Size-Controlled and Surface-Modified LDH Nanofillers Enable Multifunctional Transparent PMMA Nanocomposites with Enhanced Thermal Stability and UV Shielding

Poly(methyl methacrylate) (PMMA)/layered double hydroxide (LDH) nanocomposites were prepared via emulsion polymerization to investigate the effects of LDH platelet size and surface modification on nanocomposite structure and performance. Size-controlled MgAl-LDH and ZnAl-LDH were synthesized by aqueous co-precipitation, organically modified with various interlayer agents, and incorporated into the PMMA matrix. Structural, morphological, thermal, mechanical, and optical properties were systematically characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), scanning and transmission electron microscopy (SEM/TEM), polarized optical microscopy (POM), and UV-visible spectroscopy. The results demonstrated that nanoscale LDH dispersion could be achieved at low filler loadings (1–3 phr), whereas increasing the LDH content to 5 phr promoted significant platelet aggregation. Notably, a high degree of LDH intercalation did not necessarily translate into improved dispersion. Despite exhibiting the greatest interlayer expansion, CDS-modified LDHs showed inferior dispersion and thermal performance, indicating that interfacial compatibility rather than interlayer spacing is the primary factor governing nanocomposite morphology and properties. The optimized PMMA/LDH nanocomposites retained over 85% visible-light transmittance while exhibiting a 27% enhancement in UV-shielding efficiency, improved thermal stability as evidenced by an increased decomposition temperature (T5d), and a higher storage modulus relative to neat PMMA. These findings demonstrate that the synergistic control of LDH platelet size and surface chemistry provides an effective strategy for developing highly dispersed PMMA nanocomposites with simultaneously enhanced optical, thermal, and mechanical properties.