History of the Micelles: A Key Parameter for the Formation Mechanism of Ordered Mesoporous Carbons via a Polymerized Mesophase

New insights into the formation mechanism of a polymerized mesophase (space group Im (3) over barm) and its derived order mesostructured carbon phase (denoted FDU-16) were evidenced by a slight modification of its synthesis conditions using proton nuclear magnetic resonance (NMR) relaxometry (T-2 spin-spin relaxation time), dynamic light scattering (DLS), small-angle X-ray diffraction (SAXRD), N-2 physisorption (77 K), and scanning electron microscopy (SEM). The preparation of those materials is based on the self-assembly of a prepolymer (phenol-formaldehyde resin called resol) and a pyrolysable surfactant (Pluronic F127, PEO106PPO70PEO106). The former is the carbon precursor, and the latter is the pore structuring agent and also the porogen. The resol and the F127 solutions are usually mixed at room temperature before being heated at 65 degrees C to promote the polymerization of the resol into a polymerized mesophase that precipates. That phase is further carbonized into a mesostructured porous carbon. By a slight change of the mixing temperature around the critical micellar temperature of the F127 amphiphilic surfactant (similar to 25 degrees C), different formation pathways were observed, visually and thanks to DLS and NMR relaxometry techniques. It is proposed that the mixing temperature influences, in the early stage of the synthesis, both the resol-bound micelles (e.g., resol localization and solubility in the micelles) and the concentration of the free resol in the solution. It leads then to different kinetics of the free resol polymerization and to different evolutions of the intermediate colloidal particle populations with precipitation times ranging from 40 to 120 h. That synthesis parameter also influences the properties of the final mesostructured carbon materials by modifying their rhombododecahedral macroscopic morphology, their body-centered cubic mesostructural arrangement, and their porosity. That study highlights the high sensitivity of surfactant templated mesostructured materials toward the mixing temperature. That key parameter is rarely investigated and deserves to be systematically mentioned in the literature to avoid unexplained problems of reproducibility.