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ABSTRACT Thermomechanical properties of polymers highly depend on their glass transition temperature (Tg). Differential scanning calorimetry (DSC) is commonly used to measureTgof 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 measuringTgin such type of polymers. The changes in specific heat capacity (Δcp) atTgwere accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δcpfrom flexible polymer (0.28 J g−1K−1for polystyrene) to rigid CPs (10−3J g−1K−1for 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 ΔcpatTgby a factor of 10, confirming that backbone rigidity reduces the Δcp. Additionally, an increase in the crystalline fraction of the CPs further reduces Δcp. We conclude that the difficulties of determiningTgfor 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
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Liquid fragility maximum in lithium borate glass‐forming melts related to the local structure
Abstract The structure of liquid lithium pyroborate, Li4B2O5(J= Li/B = 2), has been measured over a wide temperature range by high‐energy X‐ray diffraction, and compared to that of its glass and borate liquids of other compositions. The results indicate a gradual increase in tetrahedral boron fraction from 3(1)% to 6(1)% during cooling fromT= 1271(15) to 721(8) K, consistent with the largerN4 = 10(1)% found for the glass, and literature11B nuclear magnetic resonance measurements. van't Hoff analysis based on a simple boron isomerization reaction BØ3O2–⇌ BØO22–yields ΔH= 13(1) kJ mol–1and ΔS= 40(1) J mol–1 K–1for the boron coordination change from 4 to 3, which are, respectively, smaller and larger than found for singly charged isomers forJ ≤ 1. With these, we extend our model forN4(J,T), nonbridging oxygen fractionfnbr(J,T), configurational heat capacity , and entropySconf(J,T) contributions up toJ= 3. A maximum is revealed in atJ= 1, and shown semi‐quantitatively to lead to a corresponding maximum in fragility contribution, akin to that observed in the total fragilities by temperature‐modulated differential scanning calorimetry. Lithium is bound to 4.6(2) oxygen in the pyroborate liquid, with 2.7(1) bonds centered around 1.946(8) Å and 1.9(1) around 2.42(1) Å. In the glass,nLiO= 5.4(4), the increase being due to an increase in the number of short Li–O bonds.
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- PAR ID:
- 10383742
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- International Journal of Applied Glass Science
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1286
- Page Range / eLocation ID:
- p. 52-68
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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