Free volume, glass transition and degree of branching in ethylene/alpha-olefin copolymers: positron lifetime, differential scanning calorimetry, wide-angle X-ray scattering and density studies

Positron annihilation lifetime spectroscopy, differential scanning calorimetry, wide-angle X-ray scattering, and density measurements were used to systematically study the variation of the glass transition temperature Tg and the mean size vh of holes (local free volumes) in n-alkyl branched polyethylenes. The samples were commercial ethylene-rich copolymers with 1-propene, 1-butene, and 1-octene comonomers. From the total specific volume V and the crystallinity Xc the specific volume of the amorphous phase Va was estimated and used to calculate the specific hole-free volume Vf. It was found that Tg and Xc decrease and V, Va, Vf, and vh increase with increasing weight fraction of comonomers. Both the real crystalline and amorphous specific volumes decrease with increasing crystallinity Xc. For not too high contents of comonomers, Tg decreases and vh increases linearly with the number and with the length of n-alkyl branches. This behavior was attributed to an increased segmental mobility caused by branching. Both Tg and vh follow linear master curves as a function of the degree of branching if this is defined as the fractional number of carbon atoms in the side chains compared with the total number of carbon atoms. A method for estimating Tg from vh measured at room temperature is shown. The number density of holes Nh shows a slight variation from 0.6 to 0.8 (±0.1) nm-3 with increasing branching.