Chemistry Reference
In-Depth Information
12
10
8
6
4
2
0
5
10
15
20
25
30
35
m
Fig. 27 Interfacial tension of a phase separated mixture of PEO 35 containing 2 wt% PDMS
m
b
PEO
n
b
PDMS
m
and PDMS 100 as a function of
m
for
n
= 37 at 100
C;
n
and
m
are the numbers of
monomeric units of the copolymer blocks [
46
]
segments was the most important parameter in determining the efficiency of the
copolymer, irrespectively of their architecture or of the size of the PEO block.
Retsos et al. [
56
] investigated the effect of the macromolecular architecture and
composition (
f
) of block copolymer additives on the interfacial tension between
immiscible homopolymers. The systems investigated were PS/PI blends in the
presence of PI
2
PS (I
2
S) and PS
2
PI (S
2
I) graft copolymers. The series of grafts
possessed constant molecular weight and varying composition. A decrease in
interfacial tension was observed with the addition of small amounts of copolymer
followed by a leveling off (plateau) as the copolymer concentration (
f
add
)
increased, illustrating the surfactant-like behavior of the graft copolymers. The
interfacial tension at interfacial saturation (plateau regime) was found to be a
nonmonotonic function of the copolymer composition
f
exhibiting a minimum
versus
f
(Fig.
28
). The dependence on
f
was understood as a competition between
the decreased affinity of the copolymer within the homopolymer phase when the
size of the “other” constituent increased, which increased the driving force of the
copolymer towards the interface, and the possibility of micellar formation. These
ideas were supported by small-angle X-ray scattering measurements, which indi-
cated the formation or absence of micelles.
Another observation in Fig.
28
concerns the fact that the interfacial tension for
the I
2
S graft with
f
PI
¼
0.36 is lower than that for the symmetric linear diblock
copolymer of the same total molecular weight (all in the plateau region of the
interfacial tension reduction). It appeared that the old rule of thumb “diblocks better
than triblocks better than grafts” should be reconsidered in the general case. The
graft with
f
PI
¼
0.36 had very similar composition with diblock SI (
f
PI
¼
0.41) and
