A 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, based on the Mandelkern relation, is in clear contradiction to ΔH m °= 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 crystallinity of the P3HT samples and revealed that the ΔH m °obtained by the Mandelkern approach (76 ± 5 J/g) could support that concluded from the Flory approach (74 ± 4 J/g).

No abstract

We investigated non-isothermal and isothermal crystallization of spin-coated poly(3-hexylthiophene) thin films prepared from the melt by in-situ ultraviolet-visible absorption spectroscopy. Analyzing the absorption spectra according to the Franck-Condon principle allowed for a quantitative assessment of the degree of crystallinity as well as the quality of order within crystalline regions of the films. Measured at room temperature, we observed a similar crystallinity for all differently crystallized films. The highest quality of order, however, was found for the P3HT film cooled slowly from the melt. These results were in full agreement with the results obtained by X-ray diffraction and calorimetry measurements. Consistently, in spite of similar crystallinities, atomic force microscopy images did not show a well-defined structure of ordered domains of preferentially aligned lamellae for the films rapidly cooled from the melt. In addition, heating the P3HT films in a specific range of temperature showed no change in crystallinity in spite of a continuous loss of order quality. Our results revealed that crystallinity and crystalline quality could behave differently while processing a semicrystalline polymer.

Poly(3‐hexylthiophene) (P3HT) as a conjugated polymer has many electrical and photovoltaic applications. However, reliable values for the equilibrium melting temperature (T°m) and for the enthalpy of fusion of a perfect P3HT crystal (ΔH°m) are still in doubt. In the published works, ΔH°m ranges from 33 to 99 J/g. Here, the melting point of P3HT in P3HT‐diluent mixtures is measured with changing the diluent concentration. By satisfying the requirements of Flory relation, “working with the same polymer crystals” and “working in the equilibrium state,” usually neglected in the literature, a reliable value of ΔH°m = 83 ± 2 J/g is obtained. The reported values for T°m of P3HT are also in a wide range, 260‑300 °C. In this work, accurate applying of the Hoffman‐Weeks relation, against the effects of lamellar thickening, yields T°m = 272 ± 2 °C rather than 300 ± 2°C reported in the literature. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 431–437