Structure and curing mechanism of high-ortho and random novolac resins as studied by NMR

1H pulse NMR and 13C CPMAS NMR measurements are carried out for novolac resins in order to investigate the structure and curing mechanism. Thermal behavior of molecular mobility obtained from spin–spin relaxation time, T2, indicates that the hydrogen bond network formed in novolacs is predominant factor of glass transition for novolacs without cross-link agents. In presence of cross-link agent, hexamethylenetetramine (HMTA), the appearance of adducts between novolacs and HMTA is recognized. At curing temperature, the rigid fraction increased as a final step of curing. Different thermal behavior of mobility is observed for random and high-ortho novolacs/HMTA. Formation of adducts occurred at lower temperature for high-ortho novolac. 13C NMR chemical shift and quantum chemical calculation suggest that high acidity of phenolic OH of o-novolac caused by intramolecular hydrogen bonding is responsible for the formation of adducts at lower temperature.