Depth-dependent orientation in thin films of bottlebrush block copolymers

We synthesized a series of symmetric bottlebrush block copolymers (BBCPs) with deuterated-polystyrene (dPS) as core block and poly(solkeltal acrylate) (PSA) as corona block using ring-opening metathesis polymerization (ROMP), where backbone length (N_BB) and grafting density (GD) was varied. Solid state conversion through acid hydrolysis of PSA block into poly(glycerol acrylate) (PGA) block converts BBCPs from hydrophobic-hydrophobic to hydrophilic-hydrophobic, bringing BBCPs from disordered state to strongly phase separated lamellae. Atomic force microscopy (AFM) results showed that linear BBCP (N_BB=1) had mixed orientation at surface, while star-like BBCP(N_BB=6), rod-like BBCP (N_BB=50) and cylindrical-like BBCP (N_BB=100) exhibited vertical orientation. Grazing-incidence small-angle neutron scattering (GISANS) showed that star-like BBCP were oriented normal to the air and substrate interfaces and in the bulk. With increasing N_BB, the preferential orientation was lost. Variation of GD for N_BB=50 had limited impact on the orientation of BBCPs thin films. By modifying the substrate from hydrophilic to hydrophobic, the dPS block (core block) selectively interacts with the substrate, which causes a parallel orientation of the microdomains, underscoring the importance of architectural control on BBCP thin film orientation.