Structural development during thermal fractionation of polyethylenes

In this study, the stepwise isothermal crystallization or thermal fractiona tion of Ziegler-Natta and metallocene based polyethylenes (ZN-PE and m-PE) with two kinds of branch lengths (ethyl and hexyl) and branch compositions were studied using simultaneous synchrotron small-angle X-ray scattering (S AXS)/wide-angle X-ray diffraction (WAXD) and differential scanning calorime try (DSC). The crystal long period and the invariant were determined by SAX S, and the variations of crystal unit cell parameters and the degree of cry stallinity were determined by WAXD. The arithmetic mean length (Ln), the we ighted mean length (Lw) and the broadness index (Lw/Ln) of the studied poly ethylenes were previously determined by DSC. Results from these studies wer e interpreted using the model of branch exclusion, which affects the abilit y of the chain-reentry into the crystal phase. Multiple SAXS peaks and step -change in crystallinity change (WAXD) were seen during heating, which corr esponded well with the crystal thickness distribution induced by stepwise c rystallization. The effects of the heterogeneity of the 1-olefin branch len gth and the distribution on the crystal long period and the invariant as we ll as the degree of crystallinity were discussed.