Calorimetry: a Valuable Tool for Emerging Smart Nanomaterials

Abstract

It is well known that calorimetry is a powerful technique to determine the complete energetics (standard free energy, enthalpy and entropy) of interaction processes. We are now assisting to the increment of the use of nano-calorimetry, which appears rather versatile because it can evidence weak interactions quickly and accurately and, furthermore, needs very small amounts of material. Unfortunately, over the years the calorimetry has been applied to restricted fields so that it has been underemployed in studying smart nanomaterials (polypseudorotaxanes, polymeric aggregates, surface functionalized nanoparticles, nanocomposites, etc.) to which a tremendous scientific interest has been recently addressed. Within this topic some significant recent results from our laboratory will be described in the following. Concerning the supramolecular chemistry field, we exploited strategies to design stable and stimuli responsive polypseudorotaxanes (Figure 1A) based on cyclodextrins (cyclic oligosaccharides) and homopolymers or block copolymers. This was done by modulating the structure of the polymer and tuning the macrocycles cavity size. The knowledge of the energetics of the cyclodextrin/polymer mixtures, determined from direct and accurate calorimetric experiments, provided not only unambiguous insights to establish the stability of the complexes and the polymer phase behaviour, but also rigorous predictive tools to trigger and to control the release processes. We designed aqueous hybrid nanoclay-polymer structures (Figure 1B) that are inexpensive, biocompatible, environmental friendly and advanced for potential specific purposes. Calorimetric experiments allowed us to determine the thermodynamics of the adsorption, which enabled us to interpret the behavior at the air/solution interface. The nanoparticles functionalization was triggered by temperature and/or inorganic salts opening up to new routes for the synthesis of smart materials. Calorimetry and thermal analysis were revealed very efficient in studying nanocomposites based on nanoclay (with disk- and tubular-like morphology) and biocompatible polymers (Figure 1C). The use of complementary techniques allowed us to define thermodynamic and structural models to correlate the mesoscopic features to the bulk properties of these materials. From all these studies one concludes that calorimetry plays a key role for the development of rather complicated supramolecular machines and molecular devices.