Diblock copolymers in a selective solvent often assemble into spherical micelles. These micelles demonstrate long range order at moderate polymer concentrations. We explore the nature of the disorder-order transition in micellar suspensions through small angle x-ray diffraction studies. The phase behavior includes body-centered cubic (bcc) and face-centered cubic (fee) lattices. We present the first phase diagram for block copolymer micelles to include both bcc and fee structures and characterize the lattice selection by a ratio of coronal layer thickness to core radius.
Polystyrene/polyisoprene (PS/PI) diblocks suspended in
decane serve as a model system
for the investigation of highly concentrated diblock copolymer
solutions. Bulk melts of PS/PI with no
solvent exhibit ordered morphologies including lamellae, close-packed
cylinders, etc. that depend on the
block asymmetry. These same diblocks self-assemble in decane at
low concentrations to form monodisperse, spherical micelles with a dense polystyrene core and a diffuse
polyisoprene corona. Strongly
interacting polymeric micelles, observed at modest polymer
concentrations, order into cubic arrays that
include both face-centered and body-centered cubic crystals depending
on the length scale of the repulsion
relative to the core dimension. These ordered morphologies of the
melt and micellar crystals provide
limiting reference states for the poorly understood high-concentration
regime studied in this work. As
we increase the polymer concentration, we observe a curious melting of
the micellar crystals before the
onset of anisotropy. Since the melting of the micellar crystal is
not predicated upon shape transitions,
we return to tethered-chain models of our spherical polymeric micelles
to qualitatively describe the
disordering process in terms of a loss of the osmotic pressure gradient
between micelles. One system
exhibits a reentrant disorder−order−disorder−order phase
transition. Finally, the development of
anisotropy in the scattering pattern is linked to shape transitions
that develop as melt conditions are
approached. We monitor the degree of anisotropy to estimate the
concentration for the onset of these
shape transitions.
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