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

Several linear polyethylene homopolymers of varied molecular weight (13 les s than or equal to M-w less than or equal to 839 kg/mol) were synthesized w ith a metallocene catalyst and characterized. The synthetic approach result ed in relatively narrow molecular weight distributions (2.3 < (M) over bar( w)/(M) over bar(n) < 3.6) as measured by size exclusion chromatography. The melt rheological data, \eta*(omega)\ were modeled by the Carreau-Yasuda eq uation. The as-polymerized polymer fluffs were compression molded into film s of quenched and slowly cooled thermal treatments, This resulted in a raug e of sample densities between 0.9302 and 0.9800 g/cm(3), due to variations in the crystal content. The thermal, morphological, and mechanical behavior s were examined for the dependencies on both molecular weight and thermal t reatment. The small-strain tensile deformation properties, Young's modulus, yield stress, and yield strain, were directly related to percent crystalli nity, 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 bre ak, were influenced by the competing effects of percent crystallinity and m olecular weight, The slit-smeared long spacings increased with molecular we ight. There was a progression from ridged and planar lamellae to curved C a nd S-shaped lamellae with increasing molecular weight. Thermal treatment ha d a large influence on the shape of the mechanical alpha-relaxation, while the crystal content affected the magnitudes of the mechanical gamma and bet a-relaxations. (C) 2000 Elsevier Science Ltd. All rights reserved.