A comprehensive study on the transformation of chemical structures in the plastic layers during coking of Australian coals

•The changes in the chemical structures take place in the plastic layers during coking.•The combination of Synchrotron IR, 13C NMR and Micro-GC is effective.•The increase of bridge bonds and looped structures across the plastic layer during coking is evident.•The amount of weak bridge bonds and looped structures is responsible for the generation of coal fluidity during coking.