Emulsion polymerization provides a convenient route to well disperse polymer/clay nanocomposites. This technique exploits the natural swelling of clay in water and may lead to significantly higher loadings than generally obtained by other techniques (melt blending or in situ polymerization). The present work has focused on nanocomposite films with laponite clay loadings of up to 50 wt %, based on emulsion polymerized polystyrene (PS)/laponite clay latexes in which the laponite platelets are attached to the surfaces of the PS latex particles. Two regimes of reinforcement were identified: below the glass transition (T_g) of the PS matrix, the observed stiffness increases are accounted for in terms of the degree of exfoliation and classical micromechanical models for mechanical reinforcement, whereas above T_g the stiffness increases were correlated with the total clay content and were underestimated by more than two orders of magnitude by these same micromechanical models. Secondary transitions at T > T_g in dynamic mechanical spectra from specimens with clay contents above 20 wt. % implied part of the matrix to show limited chain mobility in this regime. A mechanical model has therefore been proposed to explain the mechanical behaviour in the rubbery state, in which laponite and PS with locally reduced mobility form a cellular network, consistent with TEM observations of the morphology of the nanocomposites and the latexes. This cellular arrangement has also been shown to have important consequences for the microdeformation mechanisms and bulk mechanical properties at high strains, resulting in a transition from crazing to relatively coarse matrix cavitation at the scale of the original latex particles as the laponite content increases. TEM has been used to demonstrate an accompanying decrease in local matrix drawability, accounting for the strong decrease in fracture resistance observed at high laponite contents.

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