Molecular Alignment in Polyethylene during Cold Drawing Using In-Situ SANS and Raman Spectroscopy

Changes in the crystalline and mesoscale lamellar structure during plastic deformation of semicrystalline polymers have been extensively studied by X-ray diffraction techniques. However, direct measurements of single chain conformations during stretching have not been realized, although they are key to fully understand the structural transitions during cold drawing and their relation with the state of uniaxial stress. We report direct measurements of molecular alignment of a semicrystalline polymer during cold drawing by combining in-situ small-angle neutron scattering (SANS) and polarized Raman spectroscopy. The sample investigated is a linear low-density polyethylene (LLDPE) with density of 918 kg/m³ and melt index of 1.0 g/10 min. A multifaceted protocol consisting of hydrogen-deuterium exchange, followed by fractionation (by molecular weight, MW) and blending of selected deuterated fractions with protonated LLDPE, was used to elucidate, via SANS measurements, the response of the different fractions to uniaxial deformation. Under tensile deformation significant chain stretching occurs in the initial elastic regime. Further plastic deformation causes additional chain stretching, but to a lesser degree, that eventually plateaus in the strain hardening regime. Concurrently, the fraction of trans conformers increases linearly, as measured by in-situ Raman spectroscopy. The total orientation, quantified using an alignment factor, is lower for the lower MW fractions. We hypothesize through simple geometric arguments that this is directly related to the probability of forming intercrystal tie chains.