Strain-Dependent Evolution of the Rigid Amorphous Fraction of Low-Density Polyethylene under Deformation

Abstract

Understanding the deformation mechanism of the amorphous phase in semicrystalline polymers has been challenging due to the lack of a long-range ordered structure and chain dynamics heterogeneity. In this study, the amorphous chains’ reorientation and dynamics change under uniaxial elongation of low-density polyethylene (LDPE) was investigated, utilizing ¹H time-domain nuclear magnetic resonance (¹H TD-NMR) in combination with wide-angle X-ray scattering (WAXS). Depending on the chain dynamics difference, the amorphous phase can be decomposed into semirigid and mobile amorphous fractions, where the third rigid amorphous fraction appears under deformation. Such a rigid amorphous fraction, which is generated during necking (1.8 < λ < 4.7), shows much slower dynamics (similar T₂ as the crystalline phase) as compared with the other two amorphous fractions. Moreover, strain-dependent ¹H TD-NMR and WAXS results clarify different structural transformation pathways for the rigid amorphous fraction. During the stress-softening (region II, 1.8 < λ < 3.7), accompanied by decreasing crystallinity from 0.35 to 0.20 by WAXS, the rigid amorphous fraction is constantly formed (from 0 to 0.20), mainly at the compensation of the crystalline phase by lamellar fragmentation; and in the stress plateau (region III, 3.7 < λ < 4.7) with constant crystallinity at 0.20, elongated and tightly packed semirigid amorphous chains are the main source of the rigid amorphous fraction (from 0.20 to 0.25). The rigid amorphous fraction generated by the former pathway exhibits more constrained chain mobility. The strain-dependent evolution of the rigid amorphous fraction is further supported by in situ ¹H TD-NMR results in addition to the ex situ process. Current work demonstrates ¹H TD-NMR as a promising technique for elucidating the amorphous chain reorientation and dynamics change upon deformation.