Porosity of Molecularly Imprinted Polymers Investigated by 129 Xe NMR Spectroscopy.

Molecularly imprinted polymers (MIPs) display intriguing recognition properties and can be used as sensor recognition elements or in separation. In this work, we investigated the formation of hierarchical porosity of compositionally varied MIPs using 129 Xe Nuclear Magnetic Resonance (NMR) and 1 H Time Domain Nuclear Magnetic Resonance (TD-NMR). Variable temperature 129 Xe NMR established the morphological variation with respect to the degree of cross-linking, supported by 1 H TD-NMR determination of polymer chain mobility. Together, the results indicate that a high degree of cross-linking stabilizes the porous structure: highly cross-linked samples display a significant amount of accessible mesopores that instead collapse in less structured polymers. No significant differences can be detected due to the presence of templated pores in molecularly imprinted polymers: in the dry state, these specific shapes are too small to accommodate xenon atoms, which, instead, probe higher levels in the porous structure, allowing their study in detail. Additional resonances at a high chemical shift are detected in the 129 Xe NMR spectra. Even though their chemical shifts are compatible with xenon dissolved in bulk polymers, variable temperature experiments rule out this possibility. The combination of 129 Xe and TD-NMR data allows attribution of these resonances to softer superficial regions probed by xenon in the NMR time scale. This can contribute to the understanding of the surface dynamics of polymers.