CHAPTER Introduction 1.1 Composite Resins – An Alternative to Dental Amalgam Before the introduction of polymeric dental composites, dental amalgam was the material of choice for stress bearing dental fillings.1 This material was inexpensive and had the advantages of high strength, excellent wear resistance, ease of application, good adaptability, ease of manipulation and finishing.1-5 Despite its excellent clinical record, dental amalgam has several disadvantages. These include the need for removal of sound tooth structure for retention, inability to bond to tooth surface and susceptibility to corrosion. The use of amalgam has also been subjected to more and more controversies due to the fear of mercury toxicity. During the placement and removal of amalgam restorations, small amounts of mercury vapors are released leading to health and environmental concerns.6,7 Although dental healthcare workers generally not show signs of mercury toxicity, the mercury body burden of dental personnel was found to be slightly higher than the non-exposed group. Dental healthcare workers with high occupational exposure to mercury vapors were also found to be less fertile than unexposed groups.6 While issues of mercury toxicity by amalgam restorations are still being debated, the Swedish government has proposed for the elimination of amalgam as a dental restorative material since 1997,8 due to environmental concerns especially the waste management of amalgam.9 While the risks associated with mercury in dental amalgam are debatable, it is interesting to note that the use of amalgam as a restorative material has declined rapidly in the last half-decade due to aesthetic reasons. Amalgam is aesthetically unattractive, metallic in color and does not resemble the physical characteristics of tooth structure. The release of metallic ions from the amalgam restoration can also discolor the neighboring tooth structure.10 Thus, with increase in aesthetic demands by patients and clinicians, tooth-colored composite resin restorations were viewed as an attractive alternative to amalgam restorations. Dental composites which consists of monomer resins, ceramic fillers, coupling agents and initiator/catalyst systems for polymerization were first developed in the early 1960s11,12 as aesthetic alternatives for tooth restorations. One of the major improvements in resin-based composite has resulted from increased filler loading along with variation in distribution, size, shape and composition. This modification to the filler component brings improvements in wear resistance, color stability, strength, radiopacity and degree of conversion of dental composites and thus the overall improvement in clinical performance of these materials. However, despite vast improvements in composite materials and their mechanical properties, present day composite resins still have shortcomings limiting their application. Inadequate resistance to wear (loss of anatomic form) under masticatory attrition, fracture of the restorations, discoloration, marginal adaptation, secondary caries and marginal leakage due to polymerization shrinkage are some of the factors limiting the longevity of composite resins.4,13-16 Commercial dental composites exhibit 2-14 % volumetric shrinkage during the polymerization process.17-20 When composites shrink, stresses are generated at the composite/tooth interface. These shrinkage stresses can cause marginal openings if the bonding system is unable to withstand the polymerization forces and thus lead to leakage and ultimately caries. Despite the dramatic improvements in the formulation of newer generation bonding agents with enhanced marginal adaptation and bond strengths, a perfect marginal seal is still not achievable. Clinical studies carried out for resin-based composite restorations for Class I and II cavities for a period of to years have also shown that secondary and/or recurrent caries were the main reasons for restoration failure4,21,22 and polymerization shrinkage has been cited as one of the most significant factors influencing the seal between tooth structure and polymer-based restorative materials. Thus, the major and most significant drawback of composite-based resins is the shrinkage during the polymerization process. This remains one of the greatest challenges in composite resin technology and the ultimate solution to polymerization shrinkage is to develop “non-shrinking” resins. 1.2 Research Objectives With the development of dental monomers with reduced polymerization shrinkage and stress becoming a major focus of dental biomaterials research, we aim to develop novel low/non-shrinking nanocomposites based on polyhedral silsesquioxanes (SSQ) for dental applications. The objectives of this research were to: (a) Design and develop low/non-shrinking SSQ-based nanocomposites with methacrylate and epoxy functional groups. (b) Synthesize and characterize the SSQ neat resins for their chemical, thermal, physical and mechanical properties. (c) Study the effects of mixing SSQ-based nanocomposites with existing dental monomers in different compositions for improvement in physicomechanical properties. (d) Develop and characterize promising experimental nanocomposites by mixing SSQ-based nanocomposites and/or dental monomers with ceramic fillers. CHAPTER Literature Review 2.1 Chemically Cured Composite Resins Chemically cured (self or auto cured) dental composite resins were first developed in the late 1950s. They were found to be insoluble, aesthetic, insensitive to dehydration, inexpensive and easy to manipulate. Curing of the composites was initiated by mixing two pastes that brought together the initiator, dibenzoyl peroxide, and the activator, tertiary amines such as N,N-di-(2hydroxyethyl)-p-toluidine (DHEPT) or N,N-dimethy-p-toluidine (DMPT), in order to initiate the polymerization reaction (Figure 2.1).23 Curing of the composite ensures uniform polymerization throughout the bulk of restorative material. However, the materials were found to be only partially successful and are not commonly used today due to issues such as poor activator systems, poor wear resistance, high polymerization shrinkage and mis-matched coefficient of thermal expansion. These adverse physical properties prevented chemically cured composites from being the material of choice for clinicians. The lack of wear resistance prevented them from preserving restoration contour in areas subject to abrasion or attrition. They were not meant for use in high-stress areas due to low strength of the material which tended to flow under load. Their high polymerization shrinkage and coefficient of thermal expansion led to microleakage and discoloration at the margins due to percolation.24 Clinicians were also constrained by the polymerization setting time when placing and shaping the restoration. In addition, clinical studies showed that self-cured composites undergo more darkening than photo-cured composites over time.25 Thus, the aforementioned limitations of self-cured composites have led to the development of light-activated composites that offer the advantages of controlled working time and the elimination of time consuming mixing procedures that often introduce unwanted porosities to the restorations. When compared to the chemically cured composites, light-activated composites demonstrated greater strength, fracture toughness, better shade selection, color stability and higher surface polymerization conversion rates. OH HO O N O O + O Dibenzoyl peroxide DHEPT OH HO N O O O benzoyloxy radical O benzoate ion Figure 2.1. Chemical activation of dibenzoyl peroxide to produce free radical for polymerization. 2.2 Light-activated Composite Resins The beginning of modern restorative dentistry was marked by the discovery of Bowen’s Bis-GMA (2,2-bis[4-(2-hydroxy-3- methacryloxypropoxy)phenyl]-propane)/ inorganic particle formulations in the early 1960s (Figure 2.2).11,12 The introduction of this composite-based resin technology to restorative dentistry was one of the most significant contributions to dentistry in the last century. Applications for this new polymer include anterior and posterior composite resin restorations, indirect inlays/onlays, pit and fissure sealants and more wear-resistant denture teeth.26 OH OH O O O O O O Figure 2.2. Chemical structure of Bis-GMA monomers. Composite materials refer to a mixture of two or more distinctly different materials with properties that are superior or intermediate to those of the individual constituents. Dental composites are complex, tooth-colored filling materials composed of synthetic polymers, inorganic particulate fillers, initiators and activators that promote light-activated polymerization of the organic matrix to form cross-linked polymer networks, and silane coupling agents which bond the reinforcing fillers to the polymer matrix. Further additives such as stabilizers and pigments are also included. Each component of the composite is crucial for the success of the final dental restoration.23 Light-activated composite resins undergo free radical polymerization by irradiation with blue light in the wavelength range of 410 - 500 nm. Light in this region is most effectively absorbed by an α-diketone photoinitiator, usually camphorquinone (CQ), and creates an excited state that reacts with an amine reducing agent such as N,N-dimethylaminoethyl methacrylate (DMAEMA) or ethyl p-dimethylaminobenzoate (DMAB) to produce free radicals that initiate the cross-linking polymerization (Figure 2.3).27,28 The absorption spectrum of CQ lies in the 450 - 500 nm wavelength range, with peak absorption at 470 nm.29,30 CH3 H3C H3C CH3 O OH H3C hv O camphorquinone Free radical initiators H3C O O O N O N O DMAEMA Figure 2.3. Light activation mechanism. 2.3 Organic Matrix The current organic matrix used in dental composites is based on methacrylate chemistry with cross-linking dimethacrylate being most universal. Approximately eighty to ninety percent of commercial dental composites use BisGMA monomer as their organic matrix.31,32 Other base monomers used in present commercial composites include triethyleneglycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), ethoxylated bisphenol-A-dimethacrylate (BisEMA), decanediol dimethacrylate (D3MA) bis(methacryloyloxymethyl) tricyclodecane and urethanetetramethacrylate (UTMA). The chemical structures for some of these monomers are shown in Figure 2.4. The most commonly used organic matrix, Bis-GMA has a very high viscosity due to the hydrogen bonding interactions that occur between the hydroxyl groups on the monomer molecules. Therefore, Bis-GMA must be diluted with more fluid monomers to provide the proper viscosity for use in dental composites.23 TEGDMA which is less viscous and has excellent copolymerization characteristics is frequently used as the diluent monomer for UDMA and BisGMA-based composites to produce a fluid resin that can be maximally filled with inorganic filler particles. Optimal properties are produced when TEGDMA is used in a 1:1 ratio with Bis-GMA.33 Some other diluents include ethylene- and hexamethylene-glycoldimethacrylate and benzyl methacrylate.34 TEGDMA has also been replaced with UDMA and BisEMA in several products to reduce shrinkage, aging and environmental effects.35 UDMA and BisEMA have higher molecular weights and fewer double bonds per unit of weight when compared to TEGDMA that generally results in lower shrinkage. O O O O O O TEGDMA O H O O N O O O N O H O UDMA O O O O O O Bis-EMA O O O D3MA O Figure 2.4. Chemical structures of common base monomers used in dental composites. 2.4 Inorganic Fillers The use of resin matrix by itself is not a suitable restorative material as it demonstrates unsuitable physico-mechanical properties. Addition of inorganic fillers is often needed to strengthen mechanical properties, provide radiopacity and reduce thermal expansion, polymerization shrinkage and water sorption. In general, the physico-mechanical properties of composites are improved in direct relationship to the amount of filler added. Fine powders of crystalline or noncrystalline silica or silicates are normally used as fillers. The type and size of filler material used has been employed as a basis for classification of modern dental composites (Table 2.1).36 Table 2.1. 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Macromolecules 1997;30:1099 -1106. 235 [...]... curing times and expansion still exist Recent advances of dental composites for reduced shrinkage with good mechanical properties and enhanced clinical performances have been made in the area of nanotechnology for the development of dental nanocomposites 2. 8 Nanotechnology with Dental Composites Nanotechnology, also known as nanoscience or molecular engineering, is defined as the creation of functional... portions of the vibrational infrared region and a small range of absorption can be defined for each type of bond 34 While the FTIR reveals the types of functional groups present in a molecule, NMR provides information about the number of magnetically distinct atoms and is an important technique used for structural determination The number of each of the distinct types of hydrogen nuclei as well as information... smallest particle of silica, is generally obtained by hydrolysis and condensation of trialkoxy or trichlorosilanes With a unique well-defined structure, POSSTM is often used for the preparation of hybrid materials with well-defined structures. 128 It can also be chemically functionalized and behave as a platform from which to synthesize organic/inorganic nanocomposites for use in a variety of applications. .. groups such as methacrylate and epoxy will be developed Their efficiency as low shrinking light-activated nanocomposites and their roles as copolymers will also be evaluated 32 3 .2 Research Programme This research programme consisted of a total of 8 phases Phases 2 to 5 were carried out in parallel and results obtained were reported in Chapters 5 and 6 Phase One Synthesis of Octa(hydridodimethylsiloxy)silsesquioxane... reaction of the ‘octannion cube’ obtained in step one with dimethylchlorosilane Phase Two Preparation of Low /Non- shrinking SSQ-based Nanocomposites The objective of this phase was to design and develop low /non- shrinking SSQ-based nanocomposites Different SSQ-based monomers with functional groups such as methacrylate and epoxy were attached to the platform material in different equivalents and combinations... nanofilled composites will be discussed in section 2. 8 2. 5 Silane Coupling Agent Formation of a strong covalent bond between the inorganic filler particles and organic matrix is essential for obtaining good mechanical properties in dental composites.38 Failure of the filler-matrix interface will result in fracture and subsequent disintegration of the composite as a result of uneven distribution of. .. control and synthesized SSQ-based monomers Formulation of the control in this study involved the mixing of Bis-GMA and TEGDMA in a ratio of 1:1 for optimum properties.33 Photochemical curing involved the addition of 1 mol % of both visible light initiators (camphorquinone, CQ) and activators (N,Ndimethylaminoethyl methacrylate, DMAEMA) to the control and synthesized SSQ-based monomers Both the control and. .. They form true molecular dispersions when mixed into polymer formulations with no phase separation and hence represent a significant advantage over current filler technologies. 129 28 In recent years, several POSSTM molecules have been synthesized and investigated for dental applications Mono-methacrylate functionalized POSSTM (Figure 2. 16) synthesized by Gao et al.130 have been evaluated and used for. .. the polymer matrix OCH3 O Si OCH3 OCH3 O Figure 2. 5 Structure of MPTS, a typical silane coupling agent used in dental composites 11 2. 6 Limitations of Current Dental Composites The development of light-activated composite materials in the 1970s heralded a period of rapid progress in the field of tooth-colored restorations One of the most obvious changes in dental practice during the 1970’s was the way... crystalline bismethacrylates 2. 7.3 Branched and Dendritic Monomers Besides development in liquid crystalline monomers, highly branched nonliquid crystalline89- 92 and dendritic monomers93,94 have also been synthesized and evaluated for dental composites These monomers were found to have the advantages of low polymerization shrinkage, viscosity and can be incorporated into formed polymer network efficiently . to Dental Amalgam Before the introduction of polymeric dental composites, dental amalgam was the material of choice for stress bearing dental fillings. 1 This material was inexpensive and. shrinkage and stress becoming a major focus of dental biomaterials research, we aim to develop novel low /non-shrinking nanocomposites based on polyhedral silsesquioxanes (SSQ) for dental applications. . free radical for polymerization. 2. 2 Light-activated Composite Resins The beginning of modern restorative dentistry was marked by the discovery of Bowen’s Bis-GMA (2, 2-bis[4- (2- hydroxy-3-