Conjugated block copolymer (BCP)-based semiconductors have been extensively researched owing to their unique microphase-separation properties, making them promising materials for various applications, especially as optoelectronic devices. Among the methods for tailoring the nanoscale and packing morphology of conjugated polymers, adding them into BCPs has attracted significant research attention. This approach may allow them to self-organize into ordered structures and efficiently control their physical and optoelectronic properties. Since most of conjugated polymers can crystalize, BCPs containing one or more rod-like conjugated component are deemed quite suitable for studying the two important fundamental phase transitions, that is, crystallization and microphase separation..
At present, most studies on tunning the crystalline structure of conjugated rod-rod BCPs are based on post-treatment methods like solvent and thermal annealingInterestingly, an alternative method for directly tuning their crystalline structures in the solution state is by efficiently controlling the selectivity of the solvent. However, this alternative method is relatively less used despite its perceived efficiency.
Recently, Fudan University researchers: Professor Juan Peng and coworkers Yue Yin, Shuwen Chen, Shuyin Zhu, Lixin Li, Dalong Zhai, Dongqi Huang investigated the microphase-separation and crystallization of a poly(3-butylthiophene)-block-poly(3-dodecylthiophene) (P3BT-b-P3DDT) BCP by tailoring the intrinsic factors like block ratios and extrinsic factors like controlling the solvent in the solution state and thermal annealing. The research team reported that higher P3BT content, slower solvent evaporation and higher thermal annealing temperatures are favorable for microphase separation in P3BT-b-P3DDT. The research work is currently published in the journal, Macromolecules.
Due to the difference in their alkyl side chain lengths, P3BT-b-P3DDT could enable efficient tunability when the solvents are changed. Specifically, cocrystalline structures were obtained when processing from chloroform, while casting from 1,2,3-trichlorobenzene at slower evaporation rates is favorable for producing microphase separated structures. Subsequently, heating at 150 °C improved the crystallinity of P3BT-b-P3DDT cocrystals casted from chloroform. On the other hand, annealing at 220 °C produced distinct microphase separation forming P3BT and P3DDT individual crystals for all the tested P3BT-b-P3DDT BCP samples. Furthermore, a synergistic effect of block ratios, solvents, annealing temperatures, and film crystallinity on the charge mobilities for field-effect transistors was also reported.
In summary, the authors modulated the block ratios, solvents and heating processes to tailor the cocrystalline and microphase-separated behaviors of P3BT-b-P3DDT BCPs. Both intrinsic and extrinsic factors have profound effects on phase transitions and resulting charge mobilities of conjugated BCPs. Most importantly, the cocrystallization and microphase separation in P3BT-b-P3DDT can be simply and efficiently tailored by controlling the solvent evaporation in the solution state. In a statement to Advances in Engineering, Professor Juan Peng explained that their new findings provided fundamental insights on the interplay between cocrystallization and microphase separation in conjugated rod-rod BCPs, which could facilitate their optoelectronic device applications.
Yin, Y., Chen, S., Zhu, S., Li, L., Zhai, D., Huang, D., & Peng, J. (2021). Tailoring Cocrystallization and Microphase Separation in Rod–Rod Block Copolymers for Field-Effect Transistors. Macromolecules, 54(10), 4571-4581.