Role of processing conditions and molecular attributes on melt-crystallization of conjugated polymers

It is well known that molecular characteristics and processing conditions determine the crystallization process and semicrystalline structure of classical polymers, which in turn dictates their final optical, mechanical, and diffusive properties. For semicrystalline conjugated polymers, charge transport properties are also known to critically depend on semicrystalline structure. However, the interplay between chain structure, processing conditions, and final morphology remains poorly understood because ill-defined crystallization conditions are typically present during solvent processing. Here, we investigate the role of molecular characteristics and well-defined processing conditions on the crystallization process of poly-3-hexylthiophene (P3HT). In particular, we show that the role of molecular weight (Mw) on P3HT crystallization is strongly dependent on regioregularity (RR): crystallization is increasingly hindered with increasing Mw for 90% RR-P3HT but nearly invariant for 95% RR-P3HT. Furthermore, self-seeding processing is more effective in manipulating crystallization of longer, more defective P3HT chains. Overall, this work highlights a key need to report both Mw and RR in P3HT studies.