Influence of molecular weight and the change of solvent solubility on beta conformation and chains condensed state structure for poly (9,9-dioctylfluorene) (PFO) in solution

Molecular weight has a significant influence on the performance of conjugated polymer photoelectronic devices. However, so far, few detailed researches have focused on solution-related properties such as chain conformation and condensed state structure. In this work, the influence of molecular weight and the change of solvent solubility on the single-chain conformation, aggregation state structure and ordered beta conformation of Poly (9,9-dioctylfluorene) (PFO) was investigated. We found that the formation of PFO beta conformation had a direct relationship with chain length. Among the six samples with different molecular weights, the size of aggregations and the content of beta conformation increased with the increase of molecular weight after adding the poor solvent acetone into toluene solution directly. There was almost no beta conformation in aggregation or precipitation of the lowest molecular weight. We speculated that the beta conformation was mainly formed by the folding of single-chain, because polymer chain of higher molecular weight was longer and more flexible for conformational and orientational transformation. Meanwhile, we found an interesting phenomenon that the beta conformation formation was strongly affected by the changing rate of solvent solubility. The possible mechanism of the related phenomenon was revealed. This research provides not only a profound understanding of the formation process and mechanism of the ordered structures of semi-rigid conjugated polymer in solution, but also a theoretical basis for manipulating the condensed state structure of films in the application of photoelectronic devices from the perspective of conjugated polymer physics.