Harnessing Particle Size Segregation To Tune Molecular Additive Distribution in Coatings

The spatial distribution of small-molecule additives within polymer coatings plays a critical role in determining their performance, from antimicrobial activity to corrosion resistance. While size segregation during film formation has been harnessed to control the distribution of nanoparticles or polymers, its potential for controlling the molecular additive distribution remains largely unexplored. Here, we investigate how bimodal colloidal blends can direct the positioning of a model additive, nickel-(II) phthalocyanine (NiPc), during drying. Using complementary microscopy and spectroscopy techniques, we show that NiPc predominantly associates with the smaller particles in the blend due to their larger total surface area. At low and medium relative humidities, this leads to an enrichment of NiPc at the film's surface via small-on-top stratification. Slow drying at high humidity results in additive accumulation near the substrate due to aggregation and sedimentation of small particle clusters. Release studies reveal that bimodal films generally slow the initial burst release of NiPc compared to monomodal controls, enabling more sustained delivery over time. Fitting release profiles with the Korsmeyer-Peppas model confirmed Fickian diffusion as the dominant mechanism, with differences in pore structure potentially influencing diffusion rates. Overall, our findings demonstrate that particle size distribution and evaporation rate can be tuned to modulate the location and release behavior of molecular additives in coatings. This approach provides a versatile route for designing functional films with tailored performance with potential applications in medical, marine, and protective technologies.