  Block copolymer self-assembly is a fundamental process in which incompatible
blocks spontaneously form organized microstructures with broad practical
applications. Classical understanding is that the domain spacing is limited by
the contour length of the polymer backbone. Here, using a combination of
molecular design, chemical synthesis, small/wide-angle X-ray scattering,
transmission electron microscopy, and electron tomography, we discover that
this molecular picture does not hold for architecturally semiflexible block
copolymers. For strongly segregated linear-semiflexible bottlebrush-linear
triblock copolymers, the size of the bottlebrush domain can be twice the
bottlebrush backbone contour length. The mechanism of such anomalous
self-assembly likely is that the interfacial repulsion between the incompatible
blocks is large enough to pull a part of the linear end-blocks into the
bottlebrush domain. This effectively increases the bottlebrush domain size.
Moreover, the semiflexible bottlebrush widens the regime for cylinder
morphology that is associated with the volume fraction of the end blocks
$f_C^{SFB}\in(0.10,>0.40)$. This window is much wider than that for flexible
linear block copolymers, $f_C^{F} \in(0.14,0.35)$, and that predicted by recent
self-consistent field theory for linear-bottlebrush block copolymers of the
same chemistry and molecular architecture. Our experimental findings reveal
previously unrecognized mechanisms for the self-assembly of architecturally
complex block copolymers.
