Nonisothermal melting and crystallization studies of homogeneous ethylene/?-olefin random copolymers

Abstract

A study of nonequilibrium melting, nonisothermal, and isothermal crystallization behavior of ethylene/1-octene (EO) random copolymers, produced using metallocene catalysts has carried out. As branch (or defect) content increases, the nonisothermal and isothermal crystallization rates, melting temperatures, and heats of fusion decrease. There is also a branch length effect on melting temperature depression, the melting temperature depression of EO random copolymers with hexyl branches were significantly larger than those of ethylene/1-butene (EB) and ethylene/1-propene (EP) copolymers having ethyl and methyl branches, respectively. The melting temperatures of homogeneous random copolymers have been found to be always lower than those of fractions of heterogeneous copolymers, having approximately the same branch content and molecular weight. Hence, defect distribution in copolymer systems is at least as important a parameter as the defect content.

The effect of modality on linear low-density polyethylene crystallization behaviour at high and very high supercoolings

“…The well-known fact that when moving downwards from the equilibrium melting temperature towards higher supercoolings, the isothermal crystallization rate increases with the degree of supercooling is proved also by the current study. From previous investigations it is known that materials with homogeneous structure (both MMD and SCBD), mainly produced by a single-site catalyst, crystallize slower than ZN materials having heterogeneous distributions and large proportion of long ethylene sequences at a similar average molar mass and branching content [1,3,12]. The current study also proves this phenomenon at temperatures above 100 °C.…”

“…It is generally understood that close to the equilibrium melting temperature methyl branches are incorporated into the crystal, giving rise to defects, hexyl and longer branches are mainly rejected from the crystal and ethyl branches have an intermediate behaviour, depending on the crystallization mode [16,17], or that ethyl branches are unconditionally rejected [3]. The diminishing difference and ultimate unification of crystallization rates towards higher supercoolings has been explained by a changed crystallization mechanism where branches are not excluded, somewhat similar to congealing [12].…”

“…Under rapid cooling the effects of molar mass and branch content give way to that of sequence length distribution [12]. The molar mass effect gives way to that of branch content when the latter has become large enough [3,18]. Similarly, at sufficiently high supercoolings, where branches are incorporated into the crystals and behave like linear sequences, the effect of branching distribution appears to be overrun by the ultra-high molar mass constituent present in the bimodal material.…”

“…The use of Ziegler-Natta (ZN) type catalysts when synthesizing LLDPEs makes them heterogeneous both in intermolecular and intramolecular terms [1], i.e. compared to materials synthesized by single-site catalysts, the ZN materials have a broad ethylene sequence length (ESL) distribution with considerable quantities of long sequences [1,2,3]. If unimodal LLDPEs are traditionally produced by a single reactor-catalyst system then bimodal polymer materials can be produced, besides using dual catalyst systems in a single reactor [4], in two cascaded reactors at different reaction conditions and feeds [5], which is comparable to blending two polymers of a different structure, resulting in a broad molar mass distribution (MMD).…”

Melting Temperature Characteristics for Polyethylenes from Crystal Size Distribution

“…Their complex molecular characteristics result in complex melting traces. Generally, they show broad and multiple melting peaks [2,6,7].…”