Formation Mechanism of
Cubic Mesoporous Carbon Monolith
Synthesized by Evaporation-Induced Self-assembly
- Publication date
- Publisher
Abstract
The formation mechanism of the cubic mesoporous carbon,
FDU-16,
synthesized by evaporation-induced self-assembly (EISA) was investigated
at the molecular level by electron paramagnetic resonance (EPR) spectroscopic
techniques. This material is synthesized using F127 pluronic block
copolymer [poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene
oxide) (PEO<sub>106</sub>-PPO<sub>70</sub>-PEO<sub>106</sub>)] as
a structure-directing agent (template) and phenolic resol as a carbon
precursor. Using two spin probes derived from pluronics with PEO and
PPO chains of different lengths that are designed to sense different
regions of the system, we followed the evaporation and thermopolymerization
stages of the synthesis in situ. To make such studies possible, we
have used a polyurethane foam support, placed in the EPR tube, which
allows for the efficient solvent evaporation as required for EISA.
We focused on the evolution of the dynamics of the template and its
interactions with the resol during the reaction. We observed that
during the evaporation stage the resol is distributed throughout the
entire PEO blocks, all the way to the PPO–PEO interface, interacting
with them via H-bonds, thus hindering the local motion of the PEO
chains. At the end of this stage there is no polarity gradient along
the PEO blocks, as found for traditional F127 micelles in water or
during the synthesis of silica materials, and the mesostructure is
not well-defined. A polarity and a resol gradient developed during
the thermopolymerization stage where the polymerizing resol is driven
out to the outer region of the PEO corona. This produces a corona
of resin-pluronic composite and a resol-free PPO core with high mobility
of the PEO segments close to the PPO–PEO interface and restricted
mobility in the composite corona. During this stage the final structure
sets in