Peculiar Aggregation Features in Poly(3-hexylthiophene)/Chlorobenzene Solutions

We report on anomalous structure and relaxation features of the aggregate clusters fostered in a representative series of dilute poly(3-hexylthiophene)/chlorobenzene (P3HT/CB) solutions (3, 5, 8, and 10 mg/mL), as resolved by multiscale dynamic/static analysis schemes including depolarized/polarized dynamic light scattering (DDLS/DLS), static light/X-ray scattering (SALS/SLS/SAXS), and scanning transmission electron/transmission electron microscopy (STEM/TEM). DLS/DDLS analyses reveal the coexistence and dynamic equilibrium of diffusive isolated-chain species (fast mode) and nondiffusive, microsized cluster species (slow mode), largely unaffected by the polymer concentration, system temperature, sonication, laser exposure time, and, in particular, repeated filtrations during the sample preparation and measurements. The SALS/SLS/SAXS analyses further reveal that while the fast mode corresponds to isolated chains (R-g similar to 1.5 nm), the slow mode represents microsized clusters comprising a condensed core (R-g similar to 4 mu m) and loose corona (similar to 700 nm in shell thickness). These combined features and detailed analyses performed herein suggest that the core-shell cluster-which is rarely observed for homopolymer solutions-is formed during a dynamic equilibrium process wherein the isolated chains undergo condensation/decomposition on the (stabilized) core material, leading to an apparent "elastic" slow-mode relaxation whose apparent rate is set by the core-material diffusion. Only at a low temperature (15 degrees C) and high concentration (c > 5 mg/mL) does the shell material become indistinguishable from the core material, when one has a homogeneous and fairly condensed cluster that exhibits the normal diffusional behavior. The present findings provide new insight into the mechanistic aspects and precise controls of the morphological properties of P3HT solutions for future applications with polymer-based electronic devices.