The Molecular Dynamics During Gelation of Resorcinol and Formaldehyde As Studied by Dielectric Relaxation

The gelation of resorcinol and formaldehyde (RF) consists of four secondary processes with no well-defined time boundaries. The growth kinetics of RF gel processes are still many unknown areas. In this work, the sol-gel polycondensation reaction of RF was analyzed by dynamic light scattering (DLS), rheological measurements, and dielectric relaxation spectroscopy (DRS). Through in situ analysis, some molecular dynamics information of RF gelation was obtained, such as particle size change trend, gel threshold time, dipole moment change and so on. These molecular dynamics information reflects the effect of the concentration of the RF reaction solution on the growth process. That is, the low-density RF sol is in the nucleation-dominant mode, while the high-density RF sol is in the diffusion-dominant mode, rapidly forming a thick skeleton. The dielectric relaxation analysis of the RF gel process included some dipole relaxations and the interfacial relaxation. The changing trend of the interfacial relaxation was consistent with the two growth modes obtained from the rheological measurements. By analyzing the kinetics of the RF reaction solution at different growth temperatures, we obtained how to control the temperature to obtain stable RF gels. A nucleation-dominant mode was obtained for the initial growth at low temperature, followed by a diffusion-dominant mode at high temperature after the gel threshold time.