Chemistry Reference
In-Depth Information
10
6
10
6
10
4
10
4
m
n
10
2
10
2
n
= 1060
m
= 986
0.0
0.2
0.4
0.6
0.8
1.0
j
c
PVME (m)
PS (n)
Fig. 23 Combination of numbers
m
of PVME segments with numbers
n
of PS segments leading to
the critical compositions
c
. The experimental data, taken from the literature [
60 62
], were
obtained by mixing one PS sample (
n
1060,
open circles
) with PVME of different molar
mass or, vice versa, one PVME sample (
m
986,
closed circles
) with different samples of PS.
The
curves
are calculated using two modeling variants as described in the text [
59
].
Solid lines
:
the conformational relaxation does not depend on temperature;
dashed lines
: it varies linearly
with T
'
fundamentally if both components become high in molar mass. In both variants, a
was considered to depend on temperature but variant 1 keeps z independent of
T
,
whereas variant 2 applies the proportionality between a and z, i.e., treats z as a
function of
T
. Variant 1 yields two stable and one unstable critical points [
59
] (as for
the system water/PVME), whereas the demixing behavior remains normal for
variant 2. Defining theta conditions for polymer blends by analogy to the usual
definition for polymer solutions in terms of critical temperature for infinite molar
mass of the polymer according to:
lim
T
c
m
!1
Y
(66)
;
n
one obtains two different theta temperatures, where the corresponding critical
concentration is either zero or unity. Conversely, z proportional to a yields only
one theta temperature, and the corresponding critical composition remains indefi-
nite, like in the original Flory Huggins theory. The question of which of the
predictions comes closer to reality can only be answered by directed experiments.
Shape-Induced Polymer Incompatibility
Demixing of polymer blends consisting of macromolecules synthesized from the
same monomers and differing practically only in their molecular architecture plays
