Discerning Thermal Transition Behavior of Conjugated Polymers through In Situ Optical Characterization of Oriented Films

The thermomechanical properties of conjugated polymers (CPs) play a crucial role in device morphological, mechanical, and thermal stability and influence device processing parameters. However, the thermal transition behavior of CPs can be difficult to characterize, particularly as the molecular complexity increases. Here, we introduce a characterization technique that is an effective and sensitive approach to observing and defining thermal transitions in CPs. Our approach combines strain alignment with in situ polarized ultraviolet–visible absorption spectroscopy, which we name AFIPS. By monitoring changes in the absorption spectra and dichroism, we demonstrate the ability to effectively identify and characterize thermal transitions undetected using calorimetry. We use this in situ characterization method along with dynamic mechanical analysis (DMA) to probe the thermal transitions of various CPs, including F8T2, P3HT, PCDTBT, PTB7-Th, PM6, and PBnDT-FTAZ. We show that the measurement has a unique response to glass transitions (T_g), cold crystallization (T_cc), liquid crystal transitions, and melting. In this investigation, we uncover signatures of a T_g in PTB7-Th and a T_cc in PCDTBT. Using the AFIPS technique complements other thermomechanical measurements, such as DMA, while being simple and effective to implement, providing a valuable tool for polymer thin film characterization.