A low-shrinkage dental composite with epoxy-polyhedral oligomeric silsesquioxane

Abstract

Polymerization shrinkage and mechanical properties affect the service life of dental restorative composites. Polyhedral oligomeric silsesquioxanes (POSS) can be an efficient strategy to improve the shrinkage properties and mechanical properties simultaneously. In this work, epoxycyclohexyl POSS (E-POSS) was used to modify the Bis-GMA/TEGDMA dental resins. E-POSS were introduced into the acrylic resin system via ring-opening reaction, and a cationic/free radical hybrid system was obtained. Double bond conversion, glass transition temperature, heterogeneity, crosslink density of dental resins and volumetric shrinkage, water sorption/solubility, flexural strength/modulus of dental composites were investigated. As the introduction of E-POSS caused higher double bond conversion and lower crosslink density, the polymerization shrinkage was significantly reduced to 2.91% (p < 0.05) and the flexural strength increased to 117.19 MPa (p < 0.05) compared to the control sample (5.06%, 99.16 MPa, p<0.05). Lower water sorption and solubility were also observed in composites contained E-POSS (p < 0.05).

Introduction

Currently, the visible light-cured resin has been widely used in the field of dental restoration. Compared to the early amalgam, the dental restorative resins have better properties, such as superior aesthetics, less toxic and better operability(Demarco et al., 2012; Kim et al., 2007; Lempel et al., 2019). Acrylate resins are the main component of dental restorative materials. Bisphenol A-dimethacrylate glycidyl ester(Bis-GMA), triethylene glycol dimethacrylate(TEGDMA) and urethane dimethacrylate (UDMA) are the traditional commercial resins(GAO et al., 2012), but there are still several problems, such as shrinkage and microcracks affecting the longevity of caries in clinical application(Bacchi et al., 2018; Podgórski et al., 2015). The polymerization shrinkage occurred in the light curing process can increase the possibility of secondary caries and repair failure (Barkoula et al., 2017; Ferracane, 2005; Nikolaidis et al., 2019). How to decrease the polymerization shrinkage has become a major subject for the researchers. Moreover, the improvement of the mechanical properties of dental composites has been also one of the research hotspots.

At present, the research about reducing shrinkage mainly concentrated on two fields. Some scientists concentrated to improve the performance of inorganic fillers, such as particle size, surface morphology and amount of addition of inorganic fillers(Atai et al., 2012; Cai et al., 2001; Samuel et al., 2009; Wang et al., 2018; Zha et al., 2018). The other scientists were committed to the resin matrix. Several researches introduced modified monomers which generally contained one or two functional groups into the resin system by grafting to the polymer chains to reduce the polymerization shrinkage (Chen et al., 2012; Ma et al., 2013). Recently some researchers reported that epoxy monomer and oligomers could reduce the volume shrinkage, shrinkage stress and water sorption (Danso et al., 2018; Song et al., 2012). J.E.Boulden et al. (2011) also reported the thiol–ene–methacrylate exhibited improvements in all performance. These monomers could improve the performance of the resin in polymerization shrinkage because of their expansion effect, nanoscale effect, etc. Besides, POSS was also an effective method to reduce shrinkage. (Boaro et al., 2013; Wu et al., 2010).

POSS has a unique molecular structure, of which chemical structures follow the basic composition of (RSiO_1.5)_n(Hill et al., 1995; Laine, 2005). The typical structure of POSS is a kind of cage-like silsesquioxanes when n=8,10,12. Its internal inorganic silicon-oxygen core cage structure has good mechanical properties and its peripheral organic groups (R groups)have high design ability and reactivity, which can introduce POSS to the polymer matrix to prepare organic-inorganic nanocomposites. R groups are the vertex group for polyhedral molecules, including hydrogen, alkyl, alkene, aryl, arylene, etc (Kuo and Chang, 2011; Zhang et al., 2018, 2017). As shown in Fig. 1, the epoxycyclohexyl POSS (E-POSS) is a liquid mixture of silsesquioxanes, which has organic epoxycyclohexyl groups attaching at the corners of the cage. E-POSS could be cured with aromatic, aliphatic amines and photoinitiators. E-POSS and its variations can provide higher use temperature(Xiong et al., 2019), excellent water and solvent resistance, enhanced thermomechanical performance and potential shrinkage reduction properties (Canellas et al., 2019; Habib et al., 2016; Kuo and Chang, 2011). Being a liquid, E-POSS could be used as a replacement for resin matrix rather than the solid filler(Habib et al., 2016).

The cationic/free radical hybrid system is considered to be one of the research directions of dental resins(Chen et al., 2008; Luo et al., 2019). The cationic ring-opening polymerization is not susceptible to oxygen inhibition at the surface. And cationic polymerization can also allow stress relaxation and further reduce polymerization shrinkage because it happens much slower than the free-radical polymerization (Ge et al., 2015). Therefore, the hybrid resins are expected to overcome the problem of resin shrinkage.

In this work, dental resins and composites modified by E-POSS were successfully prepared. As a kind of liquid mixture with moderate viscosity, E-POSS had good compatibility in the resins. The results declared that hybrid dental composites had lower volume shrinkage and higher flexural strength & modulus with proper POSS content. The volume shrinkage could be reduced by 22% and the flexural strength could be developed to 117.19MPa.