Measuring the equilibrium melting enthalpy (ΔH0) and the equilibrium melting temperature (Tm0) of high molecular weight Poly(3-hexylthiophene)

Abstract

The close correlation between device operation and the degree of crystallinity in semicrystalline semiconducting polymers, e.g. poly(3-hexylthiophene) (P3HT), highlights the need for correct values of the crystal-related thermodynamic parameters from which reliable values of the degree of crystallinity can be calculated. The objective of this work is to provide an overview of the methods used and the equilibrium melting temperature (T_m⁰) and equilibrium melting enthalpy (ΔH⁰) values obtained to date, as well as to attempt to reach an agreement on these values. To this end, we investigate the thermodynamic parameters needed to calculate the degree of crystallinity of P3HT by calorimetry. For that, the crystallization behavior and thermodynamics of blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), a prototypical electron acceptor molecule, are investigated. The miscibility of the two components in the liquid state was confirmed by the depression of the P3HT melting temperature and the composition-dependent single glass transition temperature (T_g), which follows the Fox equation. T_m⁰ were determined for neat P3HT and its blends via Hoffman–Weeks extrapolation. These values enabled the application of the Flory–Huggins model, resulting in an ΔH⁰ of 68 J g^-1 for P3HT. This thermodynamic parameter was subsequently used to quantify the degree of crystallinity of P3HT blends. Our approach provides a robust and internally consistent framework for determining the degree of crystallinity in semicrystalline semiconducting polymer systems.