Humidity-Responsive Fluorescent Polymer Nanocomposites

In recent years, much effort has been devoted to extend applications of commodity plastics through mixing and blending with different additives. In this context, polymer nanocomposites prepared by dispersion of functional nanostructured substrates are of great interest due to the possibility to combine improvements in mechanical properties, heat resistance, flammability, gas permeability, with functional properties, which are transferred from the filler to the polymer composite [1,2]. Intercalation of guest species into layered inorganic solids is a method for producing ordered inorganic-organic assemblies with unique micro-structure controlled by host-guest and guest-guest interactions. Functional hybrid materials obtained by intercalation of specific active species between inorganic layers can be in principle dispersed into a polymer matrix thus obtaining multifunctional polymer nanocomposites. In this work, fluorescent layered double hydroxides (LDHs) were prepared by anion exchange and calcination-rehydration methods. In particular, the co-intercalation of fluorescein and alkyl sulfate anions was used as an effective strategy for preventing the aggregation of the dye reducing intermolecular quenching and obtaining hybrid LDHs that are fluorescent even in the solid state. Later, fluorescent poly(lactic acid) (PLA) and linear low density polyethylene (LDPE) nanocomposites containing 5 wt.% of fluorescein-modified LDHs were prepared by solution mixing. These materials showed thermal properties similar to those of the neat polymers and morphology depending on the level of dispersion of the inorganic layers. A blue shifted emission (20-25 nm) with respect to the corresponding co-intercalated LDHs was found for both types of nanocomposites with a more pronounced effect for PLA-based samples, revealing a further destruction of dye aggregates due to the dispersion and distribution of the hybrid into the polymer matrix. Fluorescence emission spectra of PLA and LDPE-based composites were registered for dried films and for films exposed to relative humidity of 99%. Interestingly, it was observed an enhanced fluorescence emission by increasing humidity due to the equilibrium between the dianionic and monoanionic fluorescein forms. Even if the effect was generally more marked for LDPE nanocomposites than for PLA based samples, results suggest the possible use of both materials as ON/OFF humidity indicators [3]. References: [1] F. Ciardelli, S. Coiai, E. Passaglia et al. Polym. Int. 57 (2008) 805-836. [2] S. Coiai, E. Passaglia, A. Pucci, G. Ruggeri, Materials 8 (2015) 3377-3427. [3] S. Coiai, S. Javarone, F. Cicogna et al. Eur. Polymer J. 99 (2018) 189-201.