On Defining a Simple Empirical Relationship to Predict the Pore Size of Mesoporous Silicas Prepared from PEO-b-PS Diblock Copolymers

Mesoporous silicas with large and accessible pores have been successfully synthesized using laboratory-made poly(ethylene oxide)-b-polystyrene (PEO-b-PS) copolymers as templates. The PEO-b-PS copolymers were synthesized using living/controlled radical polymerization. The porous structure (mesopore size, size distribution, and microporous volume) was characterized using small-angle X-ray scattering, electron transmission microscopy, and nitrogen sorption measurements. In this study, we have investigated the dependence of the mesoporosity on both the PS and PEO blocks length using two series of PEO-b-PS copolymers with a constant degree of polymerization of the PEO block for each series (N-PEO = 114 and N-PEO = 232). It was found that the mesopore size increases and the microporous volume decreases as the PS block length (N-PS) increases. The PEO block participates to both the micropore and mesopore formation. By fitting these experimental data, a simple empirical relationship between the pore radius and the length of the both PS block (N-PS) and PEO block (N-PEO) is found R-P (nm) = 0.36.(NPEONPS0.5)-N-0.19. This relationship is in agreement for both low- and high-molecular-weight copolymers and can be easily be used to fine-tune the mesopore size of silica materials, in a large range (from 4 to 22 nm), when using PEO-b-PS copolymers as templates. Furthermore, the influence of the synthesis temperature (between 25 and 60 degrees C) on the porous structure was also investigated and it was found that by increasing the synthesis temperature, the mesopore diameters remain relatively constant; however, the pore entrances increase in size, leading to more open pore structures.