Differences in porous characteristics of styrenic monoliths prepared by controlled thermal polymerization in molds of varying dimensions

Abstract

Nitroxide-mediated polymerization was used as a model system for preparing styrenic monolithic materials with significant mesopore contents in different mold formats, with the aim of assessing the validity of pore characterization of capillary monoliths by analysis of parallel bulk polymerized precursor solution. Capillary monoliths were prepared in 250 microm id fused silica tubes (quadruplicate samples, in total 17 m), and the batch polymerizations were carried out in parallel in 100 microL microvials and regular 2 mL glass vials, both in quintuplicate. The monoliths recovered from the molds were characterized for their meso- and macroporous properties by nitrogen sorptiometry (three repeated runs on each sample), followed by a single analysis by mercury intrusion porosimetry. A total of 14 monolith samples were thus analyzed. A Grubbs' test identified one regular vial sample as an outlier in the sorptiometric surface area measurements, and data from this sample were consequently excluded from the pore size calculations, which are based on the same nitrogen sorption data, and also from the mercury intrusion data set. The remaining data were subjected to single factor analyses of variance analyses to test if the porous properties of the capillary monoliths were different from those of the bulk monoliths prepared in parallel. Significant differences were found between all three formats both in their meso- and macroporous properties. When the dimension was shrunk from conventional vial to capillary size, the specific surface area decreased from 52.2+/-4.7 to 34.6+/-1.7 m(2)/g. This decrease in specific surface area was accompanied by a significant shift in median diameter of the through-pores, from 310+/-3.9 to 544+/-13 nm. None of these differences were obvious from the scanning electron micrographs that were acquired for each sample type. The common practice of determining the mesopore characteristics from analysis of samples prepared by parallel bulk polymerization and looking for changes in the macropore structure by visual assessment of SEMs are therefore both rather questionable, at least for monoliths of the kind used in this study.

Comprehensive study of the macropore and mesopore size distributions in polymer monoliths using complementary physical characterization techniques and liquid chromatography

“…S1, a comparison is made between the morphologies of monoliths synthesized in capillary format and in bulk, the latter being used for mercury‐intrusion porosimetry and gas‐adsorption experiments. Although similar domain sizes are observed, Byrström et al discussed that the variations in porous structure induced by differences in mold size may not be evidently visible .…”

Fast preparation of a hydrophobic monolith by redox initiation and its application in the separation of small molecules

“…It allows a useful estimate of the expected total surface areas of porous polymeric materials and the existence of a permanent mesoporous pore space at least qualitatively. 26 We ascertained that the monolithic column with no porogen having a surface area of 1.8 m 2 g À1 was not suitable for the efficient separation of small molecules. When 0.2 g of cetyl alcohol was used as a porogen the surface area increased to 279 m 2 g À1 .…”

Guidelines for tuning the macropore structure of monolithic columns for high‐performance liquid chromatography

“…It has been speculated that polymerization in confined spaces results in different macropore structures than when conducting the polymerization in large id conduits. Byström et al synthesized monolithic structures via thermally initiated polymerization reactions in 250 μm id capillary columns, and in (micro-)vials with an id of 4.4 mm and 9.7 mm, respectively [71]. While SEM images did not reveal significant differences, difference in mode pore diameter and macropore size distribution was observed applying mercury intrusion porosimetry.…”