Quantitative Measurement of Polymer Compositions by NMR Spectroscopy:Targeting Polymers with Marked Difference in Phase Mobility

Abstract

A novel scheme that combines the cross-polarization (CP) with a single-pulse (SP) experiment is developed to quantitatively analyze compositions of multiphase polymers with marked difference in phase mobility. The CPSP method is able to record both CP and SP spectra in one experiment and avoid excessive long relaxation delay, saving considerable experimental time. The quantification of CP spectrum is done by measuring the enhancement factor of CP with a reciprocity relation, while the quantification of SP is derived directly from the Boltzmann distribution in thermo equilibrium. The relative mole composition obtained from a poly(styrene)-poly(isoprene) diblock copolymer matches well with the known value.

Hybridizing cross-polarization with NOE or refocused-INEPT enhances the sensitivity of MAS NMR spectroscopy

“…However, the signals from mobile components in a typical 13 C NMR spectrum obtained with 1 H- 13 C CP are generally compromised due to their weak 1 H- 13 C heteronuclear dipolar coupling. In fact, when a short contact time is used for the CP transfer, these signals could completely disappear [40]. Therefore, it is necessary to enhance the S/N ratio for signals from mobile groups while simultaneously retaining the signals from rigid groups as obtained with CP.…”

“…Fig. 1a shows the conventional ramp-CP pulse sequence, where only a 1 H 90° pulse is applied before the CP period; whereas the CPSP pulse sequence [40], shown in Fig. 1b, requires the simultaneous irradiation of 90° pulses on 1 H and 13 C channels before the CP period.…”

“…Traditionally, an additional experiment like the single-pulse excitation of dilute nuclei or refocused-INEPT (refocused insensitive nuclei enhanced by polarization transfer) [36–39] is often performed independently of the CP experiment in order to characterize the mobile components in such systems. Recently, a new pulse sequence that combines CP with a single-pulse experiment (referred to as CPSP) [40] was used to analyze various compositions of multiphase polymers with large mobility contrast (Fig.1). Because 13 C T 1 of mobile sites are generally shorter than those of rigid ones, the 90° pulse applied before CP on the 13 C channel in the CPSP experiment flips the 13 C magnetization from mobile groups onto the xy -plane for final detection.…”

“…The theoretical details underlying this proposed CP-NOE sequence are presented by revisiting the Solomon equations [52]. We also compare the performance of CP-NOE pulse sequence with that of the CPSP method [40]. Secondly, we demonstrate the feasibility of independent acquisition of signals from rigid and mobile groups within a single experiment by combining RINEPT sequence with CP (referred to as CP-RINEPT) after the CP signal acquisition.…”

Quantitative analysis of a labeled 13C system in NMR

“…To alleviate these problems, many approaches had been proposed. They utilized the effects of variable contact times [9], Lee-Goldberg spin locking [10], cross depolarization [3,[11][12][13], and dipolar recouping (termed QUCP) [14][15][16][17]. These methods worked well on their proposed systems.…”