Poor solvent and thermal annealing induced ordered crystallites in poly(3-dodecylthiophene) films

Abstract

The influence of poor solvent and thermal annealing, and their specific roles, in the crystalline ordering of poly(3-dodecylthiophene) [P3DDT] films, which are of immense importance in their performance as semiconducting materials, were investigated using complementary techniques. Edge-on oriented crystallites (Form-II like) are enhanced in the as-cast films prepared after addition of a poor solvent. However, the coil-to-rod-like conformational transition is more prevalent compared to the crystallites, suggesting that a poor solvent predominantly helps to overcome the unfavorable conformational transition. A very large enhancement in the amount of the crystallites (Form-I-like) is observed for the films annealed above the melting temperature of the alkyl side chains, suggesting thermal annealing essentially helps to promote the diffusion of rod-like chains and to overcome the hindrance of the π–π stacking. Perfectly edge-on oriented crystallites, which are enhanced with increasing annealing temperature, start to deteriorate when the melting temperature of the polymer backbone is reached. The domain-like morphology of the as-cast film, however, remains almost unchanged upon thermal annealing suggesting the spontaneous organization of π-stacked layers through alkyl side chains to form crystallites is essentially within the small domains. The best edge-on oriented crystallites are found for the P3DDT films prepared from a solution containing a large amount of poor solvent and subsequently annealing the film at around 130 °C.