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

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journal contribution
posted on 2014-12-09, 00:00 authored by Bryan S. Beckingham, Victor Ho, Rachel A. Segalman
While polymer materials possess significant promise as components in large-area organic electronic devicessuch as thin-film transistors or photovoltaic devicesthe 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 structure–property relationships in conjugated polymers. Herein, we examine P3EHT as a model polymer for exploring crystallization in P3ATsas it has a final melting transition well below degradation in contrast to the more common P3HTusing differential scanning calorimetry (DSC) and wide-angle X-ray scattering. Notably, examination of the melting endotherms following isothermal crystallization of P3ATsnamely 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 π–π 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 P3EHT’s melting behavior is a consequence of a melt-recrystallization mechanism and observe perfection of the π–π stack direction during the melt-recrystallization process. Lastly, we utilize the obtained final melting temperatures to elucidate values for ΔHm0 and Tm0, 20 ± 4 J/g and 92 °C, respectively.