Organomontmorillonite/poly(methyl methacrylate) nanocomposites prepared by in situ photopolymerization. Effect of the organoclay on the photooxidative degradation

Abstract

Montmorillonite/PMMA nanocomposites were obtained by in situ photopolymerization. Methyl methacrylate was photopolymerized in the presence of modified clay minerals using thioxanthone (TX) and ethyl 4-(dimethylamino) benzoate (EDB) as photoinitiating system. The organomontmorillonites (SWy-1-C8-Mt and SWy-1-C16-Mt) were prepared by ion exchange of SW-1 Mt with octyltrimethylammonium bromide (C8) and hexyltrimethylammonium bromide (C16), respectively. X-ray diffraction indicated that clay/PMMA nanocomposites can have intercalated or exfoliated structures, or even a mixture of exfoliated and partially intercalated structure layers. The structure of each particular clay polymer nanocomposite (CPN) depends on the clay mineral loading and the solvent used for the preparation. The molecular weights of the SWy-1-C8/PMMA and SWy-1-C16/PMMA (1.0, 3.0 and 5.0%) obtained by photopolymerization in ethanol, were in the range of 1,000,000 to 3,000,000D, and in acetonitrile the Mw values varied from 220,000 to 270,000D. Photooxidative degradation of clay/PMMA nanocomposites has been investigated using size exclusion chromatography (SEC). Evidence was found that PMMA and CPN degrade by random chain scissions. The polydispersity increases after irradiation and the degradation rate coefficient for pure PMMA is up to 6 times larger than that for CPN. The effects of the clay mineral content, clay mineral type (clay mineral modified by surfactants with different lengths of alkyl chains) and solvent used for dispersion of organoclay on the photodegradation rate coefficients were also studied. The influence of these parameters on the photodegradation process was statistically evaluated using a two-level factorial design. The importance of the parameters was proved to follow the order: clay mineral content>clay mineral type>solvent. CPN with higher clay mineral loadings showed slower rates of oxidation. The clay mineral stabilizes the polymer against UV irradiation. SWy-1 clay mineral scatters and absorbs the incident light, decreasing the degradation rate of polymer present in the CPN.