Effect of glass transition temperature on the final morphology of nanoparticle-laden aerosol droplets

The final morphology of spray-dried microparticles is a key factor in determining their chemical and physical properties, which are in turn important for pharmaceuticals, cosmetics and food manufacturing. It is well known that both the drying kinetics of evaporating droplets and the composition of the feed solution used to produce the aerosol droplets affect the morphology of the dried particles. Herein, the effect of varying the glass transition temperature of the core-forming block on the dried particle morphology is investigated for aqueous droplets containing bespoke diblock copolymer nanoparticles. Polymerization-induced self-assembly (PISA) was used to prepare spherical nanoparticles directly in the form of aqueous colloidal dispersions, with approximately constant hydrodynamic diameters of 73 to 86 nm and glass transition temperatures ranging from -30 to 93 °C. To study individual levitated aerosol droplets (mean radius = 28-36 μm), an electrodynamic balance was used to monitor their evaporation kinetics at relative humidities (RH) of 0 to 55%. A falling droplet column was used to dry the droplets under the same conditions and scanning electron microscopy was employed to examine how the evaporation kinetics and glass transition temperature of the nanoparticle cores influenced their final morphology. Atomic force microscopy was employed to visualize the surface topography of the nanoparticles and to assess their interfacial mechanical properties. Decreasing the RH led to a increased evaporation rate and caused a higher degree of buckling in the final dried microparticles. For dilute aqueous dispersions, the nanoparticles had no impact on the aerosol evaporation kinetics but sub-ambient glass transition temperatures led to softer dried microparticles that exhibited a greater degree of deformation.