Some of the early efforts to understand rod–coil block copolymers were based on
rigid segments that were made up of a-helices formed by polypeptides, conjugated
chains or thermotropic mesogens [65]. Most of the earlier systems contained polydisperse blocks of both the rigid and coil segments, and were less amenable to
discrete nanostructured phase separation. One of the first efforts to prepare rod–
coil block copolymers with precisely controlled size of the rod-segment, utilized
a molecularly exact mesogenic segment built from biphenyl units, while the
coil segment consisted of a poly(isoprene) segment, whose molecular weight was
varied (3200, 4200 and 5400), leading to rod–coil systems with the volume fraction
of the rod ( f rod ) decreasing from 0.36 to 0.30 to 0.25, respectively (Structure A, in
Figure 15.18). Thin films of these rod–coil diblock copolymers exhibit nanophase
separation forming rod-rich and coil-rich domains with dimensions less than
10 nm. With decreasing volume fraction of the rod segment, continuous strip-like
domains break up to form discrete aggregates. Electron tomography studies proved
to be particularly useful in establishing the nature of the packing across the film
thickness. A schematic depiction of the packing of the strips that measure 6–7 nm
in width (approximately the molecular length of the rod segment) is shown in
Figure 15.19. The strips were shown to pack in layers parallel to the film plane in
the manner shown; the order in which the block copolymer domains assemble is a
consequence of the need to minimize the extent of stretching that the coils must
sustain in order to ensure uniform filling of the available space (uniform segment
density across the domain). As the volume fraction of the rod segment decreases to
0.25, small disk-like aggregates are formed that form close-packed hexagonal superlattices (Figure 15.19).
It was argued that the formation of aggregates with an intrinsically self-limited
size could be possible if steric constraints were built into the block copolymer design. Increasing the cross-sectional area of the coil block is one possible approach
to bring about the formation of such a self-limiting nanoaggregate. Incorporation of
another block of poly(styrene) to generate rod–coil triblock copolymers, in which
the coil block consists of two blocks, one a poly(styrene) block (semiflexible) and
the other a poly(isoprene) block (flexible), proved to be one such approach (Structure B in Figure 15.18) [68]. What distinguishes this particular study from many of
the earlier efforts is that it dealt with macromolecules having relatively small sizes
of each block. The triblock copolymer consisted of PS and PI segments, each having an average of just 9 repeat units, that was linked to a trimeric biphenyl segment. Films cast from this rod–coil block copolymer generated mushroom-shaped
nanostructures (Figure 15.20a), which lacked a center of inversion. Each of these
aggregates was estimated to contain about 100 molecules. Thus, the origin of these
self-limiting aggregates is believed to be the interplay between the crystallization
tendency of the biphenyl segments, the coil entropy of the flexible PI coil segment
and the steric repulsion of the semiflexible PS segments. One of the most interesting features of these structures was the macroscopic layer-wise self-assembly of
these mushroom-shaped nano-aggregates, even in the bulk film [68]. Using a series of such triblock copolymers with varying sizes of the coil diblock, a clear linear
relationship between the layer periodicity and the extended average size of the
15.7 Rod–Coil Diblock Copolymers 503
rigid segments that were made up of a-helices formed by polypeptides, conjugated
chains or thermotropic mesogens [65]. Most of the earlier systems contained polydisperse blocks of both the rigid and coil segments, and were less amenable to
discrete nanostructured phase separation. One of the first efforts to prepare rod–
coil block copolymers with precisely controlled size of the rod-segment, utilized
a molecularly exact mesogenic segment built from biphenyl units, while the
coil segment consisted of a poly(isoprene) segment, whose molecular weight was
varied (3200, 4200 and 5400), leading to rod–coil systems with the volume fraction
of the rod ( f rod ) decreasing from 0.36 to 0.30 to 0.25, respectively (Structure A, in
Figure 15.18). Thin films of these rod–coil diblock copolymers exhibit nanophase
separation forming rod-rich and coil-rich domains with dimensions less than
10 nm. With decreasing volume fraction of the rod segment, continuous strip-like
domains break up to form discrete aggregates. Electron tomography studies proved
to be particularly useful in establishing the nature of the packing across the film
thickness. A schematic depiction of the packing of the strips that measure 6–7 nm
in width (approximately the molecular length of the rod segment) is shown in
Figure 15.19. The strips were shown to pack in layers parallel to the film plane in
the manner shown; the order in which the block copolymer domains assemble is a
consequence of the need to minimize the extent of stretching that the coils must
sustain in order to ensure uniform filling of the available space (uniform segment
density across the domain). As the volume fraction of the rod segment decreases to
0.25, small disk-like aggregates are formed that form close-packed hexagonal superlattices (Figure 15.19).
It was argued that the formation of aggregates with an intrinsically self-limited
size could be possible if steric constraints were built into the block copolymer design. Increasing the cross-sectional area of the coil block is one possible approach
to bring about the formation of such a self-limiting nanoaggregate. Incorporation of
another block of poly(styrene) to generate rod–coil triblock copolymers, in which
the coil block consists of two blocks, one a poly(styrene) block (semiflexible) and
the other a poly(isoprene) block (flexible), proved to be one such approach (Structure B in Figure 15.18) [68]. What distinguishes this particular study from many of
the earlier efforts is that it dealt with macromolecules having relatively small sizes
of each block. The triblock copolymer consisted of PS and PI segments, each having an average of just 9 repeat units, that was linked to a trimeric biphenyl segment. Films cast from this rod–coil block copolymer generated mushroom-shaped
nanostructures (Figure 15.20a), which lacked a center of inversion. Each of these
aggregates was estimated to contain about 100 molecules. Thus, the origin of these
self-limiting aggregates is believed to be the interplay between the crystallization
tendency of the biphenyl segments, the coil entropy of the flexible PI coil segment
and the steric repulsion of the semiflexible PS segments. One of the most interesting features of these structures was the macroscopic layer-wise self-assembly of
these mushroom-shaped nano-aggregates, even in the bulk film [68]. Using a series of such triblock copolymers with varying sizes of the coil diblock, a clear linear
relationship between the layer periodicity and the extended average size of the
15.7 Rod–Coil Diblock Copolymers 503