In situ polymer nanoparticle densitometry via real-time 3D single-particle tracking.

Single-particle techniques have the potential to measure the heterogeneous dynamics at the nanoscale within reaction mixtures. However, new tools are needed to gain insight into the molecular structure of evolving chemical systems at the single-particle level. Here, a single-particle method for measuring the density and mass of freely diffusing polymer nanoparticles is introduced and applied to the ring-opening metathesis polymerization of norbornene. Using 3D single-molecule active real-time tracking, growing polymer nanoparticles are tracked in situ, enabling simultaneous real-time measurement of both particle size and particle density. Particle density is extracted from the descent velocity of growing nanoparticles, giving insight into particle composition at the single-particle level. Using this technique, it is found that polynorbornene nanoparticles have a density of 850 ± 30 kg/m3, validated through control measurements on polystyrene beads. Density and size analysis indicate each nanoparticle contains ∼3.5 × 109 densely packed monomers, rather than loosely packed polymer chains. Simulations show that the accuracy of this method depends on particle size, the distance traveled before contact with the coverslip, solvent properties, and the number of trajectories. Overall, this paper presents a technique that enables in situ characterization of the density and mass of individual growing polymer nanoparticles.