Free Volume, Glass Transition and Degree of Branching in Ethylene/α‐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 ν h 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 X c the specific volume of the amorphous phase Va was estimated and used to calculate the specific hole-free volume V f . It was found that T g and X c decrease and V, V a , V f , and ν h increase with increasing weight fraction of comonomers. Both the real crystalline and amorphous specific volumes decrease with increasing crystallinity X c . For not too high contents of comonomers, Tg decreases and ν h 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 ν h 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 T g from ν h measured at room temperature is shown. The number density of holes N h shows a slight variation from 0.6 to 0.8 (±0.1) nm -3 with increasing branching.