Chemistry Reference
In-Depth Information
Increasing
solvent
power
(b)
(a)
Increasing
molecular
weight
π
π
Theta
solvent
c
c
c
c
FIGURE 3.2
(a) Reduced osmotic pressure (
π/c) versus concentration, c, plots for the same polymer
sample in different solvents. (b) (
π/c) versus c plots for different molecular weight samples
of the same polymer type in a common solvent.
(
Fig. 3.2a
), however, since the second virial coefficient reflects polymer
solvent
interactions and can be related, for example, to the Flory
Huggins interaction
parameter
χ
(Chapter 5) by
1
2
2χ
LV
1
υ
2
2
A
2
5
(3-30)
υ
2
is the specific volume of the polymer,
V
1
is the molar volume of the
solute, and
Here
χ
is an interaction energy per mole of solvent divided by
RT
. When
χ5
0 and the solvent is a theta solvent for the particular polymer.
Better solvents have lower
0.5
,A
2
5
values and higher second virial coefficients.
It may be expected also that different molecular weight samples of the same poly-
mer should yield the same slopes and different intercepts when the osmotic pressures
of their solutions are measured in a common solvent. This situation, which is shown
in
Fig. 3.2b
, is not realized exactly in practice because the second virial coefficient is
a weakly decreasing function of increasing polymer molecular weight.
χ
3.1.6
Vapor Phase Osmometry
Regular membrane osmometry is not suitable for measurements of
M
n
below
about 30,000 because of permeation of the solute through the membrane. Other
colligative methods must be employed in this range, and vapor pressure lowering
