Chemistry Reference
In-Depth Information
For a certain polymer polymer pair, interfacial tension generally decreases
linearly with temperature with a temperature coefficient of the order of 10
2
dyn/(cm
C) [
10
,
19 24
]. Increasing the molecular weight of either polymer leads
to an increase in the interfacial tension; it is now recognized that, for high enough
molecular weights, interfacial tension shows a
M
n
z
dependence on the molecular
weight [
20
,
21
,
23 26
] with
z
2/3 or even
0.5 for lower molecular weights (
M
n
is the number average molecular weight).
Moreover, interfacial tension was found to decrease with increasing polydispersity
[
22
,
23
,
26
]. A number of thermodynamic theories have appeared from very early
on [
27 29
] until more recently [
25
,
30 35
], which predict the interfacial tension of
blends of immiscible polymers and its temperature and molecular weight depen-
dencies. Both the experimental and the theoretical investigations of polymer
polymer interfacial tension will be thoroughly reviewed in Sect.
3
.
Suitably chosen block or graft copolymers are widely used by the polymer
industry as emulsifiers in multiconstituent polymeric systems in order to improve
the interfacial situation and, thus, obtain an optimized product [
1
,
2
,
36
]. This is due
to their interfacial activity, i.e., to their affinity to preferentially segregate to the
interface between the phase-separated homopolymers [
37 44
]. This partitioning of
the block copolymers at the interface is responsible for the significant reduction of
the interfacial tension between the two macrophases [
45 59
], aids droplet breakup,
and inhibits coalescence of the dispersed phases [
60
,
61
]. This leads to a finer and
more homogeneous dispersion during mixing [
52
,
62 66
], and improves interfacial
adhesion [
67
,
68
] and mechanical properties via the significant increase in the
interfacial thickness between the macrophases [
38
,
69
]. For a block or graft
copolymer to be effective as an emulsifier, it is, thus, important that it is localized
to the polymer polymer interface [
37
,
38
,
40 44
], with each block preferentially
extending into its respective homopolymer phase [
39
,
70 74
]. Because block and
graft copolymers are likely to be expensive, it is of great importance to maximize
their efficiency so that only small amounts are required. The efficiency of interfacial
partitioning is predicted to depend on the molecular weights of the copolymer
blocks relative to those of the homopolymers [
70
,
75 79
], on the macromolecular
architecture/topology and composition of the copolymers [
80 98
], as well as on
the interaction parameter balance between the homopolymers and the copolymer
blocks [
99
,
100
].
However, a crucial issue that could severely influence the efficient utilization of
a copolymeric additive as an emulsifier is the possible formation of copolymeric
micelles within the homopolymer phases when the additive is mixed with one of the
components [
101
]. The micelles will compete with the interfacial region for
copolymer chains, and the amount of copolymer at the interface or in micelles
depends on the relative reduction of the free energy, with much of the premade
copolymer often residing in micelles for high molecular weight additives. The
effect of the existence of micelles on the interfacial partitioning of diblock co-
polymers at the polymer polymer interface has received some attention in the
literature [
54
,
56
,
75
,
77
,
102 105
]. As an alternative, in-situ formation of copoly-
mers (usually grafts) is utilized [
61
,
106 117
] in order to overcome “wasting” of the
1, although there are reports for
z
