Vibrational dynamics and hydrogen bond properties of beta-CD nanosponges: an FTIR-ATR, Raman and solid-state NMR spectroscopic study

Cyclodextrin nanosponges (CDNS) are cross-linked polymers with remarkable inclusion/release properties. CDNS show swelling capability and a hydrophilicity/hydrophobicity balance that can be dramatically modified by the type and quantity of cross-linking agents. Here, we focus our attention on samples of beta-cyclodextrin nanosponges (beta-CDNS) obtained by reacting beta-cyclodextrin (beta-CD) with the cross-linking agent carbonyldiimidazole at different beta-CD:cross-linking agent molar ratio. The vibrational properties of CDNS thus synthesized have been investigated by Fourier transform infrared spectroscopy in attenuated total reflectance geometry and Raman spectroscopy in the dry state at room temperature. The quantitative analysis of the O-H stretching region (3,000-3,800 cm(-1)) allowed us to obtain structural information on the role played by primary and secondary OH groups in the hydrogen bond network of the polymer. Also, the contribution of interstitial and intracavity crystallization water molecules is reported. Solid-state NMR spectroscopy is used to study the molecular mobility of the polymer by measuring the H-1 spin-lattice relaxation time in the rotating frame (T-1 rho). The T-1 rho values obtained for the polymer beta-CDNS are compared with free beta-CD. The observed relaxation parameters point out that the ester formation occurs mainly at the primary OH groups of CDs, also supporting the interpretation of vibrational spectra.