Effect of hydrophobic group in polymer matrix on porosity of organic and carbon aerogels from sol–gel polymerization of phenolic resole and methylolated melamine

Abstract

Organic and carbon aerogels were prepared by solution–sol–gel polymerization of phenolic resole and methylolated melamine followed by supercritical drying and pyrolysis. The hydrophobic group was incorporated into polymer matrix by adding m-cresol in the solution–sol–gel step and the effect of addition on porosity of organic and carbon aerogels was elucidated by nitrogen adsorption and density measurement. The ratios of m-cresol to phenolic resole (R) were changed from 0/7.5 to 2.5/7.5 while ratios of other components to phenolic resole remained unchanged. It is found that the total pore volumes of organic and carbon aerogels exhibit maxima at 1/7.5 and are determined by the total concentrations of reactants and cumulative volume shrinkages of gels from hydrogels to organic aerogels and from hydrogels to carbon aerogels respectively. The macropores in organic aerogels, developed in supercritical drying process, are determined by gel discrete particles–gel discrete particles interactions, which can be tuned by the incorporated hydrophobic groups. The micropores in carbon aerogels are generated by evolving volatile compounds in the pyrolysis process. The meso- and macropores in carbon aerogels are determined by (1) meso- and macropores in relevant organic aerogels; (2) volume shrinkages that convert macropores to mesopores and mesopores to micropores; (3) mass loss in pyrolyzing organic aerogels that increases sizes and volume of carbon aerogels; and (4) coarsening that converts mesopores to macropores to reduce interfacial energy. The mesopore size distributions of organic and carbon aerogels exhibit maxima at 1/7.5, which is also related to the actions of the hydrophobic groups. The BET surface areas of organic aerogels and external surface areas of carbon aerogels are determined by sizes and volumes of mesopores and the BET surface areas of carbon aerogels are determined by micro- and mesopores.