Particle-size-dependent properties of sulfonated polystyrene nanoparticles

The influence of sulfonation reaction time, temperature and the parent polystyrene (PS) particle size on the degree of sulfonation (DS), ion exchange capacity (IEC), morphology and glass transition temperature (Tg) of sulfonated polystyrene (SPS) particles was investigated. A longer reaction time (ca 2 h) at 40 degrees C and a smaller particle size resulted in SPS particles with a high DS. It was found that a larger PS particle size did not readily yield SPS particles with a high DS even with a longer reaction time. Contrary to the popular belief in the literature that a higher DS ensures a high IEC, we observed that the proportionality of IEC to DS is primarily controlled by the SPS particle size. Larger IEC values were obtained for larger particles rather than smaller ones despite their similar DS, owing to the presence of strong interactions between - SO3H groups within the particles in the latter case which restricts the availability of free H+ for ion exchange. The SPS particles displayed a core-shell morphology in which the outer shell appeared because of sulfonation on the PS. The DS value and the SPS particle size significantly influenced the shell thickness and thereby the morphology of the SPS particles. Copyright (c) 2012 Society of Chemical Industry