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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

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posted on 28.07.2015, 00:00 authored by Libor Kobera, Jiri Czernek, Magda Strečková, Martina Urbanova, Sabina Abbrent, Jiri Brus
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 11B–11B MAS NMR correlation techniques combined with the quantum chemical geometry optimizations and the subsequent 11B 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 (BIV). 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 11B–11B 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 11B···11B interatomic distance greater than 6 Å. A non-negligible part of the spirocyclic borate anion complexes (ca. 20%), however, existed in pairs or small clusters in which the average 11B···11B interatomic distance was less than 5.5 Å. In addition, the formation of homodimers (diester–diester) was demonstrated to be preferred over the formation of heteroclusters (monoester–diester).