Origin of Different Crystal Orientations of Ultrahigh-Molecular-Weight Polyethylene during Uniaxial Stretching: The Effect of Entanglements and Chains Tilt

Entanglement networks can significantly impact structure formation and mechanical properties of semicrystalline polymers. Solution crystallized disentangled ultrahigh molecular weight of polyethylene (sc-disUHMWPE) can exhibit ultradrawability due to the lack of chain constraints. Upon a short time of melting and recrystallization, the entanglement network can regenerate (re-UHMWPE), causing strain-hardening and the loss of ultrahigh elongation. This work systematically follows the microscopic structural variations of sc-disUHMWPE and re-UHMWPE samples during uniaxial stretching via synchrotron Small Angle X-ray Scattering (SAXS). Unlike the classic four-point scattering pattern observed in re-UHMWPE, the crystals of sc-disUHMWPE are aligned parallel to the stretching direction and exhibit a clear fibrillation scattering streak. On the other hand, ex-situ Wide Angle X-ray Scattering (WAXS) reveals that all UHMWPE chains aligned parallel to the stretching direction in the large deformation regime. The results indicate that chain tilting within the crystals determines the rotational degree of the crystalline lamellae regardless of the initial entanglement states and crystal orientation. We propose that the fibrillation process observed in sc-disUHMWPE occurs by chain slippage within the crystals rather than by melting and recrystallization.