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

Reliable value for the enthalpy of fusion of a perfect Poly(3-hexylthiophene) (P3HT) crystal (ΔH°m) is still in doubt. In the published works, ΔH°m ranging from 33 to 50 J/g, obtained from measuring the dependence of the heat of fusion on crystallinity and crystal thickness, is in clear contradiction to ΔH°m  = 99 J/g, resulted from measuring melting point depression of P3HT crystals in the polymer-diluent mixtures. In this work, we satisfied a requisite of the Flory relation, 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 Flory relation is the “dry melting temperature” of the P3HT crystals prepared in presence of the diluent and not the melting temperature of those prepared in absence of the diluent. We also employed UV-vis spectroscopy along with a Frank-Condon analysis to determine crystallinity of the P3HT samples and revealed that the two mentioned methods support each other and lead to a common value of ΔH°m = 75 ± 5 J/g.