Characterizing chemical composition of polyolefin-based copolymers from spectral features in the C-H stretching region

Abstract

The comonomer content of a series of commercial ethylene–octene (Engage®, Infuse®), ethylene–butene (Engage), and ethylene–propylene (Versify®, Nordel®) copolymers is investigated using the Raman spectroscopy. The analysis relies upon the different content of methyl, methylene, and methine groups of each copolymer and focuses on the spectral features of the C―H stretching region. Raman spectra of a series of molecules with well-defined content of methyl, methylene, and methine groups (alkanes and well-defined polymer chains) are first addressed to rationalize the complex spectral features arising from different stretching modes, their Fermi resonance, and the different molecular conformations. Results are interpreted on the base of recent work on the topic. A curve-fitting procedure is proposed to resolve contributions arising from CH 2 and CH 3 groups. The sum of intensity of bands at 2,855 and 2,865 cm −1 (symmetric C―H stretching and Fermi resonance) correlates linearly with CH 2 content whereas that at 2,880 cm −1 (symmetric C―H stretching) does with CH 3 content. With that base, Raman spectra of ethylene-based copolymers are analyzed to quantify comonomer content. Results are compared with independent results from 13 C nuclear magnetic resonance analysis with good agreement between the methods. Overall, it highlights the importance of Raman spectroscopy as versatile tool for process monitoring, quality control, or sample identification not only at academics but also in industrial environments.