Enhanced Properties of Polymethyl Methacrylate Coated with Atomic Layer Deposited Ceramic Nanofilm

Polymethyl Methacrylate is a polymer used as a denture base material since 1936. PMMA has desirable properties such as good biocompatibility, good aesthetics, ease of processing and low cost but its poor surface properties and wear resistance can lead to fracture, deformation and loss of surface accuracy. Functionalization of this material is needed since studies have proved that 68% of dentures degrade in the first few years of usage. The oral environment is very aggressive; due to fast pH and temperature changes and the presence of different kinds of bacteria, the polymer can undergo biodegradation, discoloration and increase of surface roughness. Atomic Layer Deposition technique can provide conformal and uniform thin ceramic films made of metal oxides like TiO2, ZrO2 and mixed TiO2-ZrO2 on polymeric substrates that we conjecture would reduce PMMA drawbacks. The process can be performed at low temperature like 90-120°C, which is desirable especially when working with polymers which melt at 160°C, as PMMA. This technique was selected among the other coating technologies because of its high control over composition and thickness at the Ångström scale. A variety of thicknesses and chemical compositions of the coatings, surface characterization, physical and mechanical properties of the coated PMMA were investigated with non-coated PMMA as a control, in order to have a standard for comparison of the results and detect if the properties of the coated polymer are improved or not with respect to the non-coated one. The characterization was challenging as the polymer used is weak, melts at low temperature and it is also non-conductive, which leads to difficulties in using traditional techniques such as electron microscopy. The goal was to obtain a ceramic nano-thin film on the polymer that would improve the surface properties of the substrate while maintaining the desirable bulk properties of the material. The bonding strength of the nano-thin ALD coating on the polymer was effective since after four hours of aggressive treatment the ceramics were still attached to the polymer: chemical analyses of the surface showed same results for samples freshly deposited and samples tested with both a chemical and physical treatment in a popular denture cleanser. Our ALD coating proved to be physically and chemically attached to PMMA, in agreement with other related literature results. All coated surfaces showed increased wettability compared to the uncoated polymers right after the depositions. The contact angle decreased from 80° to 25-35°. Moreover, surface wettability resulted in being stable even after extended treatments such as four hours of sonication in a denture cleanser solution. In addition to the wettability increase, a 30 nm-thick ceramic coating successfully managed to increase the PMMA surface hardness value by 60%, which can be helpful in facilitating finishing and polishing and in providing resistance to damage during cleaning of prosthodontics. On the other hand, the desirable flexural properties of the bulk material were maintained in the coated samples. Moreover, surface morphology was confirmed not to be affected significantly by the coating: the root mean square roughness value ranged from 45 0.7nm to 28 4nm in the best case. The change is not even in the micrometer range, which was anticipated, since ALD is a conformal technique and it covers all the peaks and troughs present on the surface to coat.