
Osmometers consist basically of a solvent compartment separated from a solu-
tion compartment by a semipermeable membrane and a method for measuring the
equilibrium hydrostatic pressures on the two compartments. In static osmometers
this involves measurements of the heights of liquid in capillary tubes attached to
the solvent and solution cells (
Fig. 3.1
).
Osmotic equilibrium is not reached quickly after the solvent and solution first
contact the membrane. Periods of a few hours or more may be required for the
pressure difference to stabilize, and this equilibration process must be repeated
for each concentration of the polymer in the solvent. Various ingenious proce-
dures have been suggested to shorten the experimental time. Much of the interest
in this problem has waned, however, with the advent of high-speed automatic
osmometers.
Modern osmometers reach equilibrium pressure in 10
30000
30 min and indicate
the osmotic pressure automatically. Several types are available. Some commonly
used models employ sensors to measure solvent flow through the membrane and
adjust a counteracting pressure to maintain zero net flow. Other devices use strain
gauges on flexible diaphragms to measure the osmotic pressure directly.
The membrane must not be attacked by the solvent and must permit the sol-
vent to permeate fast enough to achieve osmotic equilibrium in a reasonable time.
If the membrane is too permeable, however, large leakage errors will result.
Cellulose and cellulose acetate membranes are the most widely used types with
synthetic polymer solutions. Measurements at the relatively elevated temperatures
needed to dissolve semicrystalline polymers are hampered by a general lack of
membranes that are durable under these conditions.
Membrane osmometry provides absolute values of number average molecular
weights without the need for calibration. The results are independent of chemical
