52 LCGC NORTH AMERICA VOLUME 22 NUMBER 1 JANUARY 2004 www.chromatographyonline.com temperature-rising elution fractionation overcomes the limitations of preparative temperature-rising elution fractionation by reducing the column and sample sizes con- siderably and by continuously monitoring the amount of the eluted polymer fractions as a function of column temperature using an on-line detector. For polyethylenes, the major factor that determines melting in a solution is the degree of short-chain branching (3). Each branching point in a chain is rejected from the growing crystal, thereby limiting crystal thickness and hence reducing the melting temperature. Thus, a direct relationship exists between the melting temperature and the degree of branching, which is reflected in analytical temperature-rising elution frac- tionation curves. Moreover, Nagano and Goto (3) and Mirabella (4) have reported that a linear relationship exists between the elution temperature and the degree of short- chain branching (number of branches per 1000 carbon atoms). Therefore, analytical temperature-rising elution fractionation curves do in fact describe the short-chain branching distribution (5). The short-chain branching distribution provides some information about the mech- anism of polymerization and the nature of the catalysts. As an example, when chemists use conventional Ziegler-Natta catalysts for the polymerization of ethylene, a broad The authors describe a system for automated analytical temperature-rising elution fractionation that uses the built-in differential refractometer of a commercial gel-permeation chromatography instrument and a stop-flow method for sample crystallization. They discuss the performance of the system and its ability to fractionate different types of polyethylenes. An Improved Analytical Temperature-Rising Elution Fractionation System for Automated Analysis of Polyethylenes Adrian G. Boborodea*†, Daniel Daoust*, Alain M. Jonas*, and Christian Bailly* * Catholic University of Louvain, Unité de Physique et de Chimie des Hauts Polymères, Bâtiment Boltzmann, Place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium, e-mail boborodea@ poly.ucl.ac.be † Polytechnical University of Bucharest, Department of Polymer Physics, 149 Calea Victoriei, Sector 1, Bucharest, Romania Address correspondence to A.G. Boborodea. olymers basically are heteroge- neous materials, and heterogene- ity can be exhibited in various ways such as distribution of chain lengths, differences in chemical composi- tion, stereoregularity, and architecture (tac- ticity or branching). Branching can be char- acterized as either long chain or short chain. Temperature-rising elution fractionation is a technique that allows the analysis of semicrystalline polymers by separating frac- tions according to their crystallizability. Temperature-rising elution fractionation enables analysts to evaluate structure hetero- geneity and to characterize the distribution of short-chain branching. Elution during temperature-rising elution fractionation is governed by the melting of semicrystalline polymers in the presence of a solvent (1). Preparative temperature-rising elution fractionation can be performed to isolate fractions for additional investigations. Dur- ing this process, a large quantity of solvent is required for eluting the sample, and a large amount of non-solvent, usually meth- anol, also is used for precipitating the eluted fractions (2). This technique is quite time- consuming; it typically requires several days to perform a fractionation. The development of analytical temperature- rising elution fractionation has been an improvement upon preparative temperature- rising elution fractionation. Analytical P