Tuning Orientational Order of Highly Aggregating P(NDI2OD-T₂) by Solvent Vapor Annealing and Blade Coating

Abstract

Its inherent strong tendency to aggregate in solution is used in the following study to prepare highly anisotropic films of the n-type copolymer poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)). Solvent vapor annealing (SVA) allows to tune the size of oriented domains in spherulite-like superstructures with alignment up to several hundreds of micrometers. Blade coating (BC), on the other hand, yields square centimeter large perfectly oriented films with dichroic ratios of 18 and charge transport anisotropies up to 14. On the nanometer scale highly oriented fibers of form I are visible in the oriented areas with the fiber long axis parallel to the chain direction. We give experimental evidence that structure formation does involve liquid crystal (LC) mesophases at high solution concentrations which are frozen upon solvent removal. Temperature post-treatment of the oriented films gives, on the other hand, evidence for a classical semicrystalline nature of this polymer with spherulites consisting of crystalline and amorphous domains. These findings point to a different growth behavior than previously discussed for the well-studied p-type polymer poly(3-hexylthiophene) and suggests that the definition and distinction between liquid-crystalline and semicrystalline nature might need to be reassessed.