Mechanism and kinetics of nanostructure evolution during early stages of resorcinol-formaldehyde polymerisation.

Resorcinol and formaldehyde react in aqueous solutions to form nanoporous organic gels well suited for a wide range of applications from supercapacitors and batteries to adsorbents and catalyst supports. In this work, we investigated the mechanism and kinetics of formation of primary clusters in the early stages of formation of resorcinol-formaldehyde gels in the presence of dissolved sodium carbonate. Dynamic Light Scattering measurements showed that size of freely diffusing primary clusters was independent of both reactant and carbonate concentrations at a given temperature, reaching the mean hydrodynamic radius of several nanometres before further changes were observed. However, more primary clusters formed at higher carbonate concentrations, and cluster numbers were steadily increasing over time. Our results indicate that the size of primary clusters appears to be thermodynamically controlled, where a solubility/miscibility limit is reached due to formation of certain reaction intermediates resulting in approximately monodisperse primary clusters, most likely liquid-like, similar to formation of micelles or spontaneous nanoemulsions. Primary clusters eventually form a particulate network through subsequent aggregation and/or coalescence and further polymerisation, leading to nanoscale morphologies of resulting wet gels. Analogous formation mechanisms have been previously proposed for several polymerisation and sol-gel systems, including monodisperse silica, organosilicates and zeolites.