The effect of chain length of flexible diacid on morphology and mechanical property of modified phenolic resin

Adipic acid (DA4), suberic acid (DA6), sebacic acid (DA8), and dodecanedioic acid (DA10) were used to improve the brittleness of resol type phenolic resin (PF). Chemical reaction of PF with diacid was manifested by a shift of carbonyl stretching peak of diacid to a higher frequency in FT-IR spectra. The modified PF with diacid showed an optimum content (0.0014 mol) and an optimum chain length of diacid (n=6, i.e. DA6) to improve effectively its brittleness. The highly cured phenolic resins with DA4 and DA6 were found to have homogeneous morphologies, but those with DA8 and DA10 had phase-separated morphologies in scanning electron microscopy (SEM) images. The slightly cured PFs with DA4, DA8, and DA10 exhibited phase-separated morphologies, but that with DA6, a homogeneous morphology on SEM micrographs. Furthermore, the latter was confirmed by a proton spin–lattice relaxation time (T1H) measurement on solid-state 13C NMR spectra. It was found by X-ray diffraction and differential scanning calorimeter that the phase-separated part of modified phenolic resin was a crystalline phase of unreacted diacid. The highly improved toughness of modified PF with DA6 was attributed to the inherently homogeneous morphology with domain sizes around 20–30 nm scale found by the T1H measurement.