Understanding Solution State Conformation and Aggregate Structure of Conjugated Polymers via Small Angle X‑ray Scattering

Donor–acceptor
(D-A) conjugated polymers are high-performance
organic electronic materials that exhibit complex aggregation behavior.
Understanding the solution state conformation and aggregation of conjugated
polymers is crucial for controlling morphology during thin-film deposition
and the subsequent electronic performance. However, a precise multiscale
structure of solution state aggregates is lacking. Here, we present
an in-depth small-angle X-ray scattering (SAXS) analysis of the solution
state structure of an isoindigo-bithiophene-based D-A polymer (PII-2T)
in chlorobenzene and decane as our primary system. Modeling the system
as a combination of hierarchical fibrillar aggregates mixed with dispersed
polymers, we extract information about conformation and multiscale
aggregation and also clarify the physical origin of features often
observed but unaddressed or misinterpreted in small-angle scattering
patterns of conjugated polymers. The persistence length of the D-A
polymer extracted from SAXS agrees well with a theoretical model based
on the dihedral potentials. Additionally, we show that the broad high q structure factor peak seen in scattering profiles can
be attributed to lamellar stacking occurring within the fibril aggregates
and that the low q aggregate scattering is strongly
influenced by the polymer molecular weight. Overall, the SAXS profiles
of D-A polymers in general exhibit a sensitive dependence on the co-existence
of fibrillar aggregate and dispersed polymer chain populations. We
corroborate our findings from SAXS with electron microscopy of freeze-dried
samples for direct imaging of fibrillar aggregates. Finally, we demonstrate
the generality of our approach by fitting the scattering profiles
of a variety of D-A polymers based on thieno-isoindigo (PTII-2T),
diketopyrrolopyrrole (DPP2T-TT, DPP-BTZ, PDPP2FT-C16),
naphthalenediimide (P­(NDI2OD-T2)), and a conjugated block copolymer
P3HT-b-DPPT-T. The results presented here establish
a picture of the D-A polymer solution state structure and provide
a general method of interpreting and analyzing their scattering profiles.