Structure, Dynamics, and Host-Guest Interactions in POSS Functionalized Cross-Linked Nanoporous Hybrid Organic-Inorganic Polymers

We report the synthesis and structural studies of new hybrid organicinorganic porous materials with flexible siloxane spacers with different lengths bonded to polyhedral oligomeric silsesquioxane (POSS) cages. Positron annihilation lifetime spectroscopy (PALS) was used to measure the pore sizes of two polymers. For sample 1, which has a shorter linker, the free volume diameters were found to be 5.32 +/- 0.06 angstrom, 8.28 +/- 0.05 angstrom, and 10.42 +/- 0.05 angstrom at 25 degrees C. The first value represents the POSS cage, while the others describe the space between linkers. For polymer 2, the appropriate diameters are 4.96 +/- 0.04 angstrom and 7.96 +/- 0.06 angstrom for the POSS cage and linker spacing, respectively. This sample is extremely well organized and homogeneous. The evolution of the free volume sizes as a function of temperature in the range from -20 degrees C to +60 degrees C was investigated. Solid-state NMR spectroscopy (SS NMR) was employed to study the structure and molecular dynamics of the polymers. The signal assignment was performed using H-1 MAS (magic angle spinning), C-13 CP/MAS (cross-polarization magic angle spinning), and Si-29 CP/MAS experiments. The dynamic processes were investigated by inspection of the relaxation times, analysis of the Si-29 principal elements of the chemical shift tensors (CST), and the center-band only detection of exchange (CODEX) technique, which is specific for studies on slow molecular motions. It was found that the POSS cage in sample 2 undergoes a complex reorientation in the cross-linked polymer lattice. Both evaluated samples are capable of hosting small organic molecules in the pores formed by the spacers and the POSS after cross-linking. The hostguest interactions were investigated by SS NMR and PALS. A strong influence of the guest molecules on the dynamics of the POSS cages for sample 2 was observed. The formation of hostguest complexes is a reversible process, and guest molecules can be replaced by other species; thus, the polymers under investigation can be used as molecular containers.