Formation of D-A conjugated polymer crystals: Diffusion and conformational transition theory

Abstract

In comparison with flexible polymers, donor-acceptor (D-A) conjugated polymers have a rigid backbone and strong intermolecular π-π interactions, which result in unique crystallization behaviors including extended chain conformation, large aspect ratio crystal structure, novel rhizoid crystals and so on. To explain their crystallization behaviors, we proposed the diffusion and conformational transition (D-CT) theory here. The crystallization process includes two steps: one end of the polymer chain diffuses to the crystal growth front from the solution, and then the rest part of the polymer chain transforms into extended chain conformation. During this process, the average chain aggregate rate vA, the average diffusion rate vD and the average conformational transition vT values determine the resulting crystal morphology. The relationship between crystal morphology and the above three velocity values was established. We found that the vD/vA value determines how large the crystals can grow, while the vT/vA value determines whether the crystals could grow into the expected size. After discussing the influence of some common factors on the vD, vA and vT values, the D-CT theory can be used to explain the crystallization behaviors of D-A conjugated polymers very well. Besides, the D-CT theory may further be used to operate the crystallization process of D-A conjugated polymers, and deepens our understanding of how molecular crystals are formed.