Sensitivity-Enhanced Spin Echo Nuclear Magnetic Resonance Pulse Sequence for Quantitative Polymer Characterizations with a Nuclear Magnetic Resonance Cryoprobe

Abstract

Microstructures in polymers often impact performance attributes and physical properties. Therefore, correctly identifying and quantifying these microstructural motifs are paramount. Unfortunately, it remains a challenge to obtain an accurate quantitation of these details with NMR spectroscopy. The primary issues that prevent NMR from delivering quality results are baseline curvature from acoustic ringing, which is found with helium-cooled cryoprobes that enhance signal sensitivity, and insufficient signal-to-noise (S/N) that leads to long experimental acquisition periods. In this report, we present a sensitivityenhanced spin echo NMR pulse sequence (CPMG-INEPTRDG5) for quantitative polymer characterizations where the signal sensitivity is enhanced by a factor of 3.8 compared with using pulse sequences 90_a180 and 90_h180, requiring 1/14th the experimental time for the same S/N. As executed on our model system, the experimental uniform excitation bandwidth was 18,000 Hz (corresponding 120 ppm 13 C on 600 MHz NMR spectrometer) with ±1% error, which is excellent for quantitative NMR analyses of 13 C and very important for analyzing polymers such as polyols as they need at least 65 ppm useful uniform excitation bandwidth. It has the potential to enhance analyses of latex surface molecules, spin finish from fiber materials, impact copolymers, blends of "high melting" and "low melting" polyethylenes, and many other polymer systems, for which the 13 C chemical shift range is within 120 ppm. The relative ratios of octene tetrad (OOOO), triad (OOO), diad (OO), and monad (O) in a blend of "high melting" and "low melting" polyethylenes are measured with the new spin echo NMR pulse sequence CPMG-INEPTRDG5.