Macroscopic Vertical Alignment of Nanodomains in Thin Films of Semiconductor Amphiphilic Block Copolymers

Though several techniques have been reported on the alignment of conventional block copolymers, the macroscopic vertical orientation of semiconductor block copolymer microdomains in thin films has still not been accomplished. Here, we report the control on the alignment of nanostructures in a semiconductor amphiphilic block copolymer comprising an amorphous triphenyldiamine hole conductor block and a hydrophilic poly(styrene sulfonate) segment. Three different compositions with a hole conductor content of 57, 72, and 79 wt % were synthesized using a combination of controlled reversible addition/fragmentation transfer polymerization and "click" chemistry. All polymers feature a narrow molecular weight distribution. Cryo-TEM reveals the formation of micelles in DMF solutions of the amphiphilic copolymer having nanoscopic dimensions. The micelle size correlates well with the X-ray analysis of dried bulk samples. Atomic force microscopy (AFM) confirms the micellar structure in the as-cast films. Thermal annealing causes an aggregation of micelles but did not lead to morphologies known for conventional block copolymers. However, annealing in saturated DMF vapor induces a morphology transition and a vertical orientation of the microdomains which was determined by grazing incidence small-angle X-ray scattering and AFM. The morphology varies from lamella to cylinders with increasing content of the hole-conductor block. The orientation arises from the controlled evaporation of the solvent, a mechanism that is similar to that observed for conventional block copolymers. Our approach demonstrates the macroscopic vertical alignment of nanodomains in semiconductor block copolymers which is a key requirement for applications in hybrid devices.