2.2. The thermal properties of acrylic acid derivative terpolymer
Figure 3 (a) shows the TG curves of LA136D, PVDF, and SBR, the test is carried out in an N2 atmosphere and the calefactive rate is 10 °C min–1. From Figure 3 (a), all the binders show thermal decomposition. The thermal decomposition onset temperature (To ) is about 260°C, 270°C, and 400°C respectively for SBR, LA136D, and PVDF. The lowerTo of SBR and LA136D are reasonable because of the presence of C−C and C−N bonds in the molecule structure which with an inferior bonding energy of 347 KJ mol−1 and 305 KJ mol−1 respectively. The higherTo of PVDF is attributed to the high electronic negativity of fluorine atoms that affects the excursion of an electronic cloud of C−C and leads to the increase of thermal stability. In contrast to To , the ψ of the binders is more important in real hermetically sealed TBs. It directly relates to the P of TBs. Figure 3 (a) shows LA136D having the lowest ψ of 39.2wt %, well below the value of PVDF (77.6wt %), and SBR (99.2wt %). The difference of ψ in the three binders can be interpreted by two aspects: 1) chain session mechanisms, and 2) chemical bonding energy.[28] According to polymer science, polyacrylonitrile and poly (methyl acrylate) parts defer to the cross-linking and random-chain scission mechanisms respectively in the pyrolysis, while PVDF and SBR defer to the random-chain scission mechanisms.[23,24] Polymers with random-chain scission and cross-linking mechanisms often lead to a low ψ since the less small volatile fragments produced, while polymers with end-chain scission mechanism usually lead to high ψ due to it often involves the breaking of a small unit or group at the end of the chain, which results in many small volatiles fragments. Thus, the polyacrylonitrile and poly (methyl acrylate) parts in LA136D help it with a low ψ. On the other hand, chemical bonds with high energy are more stable at high temperatures. The double bonds (the bond energy of C=O is 799 KJ mol−1) and triple bonds (the bond energy of C≡N is 887 KJ mol−1) in LA136D have higher bonding energy than C−C (347 KJ mol−1) and C−H (411 KJ mol−1), which mainly presence in PVDF and SBR, also contributes to reducing volatile fragments at high temperature. The FTIR analysis of the non-volatile residues after 550 °C treatment of LA136D shows the presence of C−O and C−N groups (Figure 3 (c) ). It verifies that polyacrylonitrile and poly (methyl acrylate) is the key to the low ψ of LA136D. In addition, LA136D shows a low thermal decomposition rate in comparison with PVDF and SBR, as shown inFigure 3 (b) . The slow mass reduction of LA136D as the temperature increase indicates a step-by-step decomposition of the molecule groups. This is due to the reasonable setting of thermal hypo-stable groups (C−N, C−C, C−H) and thermal stable groups (C≡N, C=O) which have a different To . The low thermal decomposition rate means a low released rate of gas fragments in binder thermal decomposition, this is especially important to maintain electrode mechanical integrity in TBs operation as will be proved in the next part.