Nanoscale ZnO Coatings on Poly(methyl methacrylate) Submicron Spheres via Low-Temperature Atomic Layer Deposition for Enhanced UV-Blocking Properties

Poly(methyl methacrylate) (PMMA), a synthetic thermoplastic polymer, has been extensively used in our lives due to its advantageous properties such as high transparency, lightweight, and biocompatibility. Although PMMA exhibits great potential in sunscreens, its poor UV-blocking properties hinder its practical applications. In this study, we modified the PMMA submicron spheres (120–150 nm) with nanoscale ZnO using atomic layer deposition (ALD) in a fluidized bed reactor at a low temperature of 40 °C, with diethylzinc (DEZ) and water as precursors. We systematically investigated the ZnO-coated PMMA microspheres using FTIR, XRD, SEM, TEM, XPS, and TG to elucidate the mechanism of nucleation and growth of ZnO. The results revealed that the ZnO deposited was polycrystalline, displaying a granular morphology on the PMMA surfaces. Specifically, a nanoparticle-coated structure was observed after low-cycle ALD, while a conformal nanofilm was formed after high-cycle ALD on the PMMA spheres, suggesting an island growth mode of ZnO. The analysis of the surface chemical species and functional groups via XPS and in situ FTIR indicated that the fast nucleation of ZnO was attributed to trace amounts of hydroxyl groups and hydrogen-bonded water molecules bound on the surfaces of the emulsion-polymerized submicron PMMA spheres. After ZnO coating, the PMMA microspheres exhibited improved UV-blocking performance. The reflectance within the 280–400 nm UV range dropped sharply from ca. 95% (pristine PMMA) to 19% for the PMMA/ZnO composite which had an optical bandgap of 3.36 eV. Meanwhile, the average visible-light reflectance remained above 93%. Moreover, owing to the physical barrier effect of the ZnO coating, the derivative thermogravimetric (DTG) peak temperature increased by 30 °C and the acetone resistance improved. This study not only provides a feasible strategy for modifying polymeric particle surfaces and fabricating organic–inorganic hybrid materials but could also advance the uses of PMMA spheres in high-end cosmetics.