Chemistry Reference
In-Depth Information
1.7
1.6
1.5
1.4
2% in PS
2% in PI
1% in PS & 1% in PI
1.3
1.2
0
300
600
900
1200
1500
1800
2100
N
Fig. 26 Interfacial tension for the PS/PS
b
PI/PI systems as a function of the number of segments
(
N
) of the copolymers at a constant temperature of 140
1
C and constant 2 wt% copolymer
added to the PS phase (
open inverse triangles
). For the
N
1127 diblock, data are also shown
when 2 wt% copolymer is added to PI (
filled triangle
), and when 1 wt% is added to PS and 1 wt%
is added to PI (
open diamond
). The PS PI interfacial tension in the absence of the diblock is
denoted by a
filled square
[
54
]
whereas it decreased by further increasing the copolymer molecular weight, thus
going through a maximum (Fig.
26
).
The results were understood by considering the possibility of micelle formation
as the additive molecular weight increased, leading to a three-state equilibrium
between copolymer chains adsorbed at the interface, chains homogeneously mixed
with the bulk homopolymers, and copolymer chains at micelles within the bulk
phases. A simple model was presented that qualitatively showed a similar behavior
(see Sect.
4.3.3
). The presence of micelles for high molecular weight additives and
their absence for low molecular weights was supported by small-angle X-ray
scattering data [
55
,
268
].
Wagner and Wolf [
46
] investigated the effects of the addition of PDMS-
b
-PEO-
b
-PDMS triblock copolymers on the interfacial tension between PDMS and PEO
homopolymers. In agreement with earlier investigations, interfacial tension was
found to fall rapidly to
10% of its initial value and level off as the effective CMC
was surpassed. Moreover, the effect of the molecular weight of the PDMS block of
the triblock copolymer was studied; this effectively studied the effect of copolymer
composition without, however, keeping the copolymer molecular weight constant.
The data (Fig.
27
) showed that the interfacial tension decreased as the molecular
weight of the PDMS block approached that of the PEO block.
Subsequently, Wolf and coworkers [
49
] investigated the effect of copolymer
architecture on the interfacial tension reduction for the PDMS/PEO blend utilizing
PDMS-
b
-PEO diblocks, PDMS-
b
-PEO-
b
-PDMS triblocks, and “bottle-brush”
copolymers consisting of PDMS backbone and PEO brushes. The study showed
that for the range of molecular weights investigated, the total number of PDMS
