Simultaneous control over both molecular order and long-range alignment in films of the donor-acceptor copolymer

Control over both molecular order and long-range alignment order in films of the donor-acceptor copolymer of 3,6-bis(thiophen-2-yl)-N,N'-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT) was demonstrated via off-center spin-coating (OCSC) from its blend solution with polystyrene (PS). It was found that the dichroic ratio (DR) of OCSC blend films was dependent on both the physical process of spin-coating and the effect of PS chains. The highest DR of 2.75 was obtained via OCSC from the blend solution in oDCB at 1500 rpm. Meanwhile, both the intrachain and interchain molecular order were improved in blend films compared with neat ones, which were indicated by the red-shift of the max absorption, enhanced J-aggregation absorption, and smaller π-π stacking distance (from 3.77 to 3.70 Å). According to the results of the investigation into the macro anisotropy, micro morphology, solution rheology properties, and photophysics features of films, an overall mechanism of simultaneous control over molecular and long-range order of D-A copolymer films was proposed. On the one hand, a larger viscosity and the pseudoplastic nature of the solution tuned by choosing good solvents with high boiling points and adding PS resulted in a better chain disentanglement, better shear transfer, and a slower contact line receding velocity to induce an enhanced alignment of chains and thus fibrillar aggregates. Also, the critical contact line receding velocity for alignment dominated by the solvent evaporation rate accounted for the variation of DR with OCSC rates. A vertical phase separation accompanying the formation of aligned fibrils during OCSC was also confirmed due to the friction shear between air and solution surface. On the other hand, the negligible dependence of the blend OCSC film's photophysical and morphological features on the solvent suggested the critical role of PS in determining the better intrachain conjugation in blend films, which was attributed to multiple reasons, like limited phase separation room, a coil-toward conformation promotion, and a high surface energy. Furthermore, the enhanced π-attraction and smaller steric hindrance induced by improved intrachain conjugation accounted for the smaller π-π stacking distance in the blend films than that in the neat ones.