Important thermodynamic characteristics of poly(3-hexyl thiophene)

Poly(3-hexyl thiophene) (P3HT) is widely studied as a model material in many electrical and photovoltaic applications, and has become the benchmark polymer when studying the physics of these devices. Despite its extensive use, its precise crystalline structure and thermodynamic characteristics, such as its enthalpy of melting of an ideal crystal, crystalline density, and amorphous density, are not well characterized. This work seeks to provide more certainty in defining these thermodynamic characteristics for regioregular P3HT. This is accomplished by determining the density of rr-P3HT with various thermal histories, and thus percent crystallinity. These densities are correlated to their melting enthalpy melting (ΔHm) via DSC. This relationship estimates that ΔH0 for P3HT is between 37 and 50 J/g, that the density of the amorphous portion of semicrystalline rr-P3HT is 1.094 g/cc, and that the density of crystalline P3HT is 1.12–1.14 g/cc. Interestingly, the density of the amorphous portion of rr-P3HT differs significantly from that of regio-random P3HT. This result indicates that the local packing of the segments differs in regio-random P3HT from that in the amorphous portions of rr-P3HT and that care must be expended when equating the behavior of these two phases.