After that the gas was switched to oxygen with the flow rate of
50 mL/min. The oxidation of the sample was observed as a sharp
increase in heat flow due to the exothermic nature of the oxidation
reaction. Melt flow index (MFI) was determined with an extrusion
plastometer (Jingjian Co., Ltd., Chengde, China) according to ISO-
1133 standard at 230 C with 2.16 kg load