Effect of molecular structure distribution on melting and crystallization behavior of 1-butene/ethylene copolymers
Introduction
Copolymers of ethylene and α-olefins are known to have a very heterogeneous chain microstructure with respect to molar mass and short chain branching (SCB). Although molar mass is of importance, the amount and distribution of the SCB are dominant factors for determining the physical properties of ethylene/α-olefin copolymers. These molecular parameters of such copolymers depend in turn on catalysts used for the copolymerization, and can differ considerably from one grade of copolymer to another. Therefore, studies on the effect of molecular structure on the physical properties of ethylene/α-olefin copolymers are of great importance.
Investigation into the relationship between structure and properties of olefin copolymers requires, among other factors, analysis of crystallization and melting behavior. A number of studies have been devoted to the melting and crystallization of homogeneous copolymers of ethylene and α-olefins made with vanadium-based catalysts [1], [2], [3], [4] and copolymers made with Ziegler–Natta catalysts [5], [6], [7]. The consensus of these studies is that melting and crystallization temperatures of ethylene/α-olefin copolymers decrease considerably with increasing amounts of SCB. The effect of molar mass on melting and crystallization behaviors was found to be small [3], [4], [8]. It has also been recognized that the melting and crystallization behaviors of ethylene/α-olefin copolymers are complex due to the variations of SCB distributions in the different types of these copolymers. This is even true for homogeneous copolymers of ethylene and 1-octene synthesized using a vanadium-based Ziegler–Natta catalyst; the SCB distribution for these polymers was considered to be uniform [2], [3], [4]. This complexity is largely due to the interrelation of SCB, MSL and molar mass distributions, and this interrelation makes it difficult to independently investigate the effect of the different molecular parameters on the crystallization and melting characteristics.
Insight into the crystallization and melting behaviors of commercial ethylene/α-olefin copolymers can be obtained by studying the whole polymer and its compositional fractions. The whole polymer sample can be fractionated according to molecular parameters such as molar mass or SCB, and the crystallization and melting characteristics of the individual fractions, for which the distributions of the structural parameters are narrow, can be studied. Several authors have employed fractionation methods to investigate structural and crystallization behaviors of ethylene/α-olefin copolymers made with heterogeneous Ziegler–Natta catalysts [5], [6], [7], [9], and they concluded that these fractionation techniques are effective for exploring the relationships between the structure, and crystallization and melting characteristics of such copolymers.
In the current study, analytical and preparative temperature rising elution fractionation (ATREF and PTREF) and thermally fractionated DSC were used to investigate the molecular structure of two commercial 1-butene/ethylene copolymers. The two polymers were fractionated by PTREF, which yielded a series of samples with similar SCB content but different SCBD. The effect of SCB distributions on the melting and crystallization behavior of the PTREF fractions and the whole polymers were studied using DSC.
Section snippets
Experimental
The polyethylenes used in this study were two commercial 1-butene/ethylene copolymers; one made with a Ziegler–Natta catalyst and the other with a metallocene catalyst. The properties of the two polymers are given in Table 1. The polymers will be referred to as Ziegler–Natta copolymer and metallocene copolymer.
Fractionation of the copolymer resins was accomplished by temperature rising elution fractionation (TREF). The polymer samples in o-xylene at concentrations of 0.005–0.04 g PE/ml, along
Molecular structural distribution of Ziegler–Natta and metallocene copolymers
The SCB content is a dominant factor affecting the spherulitic texture and the lamellar morphology, and hence the melting and crystallization behaviors of ethylene/α-olefin copolymers [3], [4], [5], [6], [7], [8], [9]; the influence of molar mass is much less significant. As suggested by Peeters et al. [3], [4], based on the results with homogeneous copolymers of ethylene and 1-octene synthesized using vanadium-based Ziegler–Natta catalyst, melting and crystallization temperatures of these
Conclusions
ATREF analysis showed that the Ziegler–Natta copolymer had a broad bimodal SCB distribution, while the metallocene copolymer showed a much narrow SCB distribution. SNA-DSC analysis revealed that the Ziegler–Natta copolymer possessed a broad MSL distribution with significant amount of long methylene sequences. In contrast, the metallocene copolymer showed a much narrower MSL distribution and contained a large amount of short methylene sequence.
The difference in the SCB and MSL distributions
Acknowledgements
The authors acknowledge the support of this work by the Natural Sciences and Engineering Research Council of Canada and NOVA Chemicals Corporation.
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