Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

Thermomechanical properties of polymers highly depend on their glass transition temperature (T_g). Differential scanning calorimetry (DSC) is commonly used to measure T_g of polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuring T_g in such type of polymers. The changes in specific heat capacity (Δc_p) at T_g were accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc_p from flexible polymer (0.28 J g⁻¹ K⁻¹ for polystyrene) to rigid CPs (10⁻³ J g⁻¹ K⁻¹ for a naphthalene diimide-based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the Δc_p at T_g by a factor of 10, confirming that backbone rigidity reduces the Δc_p. Additionally, an increase in the crystalline fraction of the CPs further reduces Δc_p. We conclude that the difficulties of determining T_g for CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D-A CPs with weak transition signals.