# Harnessing Particle Size Segregation To Tune Molecular Additive Distribution in Coatings

## TL;DR

This paper shows how particle size and drying conditions can control the placement and release of additives in polymer coatings, improving their performance.

## Contribution

The study demonstrates a novel use of particle size segregation to control molecular additive distribution and release behavior in coatings.

## Key findings

- NiPc associates with smaller particles due to higher surface area, leading to surface enrichment at low humidity.
- High humidity drying causes NiPc accumulation near the substrate via small particle aggregation.
- Bimodal films reduce initial additive burst release, enabling sustained delivery governed by Fickian diffusion.

## Abstract

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.