Liquid-Crystalline Order and Film Thickness Determine the Semicrystalline Morphology in Diketopyrrolopyrrole-Based Copolymers

Lyotropic liquid crystalline (LC) phases offer a means of controlling molecular order and orientation in thin films of conjugated polymers. Surface energy, surface-induced ordering, and film thickness are additional factors determining the molecular order in thin films. Through solvent vapor annealing and in situ atomic force microscopy in the swollen state, we show that in ultrathin films of a poly(dithiazolyldiketopyrrolopyrrole-tetrafluorobenzene) (PTzDPPTzF4) alternating copolymer stacks of monomolecular-thick layers with a 2.1 nm step height form, which resemble a lyotropic smectic LC phase. Within the smectic layers, the polymer backbones are aligned parallel to the film plane, with edge-on oriented diketopyrrolopyrrole (DPP) cores. Thicker films resemble a semicrystalline morphology with lamellae consisting of blocks. Such lamellae are typical for polymers crystallizing via Strobl’s block-forming model. Our findings indicate that molecular order, molecular orientation, and the morphology of PTzDPPT...