Crystallization mechanisms for LLDPE and its fractions

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Abstract
The crystallization behavior of two ethylene/octene copolymers, which differ in hexyl branch concentration, and their fractions were assessed. Fractionation of the crystalline linear low density polyethylenes (LLDPEs) was achieved by temperature rising elution fractionation. As the column temperature was raised, the eluted fractions exhibited a reduction in branch concentration and an increase in molecular weight. This was attributed to the difference in reactivity between ethylene and octene and the subsequent depletion of the ethylene monomer in the solution process. Spherulites formed during the crystallization of the whole polymers were well developed, banded, and displayed a wide distribution of sizes. However, spherulites of the LLDPE fractions were less well developed, more uniform in size, and tended to progressively deteriorate and become smaller as the concentration of branches increased. The ethylene and octene blocks of the copolymer crystallized independently, and it was proposed that the octene portion formed short, curved lamellae in the interfacial region of the lamellae formed from the linear ethylene portion of the molecule. Decreases in d spacing for fractions with increased short chain branching corresponded with similar drops in molecular weight.