Remarkably high mobility of some chain segments in the amorphous phase of strained HDPE

Abstract

The effect of uniaxial cold drawing of HDPE on crystallinity and chain mobility in the amorphous phase is studied by 1H NMR T2 relaxometry. A special device was used to keep the necked part of drawn HPDE under strain during NMR experiments. Undrawn HDPE is composed of ∼61 wt% crystalline phase, ∼12% largely immobilized chain segments in crystal–amorphous interface and ∼27% soft amorphous phase. Drawing causes a decrease in crystallinity by ∼6 wt% and a large change in molecular mobility in the amorphous phase. The fraction of largely immobilized chain segments in non-crystalline domains increases by ∼18%. This increase is largely caused by the formation of taut-tie chain segments which transfer force between adjacent crystals. Mobility of the taut-tie chain segments is not significantly larger than in glassy polymers. Unexpectedly, ∼5% of chain segments in drawn HDPE have molecular mobility similar to that for PE melts. This unusually high chain mobility is caused by additional free volume and void formation upon mechanical stress. Crystallinity increases by 7 wt% after load release. However, molecular mobility in crystalline and amorphous domains is lower than that in the initial HDPE. A small fraction of highly mobile chain segments retains in the strain recovered sample due to incomplete recovery. This knowledge is important for better understanding the role of the amorphous phase in stress–strain behaviour of polyolefins.