Effect of cationic surfactant tail length on the emulsion gel polymerization of styrene

Styrene was polymerized for 24 h at room temperature (30°C) within different colloidal microstructures made out of fumed silica, 2,2′-azobis(isobutyronitrile) (AIBN) and aqueous solution of different cationic surfactants with varied tail length; hexyltrimethylammonium bromide (HTAB), dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB) and octadecyltrimethylammonium bromide (OTAB). Styrene in styrene/silica/CTAB showed a larger polymeric conversion (90%) compared to styrene/silica/DTAB (85%) and styrene/silica/OTAB (82%) mixtures. These polymerized styrene samples were examined using scanning electron microscopy, thermogravimetric analyzer, differential scanning calorimeter and gel-permeation chromatography. Polystyrene (PS) composites contained spherical particles with varied mean particle diameter (d). The particles in the composite became larger with increased surfactant tail length. PS/silica/OTAB particles were comparatively larger (d = 271 ± 13 nm) than PS/silica/CTAB (d = 250 ± 12 nm) and PS/silica/DTAB (d = 240 ± 12 nm) particles. Compared to silica-containing styrene mixtures with CTAB or DTAB, styrene/silica/OTAB formed PS with larger molar mass and elevated glass transition temperature, at room temperature. The molar masses and glass transition temperatures of PS obtained from PS/silica/OTAB, PS/silica/CTAB and PS/silica/DTAB were 1279, 943, 995 kg mol–1, and 105, 104, 102°C, respectively. The short hydrophobic tail of HTAB resulted in inconsistent styrene to PS conversions in the mixtures.