Impact of undercooling on epitaxially oriented liquid crystalline organizations of poly(3-hexylthiophene) below the eutectic temperature

Abstract

For widely studied hole-transporting conjugated poly(3-hexylthiophene) (P3HT), the routes of liquid crystalline (LC) organization below the eutectic point were studied as a consequence of dissolution in molten hexamethylbenzene (HMB). Based on the variation of melting points with mixing compositions, the eutectic condition and phase diagram of these two crystallizable components were disclosed. Upon significant undercooling, the face-on LC organization of conjugated polythiophene epitaxially developed on formerly established HMB crystals. This face-on organization is featured with alternate layer stacking of organized thiophene stems with loosely aggregated regions by a nanometer scale of stacking periodicity. Differently, with a few degrees of undercooling below the eutectic point, edge-on organization preferably prevailed, which results in the anisotropic platelets extending along the chain direction. Regarding these prompt epitaxial LC organizations of conjugated polythiophene, the association of bead-like or worm-like fringe micelles in the molten state emerges as being elucidative for both structural and morphological features. After the sublimation of HMB crystals, the original face-on LC organization transformed to the crystalline packing of edge-on backbones via annealing above 160 °C. This evolution of molecular packing provides a new direction for deciphering the crystallization mechanism of conjugated polymers and the thread-like morphology.