Supramolecular complexes of PNIPAAM-co-MA and imogolite to obtain thermo-responsive clay nanotubes

In this study, we investigate the colloidal properties of hybrids formed by imogolite nanotubes (INTs) and poly(N-isopropylacrylamide)-co-methacrylic (PNIPAAM-co-MA), which is a thermo-sensitive copolymer suitable for drug delivery applications. The selective adsorption of anionic PNIPAAM-co-MA onto the positively charged external surface of imogolite is driven by electrostatic attractions as evidenced by both ζ-potential and Dynamic Light Scattering (DLS) experiments. According to polarized optical microscopy (POM) and Small-angle X-ray scattering (SAXS) measurements, the specific interactions with PNIPAAM-co-MA affect the structural organization of imogolite nanotubes destabilizing the nematic phase. On the other hand, the formation of PNIPAAM-co-MA/INTs complex alters the thermo-responsive characteristics of the copolymer due to variations on the thermodynamics of the coil-to-globule transition of PNIPAAM-co-MA. Thermodynamic characterizations (Differential Scanning Calorimetry, Density and Speed of sound experiments) reveal that the lower crystalline solution temperature (LCST) of PNIPAAM-co-MA is reduced in the presence of imogolite nanotubes. The attained knowledge represents a starting point to develop thermo-responsive clay nanotubes useful for biomedical purposes by exploiting the selective coating of imogolite with PNIPAAM-co-MA.