Dynamic mechanical behavior of oriented films of high-density polyethylene

Dynamic mechanical properties of oriented films based on HDPE prepared by multistep zone annealing with draw ratios λ = 4.7, 9.9, 21.2, and 28 were studied at a frequency of 3.5, 11, 35, or 110 Hz and in a temperature range from -150 to +120°C. Intensities of the γ-relaxation and α-relaxation transitions were shown to decrease with increasing λ. For the samples with maximum λ, the corresponding temperature dependence of loss tangent shows an intense β-relaxaton peak at -37°C (frequency 3.5 Hz), which is almost never observed for oriented HDPE and ultrahigh-molecular-weight PE. Maximum tear energy of the samples with λ = 28 is indicative of enhanced interfibrillar affinity, in particular, at the expense of densification of interfibrillar amorphous regions. The molecular mobility in these regions may be responsible for the appearance of the above-mentioned β-relaxation peak. Temperature-frequency superposition of mechanical losses was carried out. Minimum values of the apparent activation energy are characteristic of the samples with maximum values of λ, which indicates a less-pronounced cooperative character of relaxation processes under these conditions.