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

The effect of uniaxial cold drawing of HOPE on crystallinity and chain mobility in the amorphous phase is studied by H-1 NMR T-2 relaxometry. A special device was used to keep the necked part of drawn HPDE under strain during NMR experiments. Undrawn HOPE is composed of similar to 61 wt% crystalline phase, similar to 12% largely immobilized chain segments in crystal amorphous interface and similar to 27% soft amorphous phase. Drawing causes a decrease in crystallinity by similar to 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 similar to 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, similar to 5% of chain segments in drawn HOPE 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 HOPE. 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. (C) 2013 Elsevier Ltd. All rights reserved.