Effect of concentration on the photoinduced aggregation of polymer nanoparticles

We describe the effect of concentration on the photoinduced flocculation and aggregation of a nonaqueous dispersion of core-shell nanoparticles (diameter = 50 nm), which consist of a tightly cross-linked core composed of poly(butyl methacrylate-co-ethylene glycol dimethacrylate) and a lightly cross-linked shell of poly(butyl methacrylate-co-ethylene glycol dimethacrylate-co-methacrylic acid). The particles could be dispersed in cyclohexane after modification of the acid groups by ester formation with 2-bromo-1-phenyl-octadecan-1-one. Photocleavage of these substituents (lambda = 310 nm) regenerated the -COOH groups and led to aggregation of the destabilized particles. Since the rate of aggregation is relatively slow in this system, we were able to study the process of particle aggregation kinetics by a combination of static and dynamic laser light scattering. Our results indicate, for particle dispersions from 0.23 mg/mL to 0.93 mg/mL, that there are three stages in the aggregation process. Initially, several particles come into contact to form small elongated clusters. Subsequently, these clusters undergo further aggregation to form larger aggregates characterized by a fractal dimension of around 2.3. This result indicates that aggregation in the second stage follows a reaction-limited cluster-cluster aggregation mechanism. At very long times, the aggregate size appears to level off, consistent with a reversible aggregation mechanism. We also found for the three different concentrations that the measured average radius of gyration <R-g,R-app> during aggregation scaled with time with an exponent of about 1.4 +/- 0.1.