Facile control of PFO β conformation in blended polymers

Polymer blends represent a viable and efficient approach for improving the performance of functional materials and semiconducting devices. Herein, we systematically investigated how the poly(9-vinylcarbazole) (PVK) impacts the chain structure and β conformation of poly(9,9-dioctylfluorene) (PFO) in both the blended solutions and their spin-coated films. At room temperature, neither the solutions nor the films exhibited any β conformation or aggregates at any PVK content. Remarkably, after a cooling treatment at −60 °C and subsequent recovering to room temperature, an increase of the PVK content from its absence to an optimal 10 wt% led to the formation of large, ordered aggregates with high content of β conformation. In contrast, a further increase of the PVK content from 10 wt% to 70 wt% significantly suppressed the formation of β conformation, aggregates, and ordered structures. We propose that at low PVK content (0–10 wt%), PVK acts as a cross-linking agent through π-π stacking between its carbazole side groups and the PFO backbone to connect PFO aggregates, while at high level of PVK (10–70 wt%), excessive PVK may disperse the PFO phase and inhibit chain's self-folding. This work shows a facile method to control the formation of β conformation of polyfluorene in solutions and films, shining light on their future application in optoelectronic devices.