Size-exclusion chromatography of low-molecular-mass polymers using mesoporous silica

In this study, the mesoporous silicas APMS-30 and MCM-41 were compared to a commercial silica, Nucleosil, in size-exclusion chromatography (SEC). Polystyrenes of various molecular masses (M) were passed through HPLC columns of the silicas (each column contained a single type of silica) and retention times were plotted as a function of log M for each type of silica. Linear fits to the data were used to identify regions of total exclusion, size exclusion, and total permeation in each of the three plots. Due to their small pore sizes, APMS-30 and MCM-41 (pore radii: 14.5 and 17.0 A from N2 physisorption, respectively) did not exhibit a total permeation region even at M= 104 g mol(-1) (styrene monomer), while Nucleosil (pore radius: 57.9 A) showed total permeation at M=644 g mol(-1). These results indicate that mesoporous silica is better than Nucleosil in SEC of polymers with low M, making it useful in a variety of SEC applications including the determination of M for small molecules such as oligomers and oligopeptides. Interestingly, calculation of the radius of gyration, Rg, of the largest polymer that still exhibits a size-exclusion effect proved to be a reasonable method of determining the pore diameters of each material, with estimated Rg = 8.9, 11.5, and 63.5 A for APMS-30, MCM-41, and Nucleosil, respectively.