4 years ago

Solvent-Regulated Mesoscale Aggregation Properties of Dilute PBTTT-C14 Solutions

Solvent-Regulated Mesoscale Aggregation Properties of Dilute PBTTT-C14 Solutions
Ching H. Wu, Chi C. Hua, Han L. Yi, Chun I Wang
Contrasting mesoscale aggregate features of a promising conjugated polymer, poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C14), that result from the use of two slightly different aromatic solvents, i.e., toluene and chlorobenzene, for a range of dilute solutions (0.5–1.2 mg/mL) are systematically explored using depolarized dynamic light scattering (DDLS), dynamic/static light scattering (DLS/SLS), small-angle X-ray scattering (SAXS), and scanning transmission electron/scanning electron/transmission electron microscopy (STEM/SEM/TEM) analysis schemes. The central findings are as follows: (1) DDLS and all EM features reveal that whereas pBTTT-C14 aggregate clusters fostered in toluene (a poorer solvent) are moderately anisotropic (cylindrical; aspect ratio ∼3) in shape, they are nearly isotropic (spherical) in chlorobenzene (a better solvent), with mean sizes in the range of a few hundred nanometers. (2) Combined DDLS/DLS/SLS/SAXS analyses indicate that the aggregate clusters in both solvent media are coexistent with a certain fraction of small rod-like species (∼10 nm in length; aspect ratio ∼2), similar or even identical to the packing units which build up the fractal network of an aggregate cluster. (3) Accurate atomistic molecular dynamics (AMD) simulations of one- and five-chain aggregate systems reveal that the solvent-induced, contrasting nanoscale/mesoscale aggregate features bear a dynamic origin, through the backbone torsional relaxation that substantially impacts the bolstering interaction force (van der Waals vs π–π) and, hence, the “anisotropic persistence” of the fundamental packing units. (4) The overall features suggest that different organic solvents may be utilized to engineer the (mesoscale) size and shape as well as the (nanoscale) packing units of the aggregate species incubated in solution and shed light on the morphological developments during thin-film fabrication that have been the focus of recent research on the pBTTT-Cn series.

Publisher URL: http://dx.doi.org/10.1021/acs.macromol.7b00790

DOI: 10.1021/acs.macromol.7b00790

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