Chemistry Reference
In-Depth Information
(attractive) interactions with the surface, one may have only adsorption onto the
surface, only micellization, or an equilibrium of chains adsorbed onto the surface
and chains in a micelle. This competition has not been investigated in detail in
copolymer/solvent and copolymer/homopolymer systems.
Chang et al. [
58
] investigated recently the effect of copolymer composition on
the interfacial tension reduction in PI/PDMS blends utilizing PI-
b
-PDMS addi-
tives. The authors utilized a series of diblock copolymers possessing constant
molecular weight of the PI block and different molecular weights of the PDMS
block (thus, different compositions) added to the PDMS phase. Ultralow values of
interfacial tension of the order of 10
3
dyn/cm were obtained for almost symmetric
diblock copolymers for additive concentrations in the plateau region. Such low
interfacial tensions had never been measured previously in polymeric systems,
whereas they had been obtained in systems of balanced small molecule surfactants,
for which the thermodynamically preferred form of aggregation is a surfactant
monolayer with no spontaneous curvature. The interfacial tension increased with
increasing PDMS block, going from a symmetric to asymmetric diblocks. At
certain copolymer composition, a discontinuity was observed with the interfacial
tension exhibiting a jump. For highly asymmetric additives, the behavior was
accounted for by a theory [
105
] that considered equilibrium between a PDMS
phase containing swollen spherical micelles and a phase of nearly pure PI. The
self-consistent field theory (SCFT) discussed the behavior of systems of nearly
balanced copolymers, which tend to form highly swollen micelles, within the
context of the Helfrich theory of interfacial bending elasticity [
275
], using elastic
constants obtained from SCFT simulations of weakly curved monolayers.
Besides the considerations regarding the thermodynamic factors that determine
the efficiency of a compatibilizer, the question of how and whether a state of
equilibrium is reached in such systems is still open. In principle, in all experimental
measurements, interfacial tension data are taken for long periods of time; “equilib-
rium” is considered to have been accomplished when the extracted values of the
interfacial tension do not change with time. These times can be very or extremely
long in the case of polymer polymer interfaces due to the normally very high
viscosities of the components of the mixtures. Actually, in these systems, one can
study the kinetics with which time-independent interfacial tensions are established.
Note that in the ternary systems it is the combined influence of hydrodynamic
relaxation and interfacial segregation of the additive that determines the kinetics of
equilibration measured. The time-dependent interfacial tension data of Stammer
and Wolf [
276
] for random copolymers added to the polymer polymer interfaces
were fitted with a double exponential function, with the two characteristic times
attributed to the viscoelastic relaxation and the compatibilizer transport to the
interface. Cho et al. [
53
] studied the segregation dynamics of PS-
b
-PDMS diblock
copolymer to the PS/PDMS polymer blend interface. The data were analyzed
within diffusion-limited segregation models proposed by Budkowski et al. [
277
]
and Semenov [
278
], as modified to treat interfacial tension data. The estimated
apparent block copolymer diffusion coefficients obtained were close to the esti-
mated self-diffusion coefficient of the PDMS homopolymer matrix. Shi et al. [
57
]
