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

Abstract

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.

Scheme 1. Experimental diagram of the microfluidic shear process.

Fig. 1. Dissolution temperature (T_d) of P3HT in toluene as a function of the isothermal crystallization temperature (T_c). The data are linearly extrapolated to the line of T_d = T_c to obtain T⁰_d at the intersection point. The concentration of P3HT solution is 0.2 mg mL⁻¹.

Fig. 2. Plots of the growth rate of P3HT crystals (G) versus the isothermal crystallization temperature of PR_0.2,60, PS_0.2,20 and PS_0.2,60. The shear rate is 1.0 s⁻¹.

Fig. 3. Variation in the reduced viscosity η_sp/c and the solution concentration c of P3HT toluene solutions.

Fig. 4. UV-vis absorption spectra of (a) PS_0.7 and (b) PS_5.0 sheared at different temperatures. (c) UV-vis absorption spectra of sheared P3HT solutions with different concentrations. The shear temperature of PS_5.0 is 80 °C, and the shear temperature of the other solutions is 60 °C. The shear rate is 1.0 s⁻¹.

Fig. 5. Effects of the shear temperature on the crystallization rate of P3HT in toluene at different concentrations. The isothermal crystallization temperature is 12 °C, and the shear rate is 1.0 s⁻¹.

Fig. 6. (a) Plots of ln G versus ln[P3HT] at different shear temperatures. The inset shows a plot of ln G versus ln[P3HT] of the resided solutions, and the residence temperature is fixed at 60 °C. (b) Variations in α with the shear temperature.

Fig. 7. UV-vis absorption spectra of (a) PS_0.2,60, (b) PS_0.7,60 and (c) PS_5.0,80 sheared at different shear rates.

Fig. 8. Plots of the crystal growth rate versus crystallization temperature for PS_0.2,60 sheared at different shear rates.