Thermodynamic Features of Perfectly Crystalline Poly(3-hexylthiophene) Revealed through Studies of Imperfect Crystals

A reliable value for the enthalpy of fusion of a perfect poly(3-hexylthiophene) (P3HT) crystal (ΔHm°) is still in doubt. In the published works, ΔHm° ranging from 33 to 50 J/g, obtained from measuring the dependence of the heat of fusion on crystallinity and crystal thickness, based on the Mandelkern relation, is in clear contradiction to ΔHm° = 99 J/g, resulting from measuring melting point depression of P3HT crystals in the polymer–diluent mixtures, based on the Flory relation. In this work, we satisfied a requisite of the Flory equation, ignored in the literature, by presenting a new conception as the “dry melting temperature”. We confirmed that the correct value for the melting temperature of the undiluted polymer to use in the equation is the “dry melting temperature” of the P3HT crystals prepared in the presence of the diluent and not the melting temperature of those prepared in the absence of the diluent. We also employed UV–vis spectroscopy along with a Franck–Condon analysis to determine crystall...