Thermal Analysis of Semiconducting Polymer Crystals Free of a Mobile Amorphous Fraction

Mobile-amorphous-free crystals of a semiconducting polymer, poly­(3-hexyl thiophene) (P3HT), were made via a flow induced crystallization technique to yield crystals that were microns long while the other two dimensions were 6 and 21 nm in size. Thermal analysis revealed a melting point depression that is described by the Gibbs–Thomson (GT) equation resulting in an interpretation that is physically sound. On the contrary, contemporary analysis of the melting enthalpy, which also shows a depression below that for an infinitely sized crystal, yields results that are nonphysical and do not agree with the GT analysis. A simple argument is made to correct this discrepancy, using the first law of thermodynamics, to include the sensible heat of the molten crystal mass required for an extrapolation to the melting enthalpy of an infinitely sized crystal. A satisfactory comparison to the results from the GT equation is now found to reconcile older, literature data to good effect. However, the comparison is not as good for the P3HT crystals made here and believed to be because of the finite size of the crystals in two dimensions coupled with the rather large size of the polymer molecules prohibiting a continuum analysis.