PBTTT-C₁₆ sol-gel transition by hierarchical colloidal bridging

A versatile conjugated polymer, poly(2,5-bis(3-hexadecyllthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C₁₆, with M_w = 61 309 g mol^-1), in a relatively good solvent (chlorobenzene, CB) medium is shown to produce gels through hierarchical colloidal bridging. Multiscale static/dynamic light and X-ray scattering analysis schemes along with complementary microscopy imaging techniques clearly reveal that upon cooling from the solution state at 80 °C to various gelation temperatures (5, 10, and 15 °C), rod-like colloidal pBTTT-C₁₆ aggregates morph into spherical ones, triggering hierarchical colloid formation and bridging that eventually turn the solution into a gel after about one-day aging. A certain fraction of primal packing units-spherical gelators (∼1 nm in mean radius)-constitute the spherical building particles (∼10 nm) noted above, which in turn constitute loose-packing aggregate clusters (∼300 nm) in the sol state. As gelation proceeds, the aggregate cluster interiors tighten substantially, and micrometer-sized clusters (∼3 μm) formed by them begin to take shape and further interconnect to form the gel network (mean porosity size ∼240 nm and spatial inhomogeneity length ∼20 μm). Rheological measurements and kinetic analysis reveal that the gelation temperature can also have a notable impact on gel microstructure, gelation rate, and mechanical strength, resulting in, for instance, a prominently nonergodic and porous structure for the soft gel incubated at a higher temperature T = 15 °C. The ac conductivity exhibits a notable upturn near pBTTT-C₁₆/CB gelation, well above those achieved by the counterpart pBTTT-C₁₄ solutions, which, in interesting contrast, cannot be brought to the gel phase under similar experimental conditions.