Spectroscopic and thermal analysis of clay mineral-organic composites.
Composites of clay minerals have been prepared with organic monomers for subsequent study upon polymerisation. Particular combinations of clays and organic species have enabled three unique systems to be studied. The first involves the intercalation of the clay minerals Kaolin and Halloysite with Phenylphosphonic Acid (PPA). The intercalation process proceeds via the use of an acetone / water entraining agent and subsequent analysis by Diffuse Reflectance Infrared Fourier transform Spectroscopy (DRIFTS), X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA) and Evolved Gas Analysis (EGA) using TG-Mass Spectrometry. This has revealed the remarkable thermal stability of these intercalates; they are stable to above 450 °C, and exhibit interlayer spacings of 15.4 A. Furthermore, the fate of the water as the entraining agent is determined and is found to be hydrogen bonded to the phosphonic acid moiety of PPA and also weakly bound to the inner surface hydroxyls in these clays. This water represents an opportunity for displacement by an organic monomer, and the lone pair of electrons on the oxygens also provide scope for coordination. Hence, the PPA intercalates were treated with the monomer N-vinylformamide (VFA) and evaluated for subsequent polymerisation.In the second system, montmorillonite was intercalated with VFA and then treated with a polar activator (Propylene Carbonate) prior to polymerisation. The polar activator interacts with the bifunctional VFA molecules causing the clay to swell further prior to polymerisation. This produces significant differences in the XRD traces with and without polymerisation of VFA in the presence of the clay. If the polar activator is absent the d[(001)] spacing is 21.0 A. If the polar activator is present then no d[(001)] is observed , in a situation reminiscent of an exfoliated nanocomposite. In the last system studied an in-situ intercalative approach was employed. Montmorillonite was treated with dialdehydes and diamines capable of reacting together to form amino resins oligomers and polymers. The molar ratio of the two reactants determines the intermediate and hence the subsequent product that is formed as characterised by DRIFTS. In a binary mixture where both reactants are simultaneously added to the clay, if the aldehyde is in excess, then dimethylol condensation products are formed. If the amine is in excess then monomethylol condensation products are formed. Sequentially adding these two reactants to clay (i.e. treating the clay first with one reactant and washing to remove any excess, then adding a second reactant) also has a bearing on the results. If the amine is added first, then the aldehyde is able to displace it from the clay interlayer. If however, the aldehyde is added first, then monomethylol reaction product is seen and this together with DRIFTS TGA and XRD analysis indicates that some of the aldehyde has been removed during the washing process.
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Sheffield Hallam University (United Kingdom).