Monolithic, "molded", porous materials with high flow characteristics for separations, catalysis, or solid-phase chemistry: Control of porous properties during polymerization

The porosity and flow characteristics of macroporous polymer monoliths that may be used to prepare separation media, flow-through reactors, catalysts, or supports for solid-phase chemistry can be controlled easily during their preparation. Key variables such as temperature, composition of the pore-forming solvent mixture, and content of cross-linking divinyl monomer allow the tuning of average pore size within a broad range spanning 2 orders of magnitude. The polymerization temperature, through its effects on the kinetics of polymerization, is a particularly effective means of control, allowing the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The choice of pore-forming solvent is also important, larger pores being obtained in a poor solvent due to an earlier onset of phase separation. Increasing the proportion of the cross-linking agent present in the monomer mixture not only affects the composition of the final monoliths but also decreases their average pore size as a result of early formation of highly cross-linked globules with a reduced tendency to coalesce. The synergy of different effects has also been observed under specific polymerization conditions using two monomer pairs, styrene-divinylbenzene and glycidyl methacrylate-ethylene dimethacrylate polymerized in close molds. Mercury intrusion porosimetry measurements, inverse size exclusion chromatography, and back pressure measured at different flow rates with the macroporous monoliths were used for the characterization of the porous properties. A good correlation between pore size and flow resistance that follows the Hagen-Poiseuille equation used previously to describe flow through a straight tube has been found.