Chemistry Reference
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ple structural variation from linear to dibranched chains generates a three-
dimensional (3D) supramolecular structural inversion from organized coil
perforations in rod layers to organized discrete rod bundles in a coil matrix.
This implies that the steric hindrance at the rod/coil interface arising from
branched coils plays a crucial role in the self-assembly of rod segments, as
well as the conformational entropy associated with coil length.
Stupp et al. reported on rod-coil copolymers consisting of an elongated
mesogenic rod and a monodisperse polyisoprene [37-39]. These rod-coil
copolymers organize into ordered structures that differ in terms of vary-
ing rod volume fraction, as monitored by transmission electron microscopy
and electron tomography. Depending on the rod volume fraction (
f
rod
), rod-
coil oligomers either form strip-like morphologies or self-assemble into dis-
crete aggregates that are organized in an hexagonal superlattice, with do-
main sizes typically between 5-10 nm. The authors also synthesized rod-coil
copolymers containing oligostyrene-block-oligobutadiene as the coil block
and rigid biphenyl units connected by ester linkages as the rod block [40-
42]. Polarized optical microscopy showed that molecule
7
undergoes a phase
transition from the smectic to the isotropic state. On the basis of TEM
and X-ray data, it was suggested that the rod-coil molecules pack into the
mushroom-shaped nanostructure with a height of 8 nm and a diameter of
2 nm. Each supramolecular nanostructure was estimated to contain approxi-
mately 23 molecules. Most importantly, this nanostructure was proposed
to impart the spatial isolation of cross-linkable oligobutadiene blocks re-
quired to form a well-defined object. Therefore, polymerization might be
confined to the volume of the supramolecular cluster. Thermal polymeriza-
tion of rod-coil molecules in liquid crystalline state produced high molar-
mass products with a very narrow polydispersity within a range of 1.15 to
1.25 and a molecular weight of approximately 70 000,asconfirmedbyGPC
(Fig. 4). The macromolecular objects obtained reveal an anisotropic shape (2
by 8 nm) similar to that of supramolecular clusters, as determined by electron
microscopy and small-angle X-ray scattering. Polarized optical microscopy
showed that polymerization of the molecules into macromolecular objects re-
sults in a strong stabilization of the liquid crystalline phase that remains up
to a chemical decomposition temperature of 430
◦
C. This result is interesting
because the self-assembly process provides a direct pathway for preparation
of well-defined molecular nanoobjects with distinct and permanent shapes
through polymerization within supramolecular structures.
Yu et al. reported the synthesis of rod-coil block copolymers contain-
ing oligo(phenylene vinylene)s (OPV) coupled to either polyisoprene or
poly(ethylene glycol) [43, 44]. These OPV showed a reversible thermotropic
liquid-crystalline transition. The liquid-crystalline texture observed using
a polarizing microscope shows a typical Schlieren pattern, which is evidence
for the presence of nematic phases. TEM and small-angle X-ray scattering re-
vealed alternating strips of rod-rich and coil-rich domains. The domain sizes
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