Crystallization kinetics of ethylene homopolymers: a new perspective from residual catalyst and resin molecular weight

Abstract

Two Ziegler-Natta and one metallocene supported catalysts were prepared that were used to synthesize ethylene homopolymers with different molecular weights. The influence of varying residual catalyst types and molecular weights on the isothermal crystallization kinetics of these polymers was studied using DSC and Avrami model to gain better understanding. The MetCat HomoPE, unlike the Z-N HomoPEs, followed the Avrami model during the entire crystallization. The increase in molecular weights did not affect (i) the decreasing trend of 1/t 1/2 (bulk crystallization rate) and K (crystallization rate constant) with the increase in T c (crystallization temperature), and (ii) the increasing trend of K with the increase in %crystallinity. However, combined with the varying residual catalysts, it differently varied the rate of change of 1/t 1/2 and K; and decreased T c . K and 1/t 1/2 turned out to be asymptotically related up to 1/t 1/2 ≈ 0.73 min−1. The residual catalyst type more predominantly affected the crystallization facileness than the molecular weight. Therefore, the Z-N 1 residual catalyst acted as heterogeneous nuclei. In Z-N HomoPE 1 and Z-N HomoPE 2, n ranged from 2.2 to 3.4, and 2.6 to 3.0, respectively as a function of T c . In MetCat HomoPE, it remained constant at 2. Therefore, the MetCat residual catalyst impinged the expected spherulitic crystal growth to a two dimensional one. All these findings were explained considering how the molecular level residual catalysts, with their characteristic surface chemistries and structures, influenced the prevailing heterogeneous nucleation process, and the aliased interaction of G (nuclei growth rate) and N (nucleation rate).