Variable-Temperature Scattering and Spectroscopy Characterizations for Temperature-Dependent Solution Assembly of PffBT4T-Based Conjugated Polymers

The solution structure of conjugated polymers (CPs) from which th films are cast is critical for tailoring the thin-film morphology thus devic performance. Here, we used multimodal variable-temperature scattering an spectroscopy tools to fully quantify the solution assembly of poly[(5,6-difluoro 2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-dialkyl-2,2′;5′,2″;5″,2‴-quaterthiophen 5,5‴-diyl)] (PffBT4T) polymers with varying side-chain lengths at differen assembly temperatures. The conformational and aggregation behaviors fo PffBT4T-based CPs were found to be very sensitive to both temperature and sid chain length using ultraviolet−visible (UV−vis) spectroscopy, nuclear magneti resonance (NMR) spectroscopy, dynamic light scattering (DLS), and small-angl neutron scattering (SANS). We found that with slightly increasing side chain lengt from 2-octyldodecyl (C8C12) to 2-nonyltridecyl (C9C13), PffBT4T-based CP show a significant decrease in aggregation-to-dissolved chain transition temperatur (10 °C), degree of aggregation, enthalpy change of aggregation, and size of th aggregates in solution. At room temperature, PffBT4T polymer strongly aggregated to form fabric structure with the film thickness of a few nanometers in thickness and hundreds of nanometers in length, as probed by atomic force microscopy (AFM), transmission electronic microscopy (TEM), and dynamic light scattering (DLS). At the elevated temperature above the aggregation-to-dissolved chain transition temperature, PffBT4T is fully dissolved and adopts a semiflexible coil conformation with the persistence length of 3.1 nm for PffBT4T-C8C12 and a slightly increased persistence length of 3.4 nm for PffBT4T-C9C13, according to temperature-dependent SANS measurements. Longer side chains of PffBT4T-C9C13 also lead to less aggregation enthalpy gain compared with PffBT4T-C8C12. This work provides a solution structure manipulating strategy of CPs and thus will inspire the molecular design and processing protocols of CPs toward higher performance electronic devices.