Melting Behavior of Poly(3-(2′-ethyl)hexylthiophene)

While polymer materials possess significant promise as components in large-area organic electronic devicessuch as thin-film transistors or photovoltaic devicesthe ability to improve the performance of these materials is critically linked to understanding and controlling the morphology, namely control of crystallinity, crystallite size, and texture. In this context, conjugated poly(3-alkylthiophenes) are a model system for studying the structureproperty relationships in conjugated polymers. Herein, we examine P3EHT as a model polymer for exploring crystallization in P3ATsas it has a final melting transition well below degradation in contrast to the more common P3HTusing differential scanning calorimetry (DSC) and wide-angle X-ray scattering. Notably, examination of the melting endotherms following isothermal crystallization of P3ATsnamely poly(3-hexylthiophene) (P3HT) and poly(3-(2'ethyl)hexylthiophene) (P3EHT)reveals a bimodal final melting peak. Differential scanning calorimetry reveals a shift in the lower temperature peak to higher temperatures as the isothermal crystallization temperature is raised and convergence into a single observed endothermic peak at high crystallization temperatures. Complementary wide-angle X-ray scattering experiments reveal an increase in crystallite perfection along the pp stack direction at higher crystallization temperatures. Thus, properties of the P3EHT crystallite populations, average size and/or perfection, can be deliberately manipulated through control of the isothermal crystallization temperature. We further determine that the bimodal nature of P3EHTs melting behavior is a consequence of a melt-recrystallization mechanism and observe perfection of the pp stack direction during the melt-recrystallization process. Lastly, we utilize the obtained final melting temperatures to elucidate values for Delta H-m(0) and T-m(0), 20 +/- 4 J/g and 92 degrees C, respectively.