Pulsed 81Br nuclear quadrupole resonance spectroscopy of brominated flame retardants and associated polymer blends

The dispersion of brominated flame retardants in polymers is monitored with Br-81 nuclear quadrupole resonance (NQR) using a pulse NQR spectrometer. The NQR spectrometer consists of a homemade 10-300 MHz single-channel NMR console coupled to a broadly tunable probe. The probe is a loop-gap resonator usable from 220 to 300 MHz and is automatically tuned over any 5 MHz region with a stepping motor and a radio frequency bidirectional coupler. Br-81 NQR spectra of several brominated aromatic flame retardants, as pure materials and in polymers, were recorded in the range of 227 to 256 MHz in zero applied magnetic field. Two factors affect the Br-79/81 NQR transition frequencies in brominated aromatics: electron-withdrawing substituents on the ring and intermolecular contacts with other bromine atoms in the crystal structure. An existing model for substituents is updated, and a point char ge model for the intermolecular contacts is developed. In this study, we exploit the Br-81 NQR transition frequency dependence on intermolecular contacts to learn how a flame retardant is dispersed in a polymer matrix. Additionally, the crystal structure for 1-bromo-4-(4-bromophenoxy)benzene and the 100 K structure of 1,2,4,5-tetrabromobenzene were determined.