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

The composition-molecular weight distribution, CD x MWD, of ethylene/alpha -olefin copolymers was measured quantitatively using two methods: (1) tempe rature rising elution fractionation (TREF) followed by size-exclusion chrom atography (SEC) of each fraction and (2) SEC followed by Fourier-transform infrared spectroscopy (FTIR) of each fraction. TREE-SEC provides a rather c omplete, 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 a dvantage of SEC-FTIR is the much shorter analysis time compared to TREF-SEC . We briefly discuss another method, TREF followed by measurements of the mol ecular 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 infor mation on the CD x MWD. However, for linear low density polyethylene-type m aterials, TREF-LS may be a more useful technique than SEC-FTIR while less t ime-consuming than TREF-SEC. Finally, we use deconvolution of the SEC-FTIR data into Flory-Stockmayer di stributions in an attempt to recover all or part of the information loss in herent 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 fai lure including limited instrumental resolution and intrinsic limitations of the SEC-FTIR technique. (C) 2001 Published by Elsevier Science Ltd.