FFC-NMR techniques for assessing the texture features of nanosponges

Abstract

Nanosponges (NSs), i.e. hyper cross linked polymeric materials obtained by reticulating cyclodextrin units 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 (average pore size, specific surface and specific pore volume) are quite difficult to estimate, and classical evaluation methodologies (N2 absorption isotherms analyzed by BET or BJH methods, or dye absorption isotherms1) have afforded questionable results. Indeed, partly due to their fair swellability, the concept itself of specific area seems quite elusive to define for these materials. In this communication we present the results of a Fast Field Cycling (FFC) NMR relaxometric investigation on a set of suitably selected NSs, aimed at providing a viable method to evaluate their texture properties. A suitable heuristic analysis of the NMRD dispersion curves enables to individuate the dynamic domains which can be led back to the structural motifs present in the materials structures. Moreover, The FFC relaxometric technique is able to afford valuable information on the mobility of water molecules inside the nanochannels of the swellable NS structure, providing indirect information on pore size distribution.1 Inspired by results gained in soil science,2 we propose to extend to nanosponge materials the concept of “connectivity”, by defining a “Pore Connectivity Index” (PCI)3,4 based on T1 realxation times distribution functions, which may constitute a valuable alternative to quantify the functional permeability of NSs.