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

Abstract

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. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1635–1644

Glass transition temperature from the chemical structure of conjugated polymers

“…The T g of conjugated polymers is notoriously difficult to identify, because many such materials are semicrystalline and have rigid backbones. Signatures of T g in differential scanning calorimetry (DSC) scans are thereby suppressed 33 , although some unambiguous values have been summarized by Müller 34 . Alternatively, rheological measurements are sensitive to changes in mechanical properties, and can unambiguously locate the T g of conjugated polymers as the peak in the loss modulus as a function of temperature 8 .…”

Backbone flexibility on conjugated polymer's crystallization behavior and thin film mechanical stability

“…34 In addition, for the fully conjugated counterpart PNDI-C0, it shows a much higher chain rigidity with l k of 196 Å, we previously observed that it shows an extremely fast crystallization (e.g., less than 1 s) which cannot be suppressed even using a fast scanning chip calorimetry at a cooling rate up to 10,000 K/s. 59 Such a rapid crystallization process is presumably due to the rigid and planar backbone and strong π-π interaction. Due to the high-chain rigidity, chain folding in the ordered structure becomes difficult and hampered, hence, it is likely that the chains adopt a fringed micelle conformation, an extended crystalline morphology, in the ordered microstructure in solid state.…”

Influence of side‐chain isomerization on the isothermal crystallization kinetics of poly(3‐alkylthiophenes)

“…The material's property of P3HT, P3(4MP)T, and P3(3MP)T were reported in our previous work [49]. b Molecular weight information and regularity have been published in previous works [49,50]. second heating scans, and the values of T g and T m along with molecular weight information are tabulated in Table 1.…”

“…Before proceeding to the isothermal crystallization measurements, cooling experiments (10-100°C/min) were carried out on the conventional DSC and suggested that the crystallization of P3ATs cannot be suppressed with a cooling rate up to 100°C/min, as shown in Supplementary Fig. S2 [50]. Hence, FSC technique was utilized.…”