Observation of two-step thermochromism in poly(3-docosylthiophene): DSC and reflection spectroscopy

Poly(3-docosylthiophene) was found to have a two-step thermochromic process for samples that are rapidly cooled from the melt. The melt-quenched samples are a red-orange color that changes first to purple and then to yellow upon heating. In contrast, slowly cooled materials are purple and have a single thermochromic transition to yellow, typical of most poly(3-alkylthiophene)s. This phase behavior is found in both regioregular and regioirregular samples. The thermal behavior of the polymers was studied using differential scanning calorimetry and visible reflection spectroscopy. The DSC thermograms of slowly cooled samples show two endotherms upon heating and three exotherms upon cooling. In contrast, rapidly cooled samples exhibit three endotherms upon heating and three exotherms upon cooling. Reflection spectra show color changes that correlate with the DSC features. A model is proposed to explain the data that involves trapping of a high-temperature mesophase under the rapid cooling conditions