The OIT measurement was performed on a TA Q20 DSC (TA Instrument,
USA) according to the standard method (ISO 11357-6,
2008). The samples were first held at 60 C for 3 min with a nitrogen
flow of 50 mL min1. Subsequently, the samples were heated
to 170 C at a rate of 20 C min1. After reaching 170 C, the samples
were held for another 3 min. At this point, the gas was switched to
oxygen at a flow rate of 50 mL min1.