Glass transition of polystyrene nanospheres under different confined environments in aqueous dispersions

Abstract

Two aqueous dispersions of anionic and nonionic polystyrene (PS) latex nanospheres were prepared via activators generated by electron transfer (AGET) atom transfer radical polymerization (ATRP) and reverse ATRP in microemulsions using sodium dodecyl benzene sulfonate (SDBS) and polyoxyethylene (20) oleyl ether (Brij 98) as surfactant, respectively. Moreover, the third aqueous dispersion of surfactant-free PS nanospheres was obtained by the dialysis of a PS microemulsion against deionized water. The molecular weight distributions (M_w/M_n) of the resultant PS with controlled-molecular weight were less than 1.7. We investigated the glass transition of these PS nanospheres in different aqueous dispersions by nano-DSC. We observed an apparent size-dependent glass transition on the surfactant-free PS nanospheres and nonionic PS latex nanospheres, and that the glass transition temperature (T_g) decreases with the reduction of size for PS nanospheres. However, the anionic PS latex nanospheres show an unambiguous glass transition near to the bulk PS. These results suggest that the effects of size and interface on glass transition may become significant as the diameter of the polymer nanospheres is decreased to less than 100 nm.