Bent and faceted single crystals of poly(vinylidene fluoride) (PVDF) were prepared by isothermal crystallization in its blends with poly(butylene succinate) (PBS). After removing PBS by CHCl 3 washing, the remaining PVDF crystals were examined by optical, atomic force, and transmission electron microscopies and Fourier transform infrared spectroscopy. PVDF single crystals are attributed to α-phase and can reach hundreds of microns with two or more bent branches. The C-and X-shaped crystals are composed of bent basal lamellae, seen flat-on, with a great number of overgrowths. The overgrowths are lozenge-shaped, small-sized single crystals, and most of them grow along the C curves of either C-shaped or X-shaped crystals, keeping the same growth direction with the basal one. Consequently, the electron diffraction patterns taken everywhere of the whole crystal exhibit always a single crystal feature but change constantly with b-axis. These may be attributed to the unbalanced growth of four (110) faces and an inhomogeneous surface order or arrangement of loops and cilia. The lozenge-shaped, large-sized monolayer crystal can be observed upon decreasing the film thickness but without overgrowth, and its shape changes with crystallization temperature. These results can help to understand the surface structure of lamellae and its proliferation process profoundly.
Thermally activated delayed fluorescence (TADF) materials have attracted extensive attention because of their 100% theoretical exciton utilization. Solution‐processable orange‐red TADF polymers are one of indispensable participants. Herein, a series of orange‐red TADF polymers with dibenzothiophene (DBT) and carbazole (Cz) units as joint backbones are synthesized. Their performance can be successfully optimized by regulating the connection positions of DBT units through backbone engineering. It is found that the pNAI37 series with DBT units embedded in the polymeric backbones at the 3, 7 sites display a better performance than those connected at the 2, 8 sites. The optimal polymer, pNAI3705, exhibits a better excited state nature, leading to the photoluminescence quantum yield of 60%. Consequently, pNAI3705 based organic light‐emitting diodes reach a maximum external quantum efficiency of 20.16%, and maintain 10.61% at 500 cd m−2, which is in first tier among orange‐red polymers. These results unambiguously suggest the potential application of the combined DBT and Cz backbones in TADF polymers. This design strategy may provide a versatile approach for optimizing the properties of TADF polymers through backbone engineering.
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