Chemistry Reference
In-Depth Information
4 Copolymers as Emulsifying Agents in Polymer Blends
4.1 Copolymer Localization at the Polymer Blend Interface
It is widely understood that the use of block or graft copolymers as emulsifying
agents or compatibilizers in polymer blends is due to their affinity to selectively
partition to the polymer polymer interface. The segments of the compatibilizer can
be chemically identical with those in the respective homopolymer phases [
37
,
38
,
40
,
45
,
48
,
54
,
56
] or can be miscible with or adhering to one of the homopolymer
phases [
251 254
]. Figure
19
depicts ideal configurations of copolymer chains at the
interface, with each block preferentially extending into the respective homopoly-
mer phase [
39
,
70
,
71
,
73
,
74
]. Other conformational models are possible, such as
segments adsorbed onto the surface of one polymer rather than penetrating it.
Conformational restraints are important [
255
,
256
], and, on this basis, a block
copolymer is expected to be superior to a graft [
257
,
258
]. A graft with one branch
is shown in Fig.
19
for the case of graft copolymers; however, multiple branches
restrict the opportunities of the backbone to penetrate its homopolymer phase. This,
of course, would not preclude adhesion of the backbone to this phase. For the same
reasons, diblock copolymers are more effective than triblocks [
87
]. The block or
graft copolymer can localize itself at the blend interface only if it has the propensity
to segregate into two phases. It is the repulsion of the unlike segments of the
copolymer and the two homopolymers that leads to the localization of the copoly-
mer at the interface. Therefore, the tendency in block and graft copolymers to
migrate at the interface depends on the balance of the interaction parameters as well
as on their molecular weights.
Fayt et al. [
259
,
260
] used transmission electron microscopy (TEM) to study the
localization of the copolymer at the polymer polymer interface. Staining a short
mid-block (isoprene) with OsO
4
permitted the direct observation of the location of
the added PS-
b
-PI-
b-
PBDH triblock copolymer to the interface between PS and low
density PE; TEM images showed the localization of the copolymer to the blend
Polymer A
Polymer B
Fig. 19 Ideal location of
block and graft copolymers at
the interface between the
homopolymer phases formed
by the immiscible polymers A
and B
