Synthesis and Characterization of Polystyrene- Encapsulated Laponite Composites via Miniemulsion Polymerization

Polymer-nanoclay composite materials have attracted considerable interest among materials scientists recently because of the pioneering work carried out by a research group in the Toyota Central Research Laboratories. The group applied a nylon-montmorillonite clay nanocomposite as an under-the-hood structural part in the engine compartment of an automobile with significantly improved tensile strength, modulus, and heating distortion temperature at no compromise of impact strength of the material. Later on, the researches were extended to other polymer-layered systems, and a range of properties were exploited, such as barrier properties, temperature responsibility, anticorrosivity, and low thermal coefficient of expansion of materials. These properties are unattainable without using the nanoclay. The nanoscale phyllosilicate platelets are derived from natural clay minerals (montmorillonite, hectorite, vermiculite, etc.), which are made of two-dimensional 1 nm layers of an aluminate sheet sandwiched between two silicate sheets. These layers are stacked together by weak ionic Summary:A novel polystyrene-encapsulated laponite composite system has been developed via a miniemulsion polymerization approach. The encapsulation mechanism and process parameters have been examined in detail using lightscattering, sedimentation analysis, wide-angle X-ray diffraction (WXRD), and transmission electronmicroscopy (TEM). The laponite was encapsulated through a miniemulsion polymerization process inwhich laponitewas predispersed in the monomer phase. The stability of both the miniemulsion and the latex depends on initiation loci, premixing procedures, intensity and time of ultrasonification and the surfactants and co-stabilizer used. Hydrophobicity of the laponite clay played a vital role in both the encapsulation of the clay and the stability of the latex. A quaternary ammonium salt, cetyltrimethylammonium bromide (CTAB), was mixed with the clay in the monomer phase prior to emulsification. As a result, the clay particles were hydrophobically modified and were intercalated. The hydrophobicity not only favored the clay dispersion in the oil droplets but also aided the entry of the monomer into the clay’s intergalleries during polymerization. Meanwhile, CTAB helped stabilize the system when it was used in conjunction with the nonionic surfactant polyoxyethylene (40) isooctylphenyl ether (TX-405). In this way, the laponite is effectively encapsulated within a polystyrene shell in a stable latex form. More importantly, the polymerization initiated in the intergalleries of the clay effectively expands the clay’s platelet array to form an exfoliated structure.