Abstract
Objectives
It has been the focus to develop low shrinkage dental composite resins in recent ten years. A major difficulty in developing low shrinkage dental materials is that their deficiency in mechanical properties cannot satisfy the clinical purpose. The aim of this study is to develop novel dental nanocomposites incorporated with polyhedral oligomeric silsesquioxane (POSS). It is especially interesting to evaluate the volumetric shrinkage and mechanical properties, improve the shrinkage, working performances and service life of dental composite resins.
Methods
The effect of added POSS on the composites’ mechanical properties has been evaluated. The weight percentages of added POSS are 0, 2, 5, 10 and 15 wt% respectively. Fourier-transform infra-red spectroscopy and X-ray diffraction were used to characterize their microstructures. Physico-mechanical properties that were investigated included volumetric shrinkage, flexural strength, flexural modulus, compressive strength, compressive modulus, Viker’s hardness and fracture energy. Furthermore, the possible reinforced mechanism has been discussed.
Results
The shrinkage of novel nanocomposites decreased from 3.53% to 2.18%. The nanocomposites incorporated with POSS showed greatly improved mechanical properties, for example, with only 2 wt% POSS added, the nanocompsite’s flexural strength increased 15%, compressive strength increased 12%, hardness increased 15% and uncommonly, even the toughness of resins was obviously increased. With 5 wt% POSS polymerized, compressive strength increased from 192 MPa to 251 MPa and compressive modulus increased from 3.93 GPa to 6.62 GPa, but flexure strength began to decline from 87 MPa to 75 MPa. This finding indicated that the reinforcing mechanism of flexure state maybe different from that of compressive state.
Conclusions
The mechanical properties and volumetric shrinkage of dental composite resins polymerized with POSS can be improved significantly. In current study, the nanocomposite with 2 wt% POSS incorporated is observed to achieve the best improved effects.
1
Introduction
The development of dental restorative materials, in particular direct resin-based filling composites, has revolutionized the field of dentistry over the past 30 years. This development has been achieved mainly through organic monomer discovery, modifications in formulation, the use of new tailor-made fillers (e.g. nanofillers or clusters for the composite) or advances in light curing equipment and efficient photo initiators . Despite these developmental advances, some problems still limit the use of composites in dental restoration. Most improvements are focused on the reduction of polymerization shrinkage , as well as the improvement of wear resistance , mechanical properties , biocompatibility , and processing properties .
Polyhedral oligomeric silsesquixanes (POSS) is one typical organic–inorganic hybrid nanocomposite, which has been developed since the end of last century. POSS is really a nanostructural chemical whose molecule is 1.5 nm isotropic in structure. POSS monomer is represented by the empirical formula (RSiO1.5) n with an inorganic silica-like core (SiO 1.5 ) surrounded by organic corner groups R.
It has been revealed that the performances of POSS-modified polymers are usually attractive. They not only have the organic characters such as good processability, toughness and low cost, but also hold excellent inorganic performances in mechanics, thermodynamics, anti-oxidation, etc. . At present, it is a very important way to get functional and good performance materials modified by POSS.
Sellinger and Laine first mentioned the possibility of POSS being used in dental restorative materials. In a latter study, the research group of Culbertson and co-workers evaluated the systems of POSS incorporated with neat resins (without filler). The results showed that miscibility between the POSS component and the matrix, especially the diluents, played a very important role in improving the properties of the formulated thermosets.
Based on the research of Culbertson and co-workers’ group , Fong et al.’s team explored novel polymeric dental restorative composites, in which POSS-MA was used to partially (or completely) replace Bis-GMA. The results showed that only a small percentage of POSS-MA substitution of Bis-GMA in the resin systems could improve the mechanical properties of the composites. However, it was a pity that there was no distinct improvement in such important properties as compressive strength, hardness and toughness.
Dodiuk-Kenig et al. believed that the type of the grafted functional group of the caged silica was the dominant factor in nanotailoring of improved dental composites and adhesives. It showed that the mechanical properties of dental composites and adhesives were improved by acrylated POSS but deteriorated by octaphenyl grafted POSS.
Many other research works have been done on the structural design of the POSS molecule, synthesizing the process of modified resins, as well as properties characterization. But we have to admit that considerably complicated techniques of synthesis and rigorous conditions of reaction are needed for most new POSS monomers. Therefore, this seriously limits the development and application of POSS in dental restorations.
In this paper, multifunctional methacryl POSS cage mixture ( n = 8, 10, 12) (from Hybrid Plastics) was explored to be added into dental composite resins to develop novel dental nanocomposites with improved properties. Multifunctional methacryl POSS molecule’s anatomy (showed by Fig. 1 ) consists of an inorganic “cage” built with silicon and oxygen, and each “Si” is attached by a methacrylate functional group. Additionally, the methacryl POSS is soluble in the composite resins.
