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 (Delta c(p)) at T-g were accurately probed for a series of CPs by DSC. The results showed a significant decrease in Delta c(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 Delta c(p) at T-g by a factor of 10, confirming that backbone rigidity reduces the Delta c(p). Additionally, an increase in the crystalline fraction of the CPs further reduces Delta c(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. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019