Square Array of Cylinders Formed in Linear–Bottlebrush–Linear Triblock Copolymers

The cylinders self-assembled by AB-type block copolymers usually pack into a hexagonal lattice because it has the highest coordination number (CN) of 6 and thus the least nonuniformity, with the stretching of the majority blocks filling the space outside the cylinders. How to obtain the square lattice of cylinders with lower CN = 4 has always been an interesting but challenging problem, despite its greater applicability in the semiconductor industry compared to the classical hexagonal cylinders. Here, we propose to use a linear–bottlebrush–linear block copolymer to stabilize the tetragonal cylinders and apply dissipative particle dynamics simulations to investigate its self-assembly behaviors. Our results reveal an optimal window for the relative length of side chains to backbone that stabilizes the tetragonal cylinders. Deviations from this window, such as overly “fat” (longer side chains with shorter backbone) or “thin” (shorter side chains with longer backbone) middle-bottlebrushes, reduce the bridging fraction or backbone stretching, respectively. The asymmetry of the linear A-blocks also plays a critical role in stabilizing the tetragonal cylinders. Highly symmetric A-blocks limit backbone stretching, while extreme asymmetry reduces the bridging fraction. Within an optimal range of asymmetry, the core–shell distribution of long and short A-blocks enlarges the A-domain and enhances backbone stretching, thereby stabilizing the tetragonal cylinders. Additionally, increased side chain rigidity facilitates the filling of far spaces in the morphology, further enhancing A-domain separation and backbone stretching. Our simulations demonstrate that the tetragonal cylinders are both thermodynamically and kinetically stable in considerable windows of the key parameters, and therefore our work may guide experiments to fabricate the unusual morphology.