Roles of solution concentration and shear rate in the shear-induced crystallization of P3HT

Microfluidic shear can induce the formation of flow-induced precursors (FIPs) of poly(3-hexylthiophene) (P3HT) in toluene. The shear temperature, solution concentration and shear rate determine the FIP content. The FIP is metastable. Upon fixing the shear rate at 1.0 s^-1 and the shear temperature at 60 °C (or 80 °C for a 5.0 mg mL^-1 solution), when the shear stress σ exceeds the critical values, a further increase in σ may destroy the formed FIP during shear, leading to the amount of FIPs first increasing when the solution concentration increases from 0.2 mg mL^-1 to 0.4 mg mL^-1 and then gradually decreasing with a further increase in the solution concentration from 0.7 mg mL^-1 to 5.0 mg mL^-1. Upon fixing the shear temperature at 60 °C (or 80 °C for a 5.0 mg mL^-1 solution), the high concentration P3HT solution has high viscosity, leading to more mechanical energy being dissipated under shear, resulting in the most suitable shear rate increases with increasing solution concentration to reduce the entropy. The reduction in entropy is related to the formation of FIPs, and thus, the most suitable shear rate at which the largest FIP content can be obtained increases with increasing solution concentration. The FIP content dramatically affects the crystallization of P3HT in toluene. Increasing the FIP content can accelerate nucleation and crystallization, and change the crystallization mechanism from a second-order reaction to a first-order reaction of P3HT aggregates.