Influence of the polymerisation time on the porous and chromatographic properties of monolithic poly(1,2-bis(p-vinylphenyl))ethane capillary columns

In order to elucidate the effect of the polymerisation time on the morphology of styrene based monolithic support materials, continuous poly(1,2-bis(p-vinylphenyl))ethane (BVPE) rods were synthesised in 1.0ml glass vials by thermally initiated free radical polymerisations of BVPE in the presence of porogens (toluene, decanol) and a,a′-azoisobutyronitrile (AIBN) as initiator at 65°C for different polymerisation times (60, 90, 150, 300 and 600min). Porosity parameters like pore-size-distribution and total porosity were investigated by mercury intrusion porosimetry, while the specific surface area of the BVPE monolithic supports was determined by N₂-adsorption (BET) measurements. An untypical bimodal pore-size-distribution comprising a high fraction of both mesopores (2-50nm) and macropores (mainly flow-channels in the micrometer range) was observed as a result of the stepwise decrease of the polymerisation time. In consequence of the significant changes of the pore-size-profile, shortening the polymerisation time also resulted in enhanced total porosity due to enlarged flow-channel diameters and increased surface area according to the presence of a considerable amount of mesopores. Results upon the porosity profile of the support are further confirmed by SEM images of monoliths polymerised for different time periods. Since mesoporosity and high surface area of the chromatographic support material play key roles in the interaction and thus retention of low-molecular-weight compounds, polymerisation time should also affect the chromatographic properties and applicability of these polymers. To study the influence of the polymerisation time towards the separation efficiency of small molecules on BVPE capillary columns (200μm I.D., 8cm), a mixture of homologous alkylbenzenes was chosen for column evaluation. In accordance with the observations of the porous properties of BVPE stationary phases, the rapid and high resolution separation of a range of low-molecular-weight compounds on monolithic BVPE supports were successfully realised. The methodical reduction of the polymerisation time has been demonstrated to be a simple and effective tool to tailor the porous properties of organic monoliths to provide novel polymer-based stationary phases with porous properties adequate for the rapid and high resolution chromatography of small organic molecules.