Amorphous Phase of Propylene/Ethylene Copolymers Characterized by Positron Annihilation Lifetime Spectroscopy

Recent advances in catalyst technology make it possible to synthesize high molecular weight propylene copolymers with a high degree of isotacticity and high levels of an α-olefin comonomer. The primary objective of this study is to systematically characterize the rubbery amorphous phase of propylene/ethylene (P/E) copolymers over a range in comonomer content. A series of new experimental P/E copolymers prepared with a group IV transition metal-based post-metallocene catalyst are compared with a series of P/E copolymers prepared with a conventional metallocene catalyst. Positron annihilation lifetime spectroscopy (PALS) is used to obtain the temperature dependence of the free volume hole size. The PALS measurements are supplemented with bulk volume−temperature measurements. It is found that the free volume hole size and the amorphous phase density at ambient temperature strongly depend on crystallinity. Densification of the amorphous phase is attributed to constraint imposed on rubbery amorphous chain segments by attached chain segments in crystals. It is now possible to attribute the reported discrepancy between conventional measurements of crystallinity from density and crystallinity from heat of melting to the crystallinity dependence of the amorphous phase density. The fractional free volume (FFV) of the amorphous phase is obtained by combining the free volume hole size with the macroscopic volume−temperature measurement. At the glass transition temperature the FFV is constant across the crystallinity range of the P/E copolymers with a value of about 0.04, in agreement with iso-free volume concepts of the glass transition.