PMMA Clay Nanocomposites

Abstract

Polymer clay nanocomposites (PCN) of poly(methyl methacrylate) (PMMA) and clay, were synthesised in-situ using a free radical suspension polymerisation technique. The weight fraction of clay in the PCN was systematically varied in order to understand the effect of clay on the physical properties of the resulting PCNs. However, unexpectedly it was found that the weight fraction of clay had a dramatic impact upon the molecular weight of the polymer matrix and a relationship between clay concentration and polymer molecular weight was established. Furthermore, it was also found that the change in molecular weight was dependent upon the clay type.

Three different clay types were investigated; an organically modified montmorillonite (Cloisite 15a), a synthetic clay (Laponite RD) and a PEO modified Laponite RD. To produce the modified Laponite RD, mono amino PEO was synthesised via anionic polymerisation using dimethyl ethanol amine as an initiator. The modification of the Laponite RD clay took place in the reaction flask prior to the suspension polymerisation of the PCN.

The PCN were characterised using size exclusion chromatography (SEC), X-ray diffraction (XRD), transmission electron microscopy (TEM) and oscillatory shear rheology. Morphological studies of the PCN showed that the extent of clay dispersion depended on the clay type. Within the PMMA/Laponite RD nanocomposites an unusual network structure was formed, which appeared to be continuous throughout the material. Thermal properties of the PCN were investigated using DSC, TGA and Microcalorimetry.

From oscillatory shear rheology, the full master curves for the PCN were obtained by applying the time-temperature superposition (TTS) principle. To quantify the effect of the clay upon the rheology, the experimental data was compared to the time dependent diffusion model of Des Cloizeaux for polydisperse polymer melts, which enables polydispersity to be incorporated through the use of the molecular weight distribution obtained via SEC.