UNDERSTANDING NANOSCALE POROSITY IN MOLECULARLY IMPRINTED POLYMERS VIA 3D FIB/SEM TOMOGRAPHY

We present combined focused ion beam/scanning electron beam (FIB/SEM) tomography as method for examining the distribution and connectivity of pores within molecularly imprinted polymers (MIPs) and their non-imprinted control polymers (NIPs). Besides µ-computer tomography (µCT), x-ray tomography, TEM tomography, FIB/SEM tomography is a highly attractive technique to obtain information on porosity and interconnectivity at the micro- and nanoscale. The advantage is also related to that large sections can be investigated. To our best knowledge, FIB/SEM tomography has not yet been extensively applied for visualizing the heterogeneity of nanoscopic pore networks, interconnectivity, and tortuosity in polymers. The present study evaluates FIB/SEM tomography for analyzing the nanoscale porosity of MIPs and their counterpart NIPs. MIPs imprinted for propranolol, a beta-blocker and commonly used model template in molecular imprinting studies - and the corresponding NIPs - were investigated establishing FIB/SEM tomography as a viable strategy complementing conventional isotherm studies. For visualizing and understanding the properties of pore networks in detail, polymer particles were stained with osmium tetroxide (OsO₄) vapor, and embedded in epoxy resin. Staining with OsO₄ provides excellent contrast during high-resolution SEM imaging. After optimizing the threshold to discriminate between the stained polymer matrix, and pores filled with epoxy resin, a 3D model of the sampled volume may be established for deriving not only the pore volume and pore surface area, but also to visualize the interconnectivity and tortuosity of the pores within the sampled polymer volume. Detailed studies using different types of cross-linkers and the effect of hydrolysis on the resulting polymer properties will also be presented. A significant difference between MIP and NIP could be found for all investigated polymers. It could be shown that the interconnectivity of the visualized pores in MIPs is significantly higher vs. the non-imprinted polymer. A comparison with commonly applied BET method will also be provided and the advantages and disadvantages of FIB/SEM tomography in comparison to such commonly applied methods will be provided.