Abstract
It is known that the solution structure of conjugated polymers may have great impact on the self-assembly behavior and structure in the subsequently cast film due to memory effect. In this study, we investigate the effect of aging the solution on the crystallization kinetics of P3HT and PBTTT-C14 in the subsequently cast film. P3HT was dissolved in chlorobenzene (CB) and tetrahydrofuran (THF), where CB is a better solvent than THF, to form macroscopically homogeneous solutions. The resultant solutions were then allowed to age for different time periods at room temperature up to two weeks. It was found that the crystallization kinetics of P3HT in the film cast from CB was essentially unperturbed by the solution aging. On the other hand, the solution aging exerted an obvious influence on the crystallization rate of P3HT in the THF-cast film, where the film cast from the solution aged for longer time exhibited a lower initial crystallinity and slower crystallization rate. The retardation of the crystallization kinetics was attribute to the development of the nanowhisker structure in the solution in the aging process. In the case of CB solution, the development of such a morphological entity by P3HT crystallization during aging was minor, such that the crystallization occurred in an essentially homogeneous amorphous matrix in the film. In the case of THF solution, the nanowhiskers formed during the aging due to poorer solvent quality and these whiskers were transferred into the film. The nanowhisker entity remained stable even in the melt state and thus imposed a topological constraint to the subsequent crystallization upon cooling. In this case, the crystallization of P3HT took place in the mesh regions constrained by the networking of the naowhiskers, so as to retard the crystallization kinetics. In the case of PBTTT-C14, the film was cast from its solution with dichlorobenzene. It was found that solution aging led to a weak retardation of the crystallization kinetics in the subsequently cast film. The effect of solution aging on reducing the crystallization kinetics amplified significantly in PBTTT-C14/PCBM blends. We also observed the formation of bimolecule crystal between PBTTT-C14 and PCBM in the fresh solution-cast film. The crystallinity and crystalline order of this characteristic structure was relatively poor, but they could be improved by the subsequent heating. The solution aging induce the crystallization of PBTTT only, indicating that the PBTTT and PCBM did not prefer to cocrysatllize in the solution with dichlorobenzene.