An outline is given of the development of temperature rising elution fractionation (TREF) technique for polymer separation on the basis of crystallizability. The TREF technique has found application mainly in the area of analysis of semi-crystalline polyolefins and its development has been significantly influenced by the desire to establish the structure of linear low density polyethylenes. These materials exhibit an unusually broad distribution of comonomer which can be readily explored by TREF. Systems have been developed which can provide fractionation on a preparative scale, where recovery of fractions for further analysis is desirable and more recently on a small scale, where rapid analysis using detectors provide refined crystallizability distributions. A description is given of these fractionation systems along with a discussion of the separation mechanism and the significance of controlling the crystallization step to achieve fractionation efficiency.

Temperature rising elution fractionation

Molecular and crystalline structures of linear low-density polyethylenes (LLDPE) were investigated by a series of characterization techniques. Molecular structural characteristics were elucidated by temperature-rising elution fractionation (TREF) and solvent-gradient elution fractionation (SGEF). A bird’s eye view and a contour map of LLDPE obtained by a combination of TREF and size exclusion chromatography exhibited a broad and multimodal chemical composition distribution (CCD), in contrast to a sharp and single CCD of conventional high-pressure low-density polyethylene (HP-LDPE). Short chain branching (SCB) was found to decrease with increase of molecular weight by SGEF technique. Thermal analysis of cross-fractions proved that a characteristic broad endothermic curve of LLDPE is attributable to its broad and multimodal CCD. Then, using DSC results, an indicative index (DI) which expresses the degree of the distribution of lamellar crystal thickness is proposed. DI was found to be sensitive both to CCD and to a kind of SCB. The crystallinity and melting temperature of cross-fractions having comparable molecular weights decrease with increasing comonomer content in the order of octene-1≍4-methyl-pentene-1>hexene-1>butene-1. From a statistical approach to the relationship between crystallinity and degree of SCB, the probability of exclusion of a bulky branching such as isobutyl from a crystalline lattice is considered to be twice as large as than that of ethyl branching.

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Structural Distribution of Linear Low-Density Polyethylenes

Crystallization analysis fractionation (Crystaf) and temperature rising elution fractionation (Tref) are analytical techniques for determining the distribution of chain crystallizabilities of semicrystalline polymers. These techniques fractionate polymer chains on the basis of the differences in their chain microstructures that affect their crystallizabilities in dilute solutions. Both techniques can be used to estimate the chemical composition distribution of copolymers and the tacticity distribution of homopolymers. This information is crucial for understanding polymerization mechanisms and constructing structure-property relationships. This review covers the theoretical aspects of both techniques, describes their basic operation procedures and applications, and discusses the mathematical models proposed for Crystaf and Tref.

Fractionation of Semicrystalline Polymers by Crystallization Analysis Fractionationand Temperature Rising Elution Fractionation

The crystallization behavior of random copolymers of ethylene

Evaluation of the heterogeneity in linear low-density polyethylene comonomer unit distribution by differential scanning calorimetry characterization of thermally treated samples

Copolymers of ethylene with relatively small amount of nine species of α-olefins, i.e., propylene, butene-1, pentene-1, hexene-1, octene-1, dodecene-1, octadecene-1, 4-methylpentene-1, and 4.6-dimethylheptene-1 were prepared with the use of ZIEGLER-NATTA catalyst and their melt properties, crystalline behaviors, and various physical properties were investigated in relation to the side-chain length and structure.



The melt index-intrinsic viscosity (MI-[η]) correlation, melt tension, and melt extensibility were found to be hardly affected by the branches. On the crystalline behavior and physical properties, the effects of branching generally became more pronounced when the branch sizes became larger. It was noticeable, however, that ethyl and propyl branches exert strong effects in comparison with their relatively small sizes. It was also observed that the ethylene/octadecene-1 copolymers have lower β-peak heights and higher β-peak temperatures in E″-temperature curves than expected from their branch size, and this was considered to be due to the crystallization of the side-chains. The properties of high pressure polyethylenes coincided, in many respects, with those of the copolymers having bulky branches.



Copolymere aus Athylen und verhaltnismasig geringen Mengen an neun α-Olefinen, d.h. Propylen Buten-1, Penten-1, Hexen-1, Octen-1, Dodecen-1, Oktadecen-1, 4-Methylpenten-1 und 4.6-Dimethylhepten-1, wurden unter Verwendung eines ZIEGLER-NATTA-Katalysators hergestellt. Die Schmelzeigenschaften, das Verhalten des Kristallzustandes und verschiedene andere physikalische Eigenschaften wurden in bezug auf Lange und Struktur der Seitenketten untersucht.



Es wurde gefunden, das die Schmelzindex-Grenzviskositatszahl (MI-[η])- Beziehung sowie die Ausdehnbarkeit und Tension der Schmelze nur wenig durch die Seitenketten beeinflust werden. Fur das Verhalten des Kristallzustandes und die physikalischen Eigenschaften ergab sich, das die Einflusse der Seitenketten generell um so starker sind, je groser die Seitenketten sind. Es war aber bemerkenswert, das Athyl- und Propylseitenketten fur ihre Grose einen starken Einflus ausuben. Bei den Athylen/Oktadecen-Copolymeren wurden niedrigere β-Peakhohen und hohere β-Peaktemperaturen in E″-Temperaturkurven, als von der Seitenkettengrose zu erwarten war, beobachtet; diese Ergebnisse konnen auf die Kristallisation der Seitenketten zuruckgefuhrt werden. Die Eigenschaften der Hochdruckpolyathylene stimme in mancher Hinsicht mit denjenigen der Copolymeren mit grosen Seitenketten uberein.

Hiroshi Watabe

Papers

Effects of branching on some properties of ethylene/α‐olefin copolymers