Assessing the nanoscale properties of polymer monoliths used for chromatography (#69)
Polymer monoliths are an increasingly common stationary phase format used for liquid chromatography (LC). Typically synthesized in situ within the column housingusing free-radical co-polymerization, macroscopically rigid polymer monoliths have distinctive structural properties ranging from the macro- to the nanoscale. While understanding these properties is a critical aspect for rationalizing their chromatographic performance, visualization of structural and mechanical characteristics at the nanoscale remains a significant challenge. In this contribution, we explore the nanoscale properties of polymer monoliths using confocal Raman spectroscopy imaging and Atomic Force Microscopy (AFM) and relate this to separation performance for LC.
Confocal Raman spectroscopy allows generation of three-dimensional representations of monoliths in both dry state and in contact with solvents. This directly demonstrates the varying accessibility of mobile phase solvent components into the polymer. AFM force-distance measurements on cross-sectioned globular features permit structural and mechanical properties to be probed at the sub-micrometer scale. This reveals a heterogeneous cross-link density distribution within individual globular features (i.e. on a scale of single micrometers with nanometer-scale resolution). This can be attributed to the typically used free-radical (co-)polymerization chemistries. These mechanical characterizations also support our extensive chromatographic data which indicate that modulation of nanoscale polymer properties in solvated environments has a substantial impact on chromatographic performance.