Micron-scale PII-2T solution aggregates in each CN and decane solution are observed under the optical microscope whereas those micron-scale aggregates are absent in other five mutual solvents (Figure ). Interestingly, the micron-scale aggregates in CN and decane adopt different shapes: needle-like structures with high aspect ratios are observed in CN whereas agglomerates composed of randomly orientated fibers of ~100 nm in diameter are seen in decane (Figures and). In contrast, when dissolved in mutual solvents (e.g. DCB), polymers aggregate into nanofibers with an averaged diameter of 32 nm as revealed by SEM images of freeze-dried solution samples (Figure ). We hypothesize that the formation of large micron-scale aggregates is due to the fact that CN and decane are selective solvents dissolving only one part of the polymer. In other words, CN is a solvent selectively dissolving the conjugated backbones whereas decane dissolves only alkyl side chains. Mutual solvents, on the other hand, dissolve both side chain and backbone resulting in much smaller, nanoscopic aggregate sizes unobservable under optical microscope.