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.