Microstructural Interpretation of Influences of Molecular Weight on the Tensile Properties of High-Density Polyethylene Solids Using Rheo-Raman Spectroscopy

The influence of the weight-average molecular weight (Mw) on the structure–properties relationship was investigated by preparing high-density polyethylenes (HDPEs) of various Mw but constant polydispersity index (Mw/Mn) values. The thickness of the lamellar crystalline layer increased with increasing Mw, whereas the amorphous characteristics, such as the amorphous layer thickness and tie-molecule fraction, remained unchanged. The microscopic deformation of these HDPEs was investigated by in situ Raman spectroscopy. The orientation parameter ⟨P2⟩ showed a power law to Mw with an exponent of −3/4 in the strain-hardening region. The Mw dependence of the molecular orientation was interpreted as the realignment of the lamellar cluster units, the size of which was determined by the end-to-end distance of the polymer chains in their melt state. It was found that higher stretching stress is applied to the crystalline chains for HDPEs of lower Mw, implying that a larger amount of taut-tie chains is formed.