Cluster Reinforced Polymers

A new type of inorganic-organic hybrid polymer is obtained by co-polymerization of organic monomers with inorganic clusters being capped by polymerizable organic groups. The obtained highly crosslinked hybrid polymers exhibit properties different to the parent polymers. Compared to functionalized (nano-)particles, the use of clusters, as nano building blocks, has several advantages:

In most clusters, all metal atoms are surface atoms, i.e. clusters can be regarded as nanoparticles without "inner" atoms. This results in the highest possible interface, and interphase in the hybrid polymers.
Each cluster in a given sample has, by nature, the same composition, size and shape. Nevertheless can clusters of varying composition, size and shape be prepared.
Clusters are big molecules, i.e. they can be dissolved in organic solvents, purified, etc., and can be characterized by the analytical tools of molecular chemistry.
Due to their molecular nature, surface modification is straightforward and can be monitored by standard analytical methods.

The two principal synthesis routes for clusters with functional ligands are the synthesis of the clusters in the presence of the (functional) capping ligands or the post-synthesis exchange or modification of the ligands. Our work is focused on carboxylate-substituted metal oxo clusters, which are easily obtained by either method by employing functional carboxylic acids. Representative examples are shown in Figure 1 (OMc = methacrylate). Two cluster features can be varied:

Composition, size and shape of the cluster core. We prepared a great variety of carboxylate-substituted mono-, di- or trimetallic metal oxo clusters of the general formula M_aO_b(OH/OR)_c(OOCR′)_d (M = Y, Ti, Zr, Hf, Nb, Ta) by reacting metal alkoxides with the corresponding carboxylic acids. Clusters of different size and shape are obtained by controlling the alkoxide / carboxylic acid ratio and the kind of OR groups in the parent alkoxide.
The organic functionality and thus the polymerization method to be used. Carboxylate-substituted metal oxo clusters with the following functionalities were prepared until present: (meth)acrylate derivatives for free radical polymerization, norbornene-2-carboxylate derivatives for ring-opening metathesis polymerization, 4-pentynoate derivatives for click reactions, 2-bromo-iso-butyrate derivatives as initiator for atoms-transfer radical polymerization (by G. Kickelbick et al., Vienna) and 3-mercaptopropionate derivatives for thiol-ene reactions (S.Gross et al., Padova).

Figure 1.
Molecular structures of the clustersZr₆O₄(OH)₄(OMc)₁₂ (left)
and Zr₄O₂(OMc)₁₂

The clusters exhibit an interesting coordination chemistry. For example, the carboxylate ligands are highly dynamic, and undergo site exchange at the cluster surface. Furthermore, the ligands can be partially or fully exchanged when cluster solutions are treated with carboxylic acids. This allows preparing clusters with two different carboxylate ligands. For example, the mixed-ligand cluster Zr₆(OH)₄O₄(propionate)₅(methacyrlate)₇ is obtained upon reaction of Zr₆O₄(OH)₄(OMc)₁₂ with six molar equivalents of propionic acid.

Radical or photochemical-initiated polymerization of the clusters, typically with a 50-200 fold molar excess of methacrylic acid, methylmethacrylate or styrene as organic co-monomers results in hybrid polymers in which the clusters very efficiently crosslink the organic polymer chains (Figure 2). A step-polymerization protocol must be applied in some cases to minimize the proportion of residual double bonds and thus to optimize the polymer properties.

Figure 2.
Graphical sketch of the structure of the hybrid polymers.

The integrity of the clusters is preserved upon polymerization. Small-angle X-ray scattering investigations (by H.Peterlik, Vienna) showed that, depending on the specific cluster-polymer combination, some aggregation of the clusters may occur. However, the cluster aggregates are usually very small and do not affect the macroscopic physical properties.

Cluster-crosslinked PMMA or PS is no longer soluble in organic solvents, but swells instead, as expected for highly crosslinked polymers. The degree of swelling decreases with an increasing cluster proportion.

One of the most important features of the cluster-based hybrid polymers is their higher thermal stability (higher onset of thermal decomposition; char formation) compared with the corresponding cluster-free polymers. PMMA crosslinked by small cluster proportions does not burn when ignited, because unzipping of the polymer is apparently suppressed. The increased thermal stability is not only caused by the crosslinking but also by a nanofiller effect of the clusters.

We were able to prepare bubble- and crack-free bulk samples with dimensions required for mechanical testing (by S.Seidler et al., Vienna) of Zr₆O₄(OH)₄(OMc)₁₂-crosslinked PS on a 30 g scale by a step polymerization protocol. The dynamic mechanical response of the hybrid polymers was characteristic for thermoplastic materials. Storage moduli (E′) at room temperature were slightly higher than that of neat PS; the increase in E′ was much more pronounced above T_g. Crosslinking with different proportions of the cluster did not change the linear thermal expansion coefficient α in the glassy state. Above T_g, α decreased with increasing cluster proportion. Changes in the tensile moduli with increasing crosslinking were not pronounced which is commonly observed for thermosetting resins in the glassy state. However, the tensile strength was significantly improved and exhibited a distinct positive correlation with increasing network density. Microhardness measurements revealed no significant changes of indentation moduli and of indentation hardness, although the scratch resistance was improved with increasing cluster proportion. In scratch tests with constant load, a reduction of pile-up and a stronger recovery was observed for the hybrid materials compared to undoped PS.

In an extension of the work, clusters with special intrinsic properties are especially interesting. For example, super-paramagnetic polymers are obtained when the super-paramagnetic cluster Mn₁₂O₁₂(acrylate)₁₆ is polymerized with ethyl acrylate as co-monomer (magnetic measurements by F.Palacio et al, Zaragoza). This work is currently extended to other magnetic clusters as well as clusters with special optical properties.

Review articles

U.Schubert, Chem.Mater. 13, 3487-3494 (2001); "Polymers Reinforced by Covalently Bonded Inorganic Clusters".
G.Kickelbick, U.Schubert, Monatsh.Chem. 132, 13-30 (2001); "Inorganic Clusters in Organic Polymers and the Use of Polyfunctional Inorganic Compounds as Polymerization Initiators".
U.Schubert, J.Sol-Gel Sci.Technol. 31, 19-24 (2004); "Organofunctional Metal Oxide Clusters as Building Blocks for Inorganic-Organic Hybrid Materials".
U.Schubert, J.Mater.Chem. 15, 3701-3715 (2005); "Chemical Modification of Titanium Alkoxides for Sol-Gel Processing".
U. Schubert in A.Abd-El Aziz, C.Carraher, C.Pittman, M.Zeldin (Eds.), Macromolecules Containing Metal- and Metal-like Elements, Vol. 7, J.Wiley & Sons, New York, 2006, ISBN 0-471-68440-6, p.55-71; "Metal Oxide Clusters as Building Blocks for Inorganic-Organic Hybrid Polymers".
U.Schubert, Acc.Chem.Res. 40, 730-737 (2007); "Organically Modified Transition Metal Alkoxides - Chemical Problems and Structural Issues on the Way to Materials Syntheses".
U.Schubert, Macromol.Symp. (2008), 267, 1-8; "Inorganic-organic hybrid polymers based on surface-modified metal oxide clusters".
U.Schubert, Chem.Soc.Rev. (2011), 40, 575-582; "Cluster-Based Inorganic-Organic Hybrid Materials".

This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

Project P12766 (1998-2001): Materials Syntheses Based on Surface-Modified Oxo Clusters and Colloids
Project P16254 (2003-2007): Inorganic-Organic Hybrid Polymers with Covalently Bonded Clusters as the Inorganic Component
Project P19199 (since 2007): Polymers from Functional Clusters as Nanosized Building Blocks
Project P22915 (2011-2014): Anisotropic Functionalization of Clusters