A series of amorphous
melamine-based polymer networks synthesized
by Schiff base chemistry (SNW) were successfully prepared by varying the strut length. The
pore structure was analyzed by gas adsorption and positron annihilation
methods. Positron lifetime measurements indicate the existence of
ultramicropores and also larger mesopores in the SNW materials. The
sizes of micropores and mesopores are almost the same in these samples,
which are about 0.7 and 16.5 nm, respectively. The relative number
of micropores increases in the order of SNW-1 < SNW-2 < SNW-3,
while the number of mesopores increases in the reverse order. N2 adsorption/desorption measurements also reveal micropores
and mesopores in these materials. However, it gives an underestimation
of the micropore volume. Benefiting from the abundant nitrogen content
and high microporosity, the SNW materials exhibit exceptionally high
CO2 capture ability, which reaches a maximum value of 18.3
wt % in SNW-3 at 273 K and 1 bar, followed by SNW-2 and SNW-1. This
order is exactly the same as the order of micropore volume revealed
by positron annihilation measurement, suggesting that micropores play
a crucial role in the CO2 uptake. Our results show that
positron can provide more precise information about the structure
of micropores and thus can offer an accurate prediction for the adsorption
capacity of complex porous materials.