SWELLING EQUILIBRIUM OF SMALL POLYMER COLLOIDS - INFLUENCE OF SURFACE-STRUCTURE AND A SIZE-DEPENDENT DEPLETION CORRECTION

Small polymer latex particles in the size range 15 nm < R < 100 nm wit h different types of covalently bound surface stabilizing groups are m ade via emulsion polymerization using diverse functionalization techni ques like incorporation of polar comonomers, surface-active initiators , or polymerizable surfactants. These colloidal dispersions in water a re swollen with toluene in the absense of free surfactants until equil ibrium swelling is reached. The equilibrium size of the swollen latice s is determined by combining densitometry and dynamic light scattering . For all surface groups, we observe a pronounced dependence of the sw elling ratio on particle size where absolute values of swelling are mu ch lower than described by classical theories. A modified description is presented which considers size-relevant effects (such as the Kelvin pressure and depletion) by an additional osmotic pressure term which increases with the inverse of the particle size. Other classical effec ts of colloids such as a size dependence of the thermodynamic interact ion parameter or the inter face energy can be excluded by quantitative considerations. The presented modified swelling equation describes th e experimental data well and is expected to have some broader relevanc e for swelling phenomena of all colloidal systems. Charge stabilizatio n as compared to steric stabilization results in higher swelling ratio s and significant lower values for the interface energy. This underlin es the importance of coulombic repulsion between fixed charges on the same particle for the swelling process as well as the existence of a ' 'surface anchoring'' effect, both of which are not considered in curre nt concepts of the swelling process, too.