Structural details and digital memory performances of difluorene-containing diblock copolymers in nanoscale thin films

Abstract

Amphiphilic poly(4-di(9,9-dihexylfluoren-2-yl)styrene)-b-poly(2-vinylpyridine)s (PStFl2(m)-b-P2VP(n)) in two different compositions and their homopolymers were synthesized: PStFl2(11)-b-P2VP(89) (50/50, volume ratio), PStFl2(12)-b-P2VP(33) (75/25), PStFl2, and P2VP. They were thermally stable up to around 350 degrees C. In nanoscale thin films, the diblock copolymers exhibited various phase-separated nanostructures depending on the composition and film process condition: random two phases, horizontal hexagonal P2VP cylinders, and hexagonally-close packed (HCP) P2VP spheres. Surprisingly, the hexagonal cylinder and HCP sphere structures are quite different from those of common diblock copolymers with similar compositions. The structural details of these thin film morphologies were investigated by synchrotron grazing incidence X-ray scattering. The thin film morphologies were found to make influences on the electrical memory performances of the polymers. In particular, the switching-ON voltage was influenced by the nanostructures and the film layer thickness as well as by the composition. Overall, the diblock copolymer films demonstrated excellent p-type permanent digital memory behaviors with unipolarity, long retention time, high ON/OFF current ratio and low power consumption. These memory behaviors were governed by a trap-limited space charge limited conduction mechanism combined with ohmic conduction and a hopping process. (C) 2015 Elsevier Ltd. All rights reserved.