Macromolecules, 2016, vol 49, 22, pp. 8653-8660
The structural transitions in solution-cast film of poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT) copolymer have been systematically studied by a combination of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), in situ Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy, and in situ wide-angle X-ray scattering (WAXS). The glass transition temperature of the main chain as well as the melting and crystallization temperatures of the polymer were determined as 10, 238, and 221 °C, respectively. The out-of-plane deformation vibration of PDCBT Cβ–H groups in FTIR has been manifestly assigned for the first time. A broad endothermic peak between 30 and 120 °C was observed during DSC heating process and was attributed to the enthalpic relaxation of the twist glass transition, which resulted in a small negative effect on the power conversion efficiency (PCE) of solar cells. A reorganization process in the crystalline region was observed in WAXS upon heating, including the increase of grain size along the a-axis.