Molecular-Weight Dependence of Tensile Properties of Nearly Monodisperse Linear Polyethylene Solids Synthesized Using a Phenoxyimine Catalyst

The molecular-weight dependence of the tensile deformation of semi-crystalline polymers is complicated by their supermolecular structure. In this study, we systematically investigated the tensile properties of linear polyethylene (PE) using a unique series of nearly monodisperse samples ( M w / M n = 1.06–1.11) synthesized using a phenoxyimine catalyst, together with conventional polydisperse counterparts. This model system enables clear separation of the influences of the molecular weight and its distribution. Small-angle X-ray scattering revealed that the lamellar long period increased with increasing molecular weight for both sample series, but the molecular-weight dependence was more pronounced for nearly monodisperse PE. Based on the lamellar dimensions and molecular-weight distribution, the fractions of tie molecules connecting several lamellar crystals were evaluated. A critical molecular weight of approximately 1.5 × 10 5 was identified, above which tie molecules connecting lamellar clusters dominate the mechanical response. Notably, this molecular-weight threshold coincides with the observation of a β-relaxation process in the dynamic mechanical spectra and a distinct yielding behavior change. Uniaxial tensile tests showed that nearly monodisperse PE exhibits pronounced broadening of the yielding region with increasing molecular weight, whereas polydisperse PE shows conventional sharp yielding. The yielding parameters and rheo-Raman spectra indicated that deformation beyond the first yield point is dominated by interlamellar amorphous chains, particularly for high-molecular-weight chains. These results show that the molecular-weight distribution governs the manifestation of the intrinsic molecular-weight-dependent deformation mechanisms of semi-crystalline PE.