Chemistry Reference
In-Depth Information
1.8
T=140 °C
(PS+I
2
S) / (PI+I
2
S)
(PS+SI) / (PI+SI)
1.6
1.4
1.2
1.0
(PS+I
2
S) / PI
PS / (PI+I
2
S)
(PS+SI) / PI
PS / (PI+SI)
0.8
PS / (PI+S
2
I)
(PS+S
2
I) / PI
0.6
0.0
0.2
0.4
0.6
0.8
1.0
f
PI
Fig. 28 Interfacial tension for the PS/I
2
S/PI systems as a function of the composition of the graft
copolymers at a constant temperature of 140
C and constant 2 wt% copolymer added to the PS
(
open circles
), or to the PI phase (
filled circles
), or when 1 wt% was added to the PS and 1 wt% to
the PI phases (
black and white circle
). Also shown are the interfacial tension data for PS/SI/PI at
140
C, i.e., with the addition of 2 wt% of the SI diblock copolymer to the PS (
open inverse
triangle
), or to the PI phase (
filled inverse triangle
), or when 1 wt% was added to PS and 1 wt% to
the PI phases (
black and white inverse triangle
). The
squares
are the interfacial tension data
for PS/S
2
I/PI at 140
C when 2 wt% of S
2
I was added to the PS phase (
filled square
) or to the PI
phase (
open square
). The
dashed line
indicates the PS/PI interfacial tension in the absence of
additives [
56
]
very similar molecular weight, but it was more interfacially active, which most
probably was an architecture effect. The better efficiency of the graft copolymer
versus that of the diblock was not anticipated theoretically [
85
] (when micelles
were not considered) but it was in agreement with an early study [
269
] on PS(PEO)
2
grafts versus PS-
b
-PEO diblocks of similar molecular weights added to water/
organic solvent systems. It is believed that this is due to the higher tendency of
the diblock to form micelles.
Furthermore, an important finding was that the final interfacial tension at
saturation depended on the side of the interface to which the I
2
S graft copolymer
was added. When the I
2
S was added to the PI homopolymer, the interfacial tension
reduction was more significant, i.e., the apparent interfacial activity of the additive
was higher. This pointed to a local equilibrium that can only be attained in such
systems: the copolymer reaching the interface from one homopolymer phase does
not diffuse to the other phase. For the symmetric SI diblock, the interfacial tension
at saturation does not depend on whether the additive is premixed with the PS or to
the PI phase, i.e., in that case adding the copolymer to the drop or the matrix phase
did not make any difference. Thus, the SI data allowed the authors to rule out one of
the possible explanations discussed by Hu et al. [
48
], who had suggested that such
an effect could be due to the presence of a larger reservoir of diblock when added to
the matrix phase, versus a depletion when it is added to the drop phase. When using
the respective S
2
I graft copolymers, a mirror image behavior was obtained, i.e.,
addition of the S
2
I graft to the PS side followed the behavior of the I
2
S added to PI
