APPLICATION OF FFC-NMR TECHNIQUES FOR THE STUDY OF THE FEATURES OF NANOSPONGES

Fast-Field-Cycling (FFC) NMR relaxometry is a versatile and powerful tool for studying the microscopic dynamics of molecular systems, provided that a suitable analysis of the longitudinal relaxation kinetics and the relevant NMRD (i.e. longitudinal relaxation rate R1 vs. Larmor frequency νL) dispersion curves is performed. In particular, FFC-NMR can afford valuable information on the texture properties (average pore size, specific surface and specific pore volume) of porous materials, once they have been saturated with water. Thus, it appears a technique of choice for assessing the otherwise elusive texture properties of nanosponges (NSs). These hyper-crosslinked polymers, obtained by reticulating cyclodextrins with suitable linker units, constitute an emerging class of functional materials due to their easy synthesis and chemical modification, and to their tunable absorption and controlled release abilities as well. NSs are supposed to possess a thick network of nanochannels in their highly disordered structure. However, their textural features are quite difficult to estimate partly because of their fair swellability, and classical evaluation methodologies such as N2 absorption isotherms analyzed by BET/BJH methods, or dye absorption isotherms, have afforded questionable results. Here we present some results from FFC-NMR investigations performed on a set of suitably selected NSs, aimed at providing a viable method to evaluate their texture properties, as well as the functional mobility of a water medium within their pore network. In particular, inspired by soil science, we tried to extend to NSs the concept of “connectivity”,[1] by defining a “Pore Connectivity Index” (PCI)[2] based on T1 realxation times distribution functions, which may constitute a valuable alternative to quantify the permeability of NSs. Moreover, by assessing the mobility of water molecules in terms of the relevant correlation times τc, estimated by means of a new heuristic analysis method of NMRD curves,[3] we attempted to rationalize their controlled release abilities.