Latex particle homogeneity and ageing

A polystyrene (PS) latex was examined by preparative centrifugation employing NaCl-sucrose density gradients. Three fractions were found, with dissimilar isopycnic densities and sensitivities to the salt concentration. However, after 12 months of ageing at room temperature, this latex displays a more homogeneous behavior: a single, broad isopycnic band is observed (instead of the three bands in the young latex) on centrifugation employing the same NaCl-sucrose gradients. Latex particle heterogeneity is thus spontaneously decreased by ageing. The origin of this heterogeneity within a homopolymer latex as well as its decrease upon ageing are discussed taking into consideration that latex particle surfaces are neither smooth nor rigid and that hydrophilic group migration is possible, both within a single particle and among different particles.1081838