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

Abstract 1 H pulse NMR and 13 C 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, T 2 , 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. 13 C 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.