Nanoindentation on crystal/amorphous polyethylene: Molecular dynamics study

Nanoindentation tests are conducted on a folded-chain crystal and an amorphous block of polyethylene by molecular dynamics simulation. Two tests are performed on the “flat-on” and “edge-on” surfaces of the crystal; however, there is no marked difference in both tests. The change in the potential energy and torsional angles reveals that molecular chains are dragged beneath the tip and straightened under rapid indentation, and they buckle again during relaxation at the maximum depth. For the amorphous polyethylene, notes that the potential energy becomes lower than that in the initial state during the loading while it reverts during the hold, i.e., without an increase in external load. The amorphous block is locally ordered by local deformation as well as the crystals, resulting in a decrease in “entropy”. Thus, the role of the entropy is exposed in the indentation of the amorphous polyethylene.