Characterization of the combined molecular weight and composition distribution of industrial ethylene/α-olefin copolymers

The composition-molecular weight distribution, CD x MWD, of ethylene/alpha -olefin copolymers was measured quantitatively using two methods: (1) temperature rising elution fractionation (TREF) followed by size-exclusion chromatography (SEC) of each fraction and (2) SEC followed by Fourier-transform infrared spectroscopy (FTIR) of each fraction. TREE-SEC provides a rather complete, accurate, and quantitative representation of the CD x MWD. On the other hand, SEC-FTIR leads to loss of information on some details of the CD x MWD. The extent of the loss depends on the material examined. The main advantage of SEC-FTIR is the much shorter analysis time compared to TREF-SEC. We briefly discuss another method, TREF followed by measurements of the molecular weight of the fractions by light scattering, LS - either off-line or on-line. We point out that this technique also leads to some loss of information on the CD x MWD. However, for linear low density polyethylene-type materials, TREF-LS may be a more useful technique than SEC-FTIR while less time-consuming than TREF-SEC. Finally, we use deconvolution of the SEC-FTIR data into Flory-Stockmayer distributions in an attempt to recover all or part of the information loss inherent in the SEC-FTIR method. The attempt was not successful when applied to the data presented in this work. We discuss several reasons for this failure including limited instrumental resolution and intrinsic limitations of the SEC-FTIR technique. (C) 2001 Published by Elsevier Science Ltd.