This part of the scientific activity is devoted to the synthesis and characterisation (chemico-physical, structural, morphological and functional) of organic-inorganic hybrid materials based on early transition metal oxoclusters.[1-3] These are neutral polynuclear complexes displaying an inorganic core consisting of M-O-M bridges (M=Zr, Hf, Ta, Ti-Hf, Hf-Ti-Zr, Ba-Ti, etc.), and surrounded by a variable number of functional moieties (methacrylates, thiols..) which are prone to be co-polymerised with suitable monomers. The presence of a covalent bond between the inorganic building blocks and the macromolecular backbone strongly enhances the structural and functional properties of the final hybrid materials.[4-6] These oxoclusters have been proved to be versatile inorganic building blocks for the synthesis, not only of hybrid materials, but also of MOF[7, 8] and nanostructured colloids, as well as interesting structural models for the exploration of early transition metal chemistry (see Figure 1).[10, 11]
Some of the obtained clusters were used to develop inorganic-organic hybrid materials by covalent embedding of the functionalised oxoclusters into polymer matrices through copolymerisation reactions (typically thermo- and photo-initiated free radical polymerisation) with suitable monomers, according the synthetic strategy depicted in Figure 2.[3-5, 12]
These materials have proven to be endowed with enhanced thermo-mechanical properties,[4-6, 13,14] as well as with dielectric properties, making them interesting candidates for the development of dielectric thin layers. Their structural properties as protective layers for different substrate materials (steel, wood, ceramics, aluminum, plastics) were also explored. More recently, in collaboration with Prof. Mauro Carraro (Dipartimento di Scienze Chimiche, Università di Padova), we have also pointed out the catalytic activity of both the transition metal oxoclusters as well as of the resulting hybrids,[18,19] acting as heterogeneous catalysts for oxidation reactions. The catalytic activity has been tested on both bulk oxocluster-reinforced hybrid materials as well as on the same systems prepared by miniemulsions at Max Planck Institut für Polymerforschung in Mainz.
Thanks to the support of SABIC Europe (Sittard, The Netherlands), we have recently synthetised and thoroughly characterised oxocluster-based shape memory thermoresponsive hybrid materials.
In particular, our research was focused on the synthesis of different methacrylate-functionalized zirconium oxoclusters and their covalent incorporation in a butylacrylate (BA)/polycaprolactone dimethacrylate (PCLDMA) copolymer, by following the strategy sketched in the following Scheme:
In the resulting hybrids the shape recovery and the shape fixity rate were studied in order to observe if the shape memory properties is maintained, going from simple copolymer to non-covalent bond-based hybrid, and to covalent bonds-based hybrid (progressive rise of bond strength). In all hybrid samples, the shape memory properties exhibited in the pristine copolymer are maintained even if a higher stiffness of the hybrid samples are observed, due to the crosslinking induced by the oxocluster. In all samples the recovery and fixity rate are good and over 90 %, also evidencing as the oxocluster does not hinder the shape memory properties in hybrid materials. In addition, the introduction of an inorganic phase and the progressive more stable interaction between organic and inorganic part, from simple copolymers to covalent hybrids, leads to an enhancement of the thermo-mechanical properties of the samples. The final materials were characterised through a multitechnique spectroscopic approach involving Solid State Nuclear Magnetic Resonance (SS-NMR) and X-ray Absorption (XAS) spectroscopies FT-IR, whereas thermo-mechanical properties were studied by TGA-DSC and swelling tests, dynamical-mechanical analyses (DMA).
This is an home made video showing the shape-memory properties of the obtained materials:
The SABIC company is gratefully acknowledged for the financial support of part of this activity.
Additionally, the Group has also pioneered a new XAS-based approach to investigate hybrid materials, in particular the structural integrity of the inorganic building blocks embedded in the polymer matrix.