Abstract. In this work the effect of the compatibility between organoclays and styrene on the flammability of polystyrene/clay nanocomposites obtained through in situ incorporation was investigated. The reactions were carried out by bulk polymerization. The compatibility between organoclays and styrene was inferred from swelling of the organoclay in styrene. The nanocomposites were characterized by X-ray diffraction and Transmission Electron Microscopy. The heat release rate was obtained by Cone Calorimeter and the nanocomposites were tested by UL94 horizontal burn test. Results showed that intercalated and partially exfoliated polystyrene/clay nanocomposites were obtained depending on the swelling behavior of the organoclay in styrene. The nanocomposites submitted to UL94 burning test presented a burning rate faster than the virgin polystyrene (PS), however an increase of the decomposition temperature and an accentuated decrease on the peak of heat release of the nanocomposites were also observed compared to virgin PS. These results indicate that PS/clay nanocomposites, either intercalated or partially exfoliated, reduced the flammability approximately by the same extent, although reduced the ignition resistance of the PS.

During the in‐situ incorporation of an organophilic clay into the polymer matrix in a free radical polymerization, the radical should penetrate the basal space between the layers of the clay increasing the distance between these layers and leading to a nanocomposite with an intercalated or exfoliated morphology. The final morphology depends on the affinity of the monomer for the cation used to change the organophilicity of the clay. In this work, Foster swelling is applied as a method to obtain a degree of affinity of the clay for the monomer. The objective of this work is to study the effect of the Foster swelling degree on the final polystyrene/clay composite obtained by free‐radical polymerization of styrene containing organophilic clay. Some commercial organophilic clays with different Foster swelling degree were employed and the incorporation of the clays into polymer was carried out in bulk styrene polymerization reactions. The X‐ray diffraction (XRD) reveals the basal spacing of the clays before and after in‐situ incorporation indicating the morphology of the nanocomposite. The nanocomposites were also characterized by thermal gravimetric analysis (TGA). Results showed a correlation between Foster swelling degree and the morphology obtained. High values of Foster swelling degree lead to exfoliated polystyrene/clay nanocomposites, whereas decreasing this value intercalated, partially intercalated and only dispersed composites were obtained depending on the degree of affinity of the monomer for the clay.