Revealing the Effect of Branched Side Chain Length on Polymer Aggregation and Paracrystallinity for Improved Mobility–Stretchability Properties

Alkyl side chain engineering has long been used to improve the solubility of conjugated polymers. However, systematic studies on the effect of different alkyl branch lengths on morphological modulation and mobility–stretchability properties are still lacking. Here, we synthesize three fluorinated benzothiadiazole-based polymers (PC12BTDFT, PC16BTDFT, and PC20BTDFT) with different alkyl branch lengths and evaluate their structure–aggregation–stretchability correlations. The results show that longer side chains alleviate severe aggregation and form short-range aggregates, leading to a higher stretching ability without sacrificing high mobility (∼0.1 cm2 V–1 s–1). Also, a higher degree of paracrystallinity produced by longer side chains improves the release of tensile stresses, resulting in more significant retention of crystalline structural domains and the maintenance of higher mobility after being subjected to mechanical forces, as evidenced by the significant difference in mobility after 60% strain (7.64 × 10–4 cm2 V–1 s–1 for PC12BTDFT and 0.011 cm2 V–1 s–1 for PC20BTDFT). Our results show the effect of the side chain length on the aggregation behavior and microstructure between polymer chains. Furthermore, a clear dependence between paracrystallinity and stretchability is identified from the changes in morphology and charge transport behavior, providing a valuable perspective for the design of high-performance stretchable conjugated polymers.