over the entire feature supports the spherical nature of micelles.
These transitions in aggregation structures can be intuitively
understood in terms of the packing requirements of the OPE block
in the selective solvent toluene. With the progressive increase of the
polymer concentrations, the spontaneous formation of large
aggregates with a dense concentric lamella structure is favored
because it considerably decreases the interface area between
toluene and the aggregating OPE blocks.12 The proposed lamellar
structure is further proved by the detection of onion-like micelles
coexisting with the spheres from a higher polymer concentration at
2.5 mg mL21 (Fig. 2C). The comparable shrinking rates of OPE
and PS layers and muti-lamellar structures result in the onion
architecture, which is considered to be controlled largely by the
same thermodynamics that lead to the formation of the lamellar
phase in bulk.13 The thickness of the outermost shell as determined
from TEM is very uniform, at around 15 nm, still corresponding
well with the bilayer structure of the folding OPE–PS–OPE.
When the initial polymer concentration is increased to
4.0 mg mL21, another structural transition occurs, as shown by
the formation of worm-like micelles in Fig. 2D. The resulting
morphological change is presumably due to coalescence of spheres
into cylinders. The micrograph suggests a relatively narrow size
distribution of the cylindrical micelle diameters. Consequently, the
aggregate emission peak is further red-shifted from 415 to 421 nm,
accompanied by quenched emission intensity and a reduced
quantum yield because of the presence of densely packed
aggregates. Upon a further increase of the initial polymer
concentration to 5 mg mL21, interconnected rod-like fibers appear
as shown in Fig. 2E. The observed aggregate characteristics are
proposed to result from the adhesion, collision and fusion of
worm-like micelles at high polymer concentrations.
aggregate can be obtained from either dilute solution or dilution of
relatively concentrated solution with a different morphology.
In summary, we present here a simple solution-based route for
the control of morphology in a rod–coil–rod conjugated polymer.
As elucidated in TEM studies, the transitions in aggregation
morphologies are found to be correlated with the initial
concentration of the polymer. Our understanding of this subject
is still qualitative, but the evidence obtained suggests that one can
exercise control over conjugated polymer morphologies through
the interplay between nonspecific interactions, i.e. demixing of
rigid and flexible polymer parts, and self-organization driven by
specific attractive forces such as aromatic p-stacking and hydrogen
bonding. The study of the formation of multiple phase behavior
within a single structure is interesting because it may provide
unprecedented insight into the factors controlling the self-
assembling architecture of conjugated block copolymers. It also
allows for the study of electronic and optical property correlations
as a function of structure. These studies, as well as complete
investigations of these unique assemblies, are current in progress.
This work was supported by The Pennsylvanian State
University.
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4788 | Chem. Commun., 2005, 4786–4788
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