Manipulating the glass transition behavior of sulfonated polystyrene by functionalized nanoparticle inclusion

Abstract

Nanoscale interfaces can modify the phase transition behaviors of polymeric materials. Here, we report the double glass transition temperature (T_g) behavior of sulfonated polystyrene (sPS) by the inclusion of 14 nm amine-functionalized silica (NH₂–SiO₂) nanoparticles, which is different from the single T_g behaviors of neat sPS and silica (SiO₂)-filled sPS. The inclusion of 20 wt% NH₂–SiO₂ nanoparticles results in an increase of T_g by 9.3 °C as well as revealing a second T_g reduced by 44.7 °C compared to the T_g of neat sPS. By contrast, when SiO₂ nanoparticles with an identical concentration and size to NH₂–SiO₂ are dispersed, sPS composites possess a single T_g of 7.3 °C higher than that of the neat sPS. While a nanoscale dispersion is observed for SiO₂ nanoparticles, as confirmed by microscopic and X-ray scattering analyses, NH₂–SiO₂ nanoparticles show the coexistence of micron-scale clustering along with a nanoscale dispersion of the individual nanoparticles. The micro-phase separation contributes to the free volume induced T_g reduction by the plasticization effect, whereas the T_g increase originates from the polymer segment mobility constrained by nanoconfinement and the rigid amorphous fractions deriving from strong polymer–particle interactions.