Phase behavior and micellization kinetics of diblock copolymer surfactants in selective
solvents influence many processes. We study the driving forces behind the
self-assembly of a diblock copolymer AB, consisting of a solvent-philic block (B) and
a solvent-phobic block (A), in selective solvents (S). We investigate this system using
self-consistent field theory (SCFT), which is a coarse-grained, approximate theory
with a proven track record for polymer mixtures. It discards the effects of fluctuations.
Micellar transformations between spherical, cylindrical, and bilayer curvatures are
tracked in the dilute regime. We determine thermodynamic and structural properties
of these isolated aggregates such as the critical micelle concentration (CMC), the
critical micelle temperature (CMT), the solvent penetration of the core, and the core
radius of micellar morphologies within the context of SCFT.
We also investigate the morphological variation from ordered phases, found in the
concentrated regime, to isolated aggregates upon copolymer depletion. Depleting this
blend of surfactant causes these stable structures to swell and ultimately unbind. The
unbinding transition of the ordered phases is compared with the morphology transformations
observed in the dilute regime. We also delineate two phase coexistence
regions between ordered phases, and ordered phases and a solvent rich macrophase.
Furthermore, we quantify the effective interactions between the aggregates themselves.
Intriguingly, for spherical micelles, the free energy of BCC, and FCC phases
can be described in terms of a single effective pair potential that depends on micellar
aggregation number, however, this aggregation number changes significantly with the
concentration and temperature.
The kinetic barriers to association and dissociation of diblock copolymers in various
selective solvents are calculated. We study the variation of these kinetic barriers
for both block copolymers in small molecule solvents and block copolymers in a homopolymer
matrix. The kinetic barriers are found to be very sensitive to temperature
and surfactant concentration. They also become prohibitive except in a modest range
of temperature near the CMT, or in sufficiently highly supersaturated or subsaturated
solutions near the equilibrium CMC. The dependence of kinetic barriers upon the chain lengths and solvent quality is also studied.