Surface mechanical properties of transparent poly(methyl methacrylate)/zirconia nanocomposites prepared by in situ bulk polymerization

Poly(methyl methacrylate)/zirconia (PMMA/ZrO₂) nanocomposites with ZrO₂ content as high as 15 wt% were prepared by modifying non-aqueous synthesized ZrO₂ nanoparticles with methacryloxypropyltrimethoxysilane (MPS) in tetrahydrofuran, dispersing MPS-functionalized ZrO₂ nanoparticles in MMA and following in situ bulk polymerization with controlled pre-polymerization time. The MPS-functionalized ZrO₂ nanoparticles showed an efficient crosslinking role in the polymerization, leading to a complete gel of PMMA at 5 wt% of ZrO₂ content. Homogeneous dispersion of the ZrO₂ nanoparticles at primary particle size level was observed in all nanocomposites, which results in good clarity of the obtained nanocomposites. Hardness tests (pendulum hardness tests and indentation tests) and anti-scratch tests (abrasion tests and nano scratch tests) were employed to probe the surface mechanical properties of the nanocomposites. The properties of nanocomposites as a function of ZrO₂ content, revealing from various characterization techniques, are not consistent and discussed in detail. At low ZrO₂ content, the mechanical properties are enhanced by the formed crosslinking structure. However, remarkable improvements of hardness and scratch resistance of PMMA were achieved when 15 wt% of ZrO₂ content was embedded.