A new method to improve poly(3-hexyl thiophene) (P3HT) crystalline behavior: decreasing chains entanglement to promote order-disorder transformation in solution.

We promoted order-disorder transformation of poly(3-hexylthiophene) (P3HT) in solution by ultrasonic oscillation which substantially improved crystallinity in its pure film. P3HT with low molecular weight (M(w)) dispersed very well in p-xylene solvent and few aggregates generated in the solution. For P3HT with high M(w), the results suggested the coexistence of two phases: disordered coils in solution and ordered microcrystals in suspension. Upon ultrasonic oscillating, more ordered precursors generated in solution due to increased self-assembly from disordered to ordered configuration, which resulting from decreased extent of chains entanglement existing in high molecular system, and red shift at absorption maximum and growing intensity of the pi-pi* absorption at ca. 604 nm were observed in solution. The films prepared from the oscillated solution then showed increased degree of crystallinity, pi-pi interactions and homogeneously distributed nanofibrils, which should be attributed to the ordered precursors constructed in solution. Furthermore, the best crystallinity of the film was obtained at the oscillating time of 4 min, showing the equilibrium state between the increased content of crystalline molecules and the shortened crystalline length. This simple method paves the way for the decreasing chains entanglement during crystallizing process of conjugated polymer in solution, and it enriches the ways to improve crystalline order in thin films comprising crystallizable polymers.