The influence of molecular weight and thermal history on the thermal, rheological, and mechanical properties of metallocene-catalyzed linear polyethylenes

Abstract

Several linear polyethylene homopolymers of varied molecular weight (13≤ M ̄ w ≤839 kg/mol) were synthesized with a metallocene catalyst and characterized. The synthetic approach resulted in relatively narrow molecular weight distributions (2.3< M ̄ w / M ̄ n <3.6) as measured by size exclusion chromatography. The melt rheological data, |η ∗(ω)| were modeled by the Carreau–Yasuda equation. The as-polymerized polymer fluffs were compression molded into films of quenched and slowly cooled thermal treatments. This resulted in a range of sample densities between 0.9302 and 0.9800 g/cm 3, due to variations in the crystal content. The thermal, morphological, and mechanical behaviors were examined for the dependencies on both molecular weight and thermal treatment. The small-strain tensile deformation properties, Young's modulus, yield stress, and yield strain, were directly related to percent crystallinity, independent of molecular weight. However, increasing molecular weight led to a suppression of the peak in the stress–strain curves at the yield point. The large-strain deformation properties, toughness and strain at break, were influenced by the competing effects of percent crystallinity and molecular weight. The slit-smeared long spacings increased with molecular weight. There was a progression from ridged and planar lamellae to curved C and S-shaped lamellae with increasing molecular weight. Thermal treatment had a large influence on the shape of the mechanical α-relaxation, while the crystal content affected the magnitudes of the mechanical γ and β-relaxations.