Anomalous Self-assembly of Architecturally Semiflexible Block Copolymers

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.