Porous polymer monoliths: Morphology, porous properties, polymer nanoscale gel structure and their impact on chromatographic performance

Porous monoliths based on organic precursors undergoing free-radical cross-linking polymerization in porogenic solvents emerged approximately two decades ago as an alternative stationary phase material for diverse applications including liquid chromatography. Though having a profound difference in morphology to their earlier generation polymer bead-based counterparts, they are often based on similar chemistries and as such show certain peculiarities with respect to transport and performance in liquid chromatography applications. Polymer monoliths typically consist of a globule-like, three-dimensionally adhered backbone, which is in a contrast to the silica monoliths having a bi-continuous mesoporous skeleton. Both material types possess large flow-through pores making them desirable for high performance liquid chromatography and other flow-through applications. The current review is devoted to a critical appraisal of the major challenges that researchers face in the retrieval of the never-ending demand of efficiency at often forgotten and desired selectivity and retention in separations using porous polymer monoliths. Therefore, an attempt is made to establish profound links of polymer monoliths to their earlier generation polymer-based particulate beds and differences to silica-based materials. These links are associated with an emerging morphological understanding of the polymer monoliths porous flow-through pore structure, the nanoscale backbone chemistry, and related chromatographic performances in both theoretical and experimental studies. Associated with this understanding, existing attempts in improving flow and transport performance of polymer monoliths are described and discussed. Such developments are addressing morphological concerns with respect to homogeneity and detailed design of pore space, but also tailoring backbone nanostructural chemistry to modulate mass transfer.