Effect of solvent quality on chain conformation and optoelectronic properties of conjugated polymers by light scattering

Conjugated polymers (CPs) are suitable for next-generation organics electronics due to its advantages like tunable constitution and solution processibility. However, as for now there are limited knowledge about the solvent quality between the conjugated polymers and various solvents. Characterizations are mostly based on visual inspection of disappearance of large piece of the polymer chain. Even for those chain are visually dispersed, there is a lack of in-depth knowledge about if the polymers are actually dissolved to form individual chain or simply “dispersion” of small insoluble aggregates in solution. In this work, we report a static light scattering approach to quantify the solvent quality for poly(3-alkylthiophene-2,5-diyl) (P3ATs) and a donor-acceptor polymer PffBT4T-C9C13 with different length of sidechains in several commonly used organic solvents (chlorobenzene, chloroform, tetrahydrofuran, and toluene). By comparing the second virial coefficient A₂, the effect of sidechain length on conjugated polymer’s interaction with solvent was investigated on CPs solutions in various solvents and temperatures. We found that for non-polar solvent, solvent quality increases as alkyl sidechain length increases and temperature increases, as the A2 value increased from -5.89 to 1.35 ×10^-3 cm^-3mol/g² as sidechain length changed from C6H13 to C12H25. However, PffBT4T-C9C13 are less soluble with a negative A₂ on the order of 10^-6 cm^-3mol/g² in all the solvent and temperature range we tested, owing to the more rigid and polar backbone. Our work sheds light on the polymer solvent interaction and can provide useful knowledge to rationally guide the development of new conjugated polymers as well as selection of solvents to process semiconducting polymers.