Structure and Distribution of Cross-Links in Boron-Modified Phenol-Formaldehyde Resins Designed for Soft Magnetic Composites: A Multiple-Quantum 11B-11B MAS NMR Correlation Spectroscopy Study

Despite the extensive use of boron-modified phenol-formaldehyde polymers as insulating materials in soft magnetic composites (SMCs), the structure and arrangement of the inorganic cross-linking units in these systems have not been fully elucidated. To clarify the structure, configuration, and distribution of the boron cross-links in these materials, phenol-formaldehyde resins modified by boric acid were synthesized and characterized using,advanced multiple-quantum B-11-B-11 MAS NMR correlation techniques combined with the quantum chemical geometry optimizations and the subsequent B-11 NMR chemical shielding calculations. The analyses of the resulting spectra revealed a well evolved (high-density) phenol-formaldehyde polymer network additionally strengthened by nitrogen and boron cross-links. The boron-based cross-links were attributed to monoester (ca. 10%) and diester (ca. 90%) complexes (six-membered spirocyclic borate anions) with strictly tetrahedral coordination (B-IV). During the thermal treatment, the monoester and diester borate complexes underwent additional transformation in which the spirocyclic borate anions were more tightly incorporated into the polymer matrix via additional N-type (amino) cross-links. A B-11-B-11 double-quantum correlation MAS NMR experiment revealed that the majority of the monoester and diester borate complexes (ca. 80%) were uniformly distributed within and effectively isolated by the polymer matrix, with an average B-11-B-11 interatomic distance greater than 6 angstrom. A non-negligible part of the spirocyclic borate anion complexes (ca. 20%), however, existed in pairs or small clusters in which the average B-11 center dot center dot center dot B-11 interatomic distance was less than 5.5 angstrom. In addition, the formation of homodimers (diester-diester) was' demonstrated to be preferred over the formation of heteroclusters (monoester-diester).