Chemistry Reference
In-Depth Information
product might also display some new features triggered by the particular phase
morphology.
In such systems, a satisfactory overall physico-mechanical behavior will cru-
cially depend on two demanding structural parameters: (1) a proper interfacial
tension leading to a phase size that is small enough to allow the material to be
considered as macroscopically “homogeneous”; and (2) an interfacial adhesion
strong enough to assimilate stresses and strains without disruption of the established
morphology. Both these structural parameters critically depend on the interfacial
tension between the two macroscopic phases. Block or graft copolymers are widely
used as emulsifying agents or compatibilizers in blends of immiscible polymers due
to their affinity to selectively partition to the polymer polymer interface, thus
reducing the interfacial tension. In this article, an attempt has been made to present
a review of the experimental and theoretical investigations of polymer polymer
interfacial tension in the absence and in the presence of block copolymer emulsify-
ing agents.
The variety of experimental methods that have been utilized to efficiently
measure the polymer polymer interfacial tension have been briefly reviewed,
with emphasis on the static methods (pendant drop, with the approach being very
similar to the case of sessile drop) that have been widely used for polymeric liquids.
The breaking thread method and the IFR method have been frequently used as well,
especially for high molecular weight polymers.
Polymer polymer interfacial tension measurements showed that interfacial ten-
sion decreases with increasing temperature (for polymer systems that exhibit USCT
behavior
2
), with a temperature coefficient of the order of 10
2
dyn/(cm
C).
Interfacial tension increases with increasing molecular weight and exhibits a
g ¼ g
1
1
n
dependence, with
g
1
being the interfacial tension in the
limit of infinite molecular weight. It is generally found that the exponent
z
k
int
M
z
!
1in
the limit of high molecular weights.
We have reviewed the theories of polymer polymer interfaces. We began by
presenting the early semiempirical attempts. Then, we discussed in some detail the
microscopic theories of polymer interfaces, with emphasis on the theories of
Helfand and coworkers as well as on subsequent theories. One should emphasize
here the significant influence of the original Helfang Tagami theory on the field of
polymer interfaces. The expression for the interfacial tension in the limit of infinite
molecular weights,
(
w
/6)
1/2
r
0
bk
B
T
(
40
), has been utilized extensively for
evaluation of the polymer polymer interfacial tension; the same holds for the
expression for the width of the interface (
38
), again in the limit of infinite molecular
weights. The rest of the theoretical section on polymer polymer interfaces focused
on the square-gradient approach and its utilization to predict the temperature and
g ¼
2
For polymer blends exhibiting lower critical solution temperature (LCST) behavior, e.g., the
system polystyrene/poly(vinyl methyl ether), one may anticipate the opposite behavior for purely
phenomenological reasons. Interfacial tension should increase with increasing temperature in the
two phase region since the tie lines become longer with increasing temperature in that case
