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

Abstract

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 β conformation of Poly (9,9-dioctylfluorene) (PFO) was investigated. We found that the formation of PFO β conformation had a direct relationship with chain length. Among the six samples with different molecular weights, the size of aggregations and the content of β conformation increased with the increase of molecular weight after adding the poor solvent acetone into toluene solution directly. There was almost no β conformation in aggregation or precipitation of the lowest molecular weight. We speculated that the β 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 β 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.