Influence of Bridging Atom and Side Chains on the Structure and Crystallinity of Cyclopentadithiophene-Benzothiadiazole Polymers

We use grazing-incidence wide-angle X-ray scattering (GIWAXS) and molecular modeling to understand the difference in crystallization of several cyclopentadithiophene-benzothiadiazole polymer derivatives. We observe using GIWAXS that when the carbon bridging atom is substituted by a silicon atom, the pi-pi stacking distance is decreased while the lamellar stacking distance is increased. Using molecular modeling, we calculate the potential energy surfaces of an ordered array of oligomers as a function of pi-pi stacking and lamellar stacking distances and find two local minima for both the carbon and silicon analogues. This finding is consistent with the GIWAXS observations. We suggest that it may be possible to crystallize the carbon and silicon versions in the same crystal structure by varying the processing conditions. We derive new potential parameters from quantum chemical calculations for side chains motions and implement those within a new force field for molecular dynamics. We find that the side chains are more flexible in the case of the silicon bridging atom. We propose that the flexibility enhancement may influence both thermodynamics and kinetics of crystallization and may result in crystallization of the polymer in the first or the second energetically favored crystal structures. This interpretation is supported by the finding of only one minimum in potential energy for longer, less bulky, and, thus, more flexible side chains for the carbon analogue.