Synthesis and characterization of non shrinking nanocomposites for dental applications 2

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. Classification of dental composites by filler particle size.36 Type Filler Size (μm) Megafill 0.5 - mm Macrofill 10 – 100 √ Midifill – 10 √ √ √ Minifill 0.1 – √ √ √ Microfill 0.01 – 0.1 √ Nanofill 0.005 – 0.01 √ Heterogeneous Hybrid Homogeneous √ √ √ One of the most significant improvements in the evolution of commercial composites has been the modifications to the filler. Optimization of filler particle 10 REFERENCES 1. Lavelle CL. A cross-sectional survey into the durability of amalgam restorations. J Dent 1976;4:139-143. 2. Paterson N. The longevity of restorations. A study of 200 regular attenders in a general dental practice. Br Dent J 1984;157:23-25. 3. Roulet JF. Benefits and disadvantages of tooth-coloured alternatives to amalgam. J Dent 1997;25:459-473. 4. Mjor IA. The reasons for replacement and the age of failed restorations in general dental practice. Acta Odontol Scand 1997;55(1):58-63. 5. Smales RJ, Webster DA. Survival predictions of amalgam restorations. J Dent 1991;19:272-277. 6. Horsted-Bindslev P. Amalgam toxicity-environmental and occupational hazards. J Dent 2004;32:359-365. 7. Roberts HW, Leonard D, Osborne J. Potential health and environmental issues of mercury-contaminated amalgamators. J Am Dent Assoc 2001;132:58-64. 8. Anusavice KJ, Soderholm K, Grossman DG. Implications of amalgam and ceramic degradation in the oral environment. MRS Bulletin 1993;18:64-72. 9. Arenholt-Bindslev D. Dental amalgam - environmental aspects. Adv Dent Res 1992;6:125-130. 10. Anseth KS, Newman SM, Bowman CN. Polymeric dental composites: Properties and reaction behavior of multimethacrylate dental restorations. Adv Polym Sci 1995;122:177-217. 11. Bowen RL; Dental filling material comprising vinyl-silane treated fused silica and a binder consisting of the reaction product of bisphenol and glycidyl methacrylate. US Patent 3066112. 1962. 12. Bowen RL; Silica-resin direct filling material and method of preparation. US Patent 3194783-3194784. 1965. 13. Full CA, Hollander WR. The composite resin restoration. A literature review part III. What the future holds for children. J Dent 1993;60:57-60. 14. Ferracane JL. Using posterior composites appropriately. J Am Dent Assoc 1992;123:53-58. 220 15. Yap AU, Soh MS. Post-gel polymerization contraction of "low shrinkage" composite restoratives. Oper Dent 2004;29:182-187. 16. Mjor IA, Qvist V. Marginal failures of amalgam and composite restorations. J Dent 1997;25:25-30. 17. Emami N, Soderholm KJ, Berglund LA. Effect of light power density variations on bulk curing properties of dental composites. J Dent 2003;31:189-196. 18. Soh MS, Yap AU, Siow KS. Post-gel shrinkage with different modes of LED and halogen light curing units. Oper Dent 2004;29:317-324. 19. Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dent Mater 1999;15:128-137. 20. Feilzer AJ, Dooren LH, de Gee AJ, Davidson CL. Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface. Eur J Oral Sci 1995;103:322-326. 21. Letzel H. Survival rates and reasons for failure of posterior composite restorations in multicentre clinical trial. J Dent 1989;17:10-17. 22. Jokstad A, Mjor IA, Qvist V. The age of restorations in situ. Acta Odontol Scand 1994;52:234-248. 23. Ferracane JL. Current trends in dental composites. Crit Rev Oral Biol Med 1995;6:302-318. 24. Sturdevant CM, Roberson TM, Heymann HO, Sturdevant JR. The art and science of operative dentistry. St Louis: Mosby; 1995. 25. Tyas MJ. Color stability of composite resins: a clinical comparison. Aust Dent J 1992;37:88-90. 26. Leinfelder KF. New developments in resin restorative systems. J Am Dent Assoc 1997;128:573-581. 27. Dart EC, Nemcek J; Photopolymerisable composition. US Patent 4071424. 1978. 28. Craig RG. Chemistry, composition and properties of composite resins. Dent Clin North Am 1981;25:219-239. 29. Lee SY, Chiu CH, Boghosian A, Greener EH. Radiometric and spectroradiometric comparison of power outputs of five visible light-curing units. J Dent 1993;21:373-377. 221 30. Denehy GE, Pires JAF, Cuitko E, Swift EJ Jr. Effects of curing tip distance on light intensity and composite resin microhardness. Quintessence Int 1993;24:517-521. 31. Peutzfeldt A. Resin composites in dentistry: the monomer systems. Eur J Oral Sci 1997;105:97-116. 32. Ruyter IE, Øysæd H. Composites for use in posterior teeth: Composition and conversion. J Biomed Mater Res 1987;21:11-23. 33. Stannard JG, Sornkul E, Collier R. Mechanical properties of composite copolymers. J Dent Res 1993;72 (Spec Iss):135. 34. Ruyter IE, Nilsen J. Chemical characterization of six posterior composites. J Dent Res 1993;72 (Spec Iss):177. 35. Yap AU, Low JS, Ong KL. Effect of food-simulating liquids on surface characteristics of composite and polyacid-modified composite restoratives. Oper Dent 2000;25:170-176. 36. Bayne SC, Heymann HO, Swift EJ Jr. Update on dental composite restorations. J Am Dent Assoc 1994;125:687-701. 37. Dogon IL. Present and future value of dental composite materials and sealants. Int J Technol Assess Health Care 1990;6:369-377. 38. Bowen RL. Properties of silica reinforced polymer for dental restoration. J Am Dent Assoc 1963;66:57-64. 39. Yearn JA. Factors affecting cure of visible light activated composites. Int Dent J 1985;35:218-225. 40. Ferracane JL, Condon JR. Post-cure heat treatments for composites: properties and fractography. Dent Mater 1992;8:290-295. 41. Carvalho RM, Pereira JC, Yoshiyama M, Pashley DH. A review of polymerization contraction: the influence of stress development versus stress relief. Oper Dent 1996;21:17-24. 42. Davidson CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. J Dent 1997;25:435-440. 43. Yap AU, Wang HB, Siow KS, Gan LM. Polymerization shrinkage of visible light-cured composites. Oper Dent 2000;25:98-103. 44. Sakaguchi RL, Sasik CT, Bunezak MA, Douglas WH. Strain gauge method for measuring polymerization contraction of composite restoratives. J Dent 1991;19:312-316. 222 45. Price RB, Rizkalla AS, Hall GC. Effect of stepped light exposure on the volumetric polymerization shrinkage and bulk modulus of dental composites and an unfilled resin. Am J Dent 2000;13:176-180. 46. Feilzer AJ, de Gee AJ, Davidson CL. Curing contraction of composites and glass-ionomer cements. J Prosthet Dent 1988;59:297-300. 47. Burgess JO, Walker R, Davidson JM. Posterior resin-based composite:review of the literature. Pediatr Dent 2002;24:465-479. 48. Davidson CL, de Gee AJ. Relaxation of polymerization shrinkage stresses by flow in dental composites. J Dent Res 1984;63:146-148. 49. Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987;66:1636-1639. 50. Sheth JJ, Fuller JL, Jensen ME. Cuspal deformation and fracture resistance of teeth with dentin adhesives and composite. J Prosthet Dent 1988;60:560569. 51. Qvist V, Qvist J, Mjör IA. Placement and longevity of tooth-colored restorations in Denmark. Acta Odontol Scand 1990;48:305-311. 52. Eick JD, Welch FH. Polymerization shrinkage of posterior composite resins and its possible influence on postoperative sensitivity. Quintessence Int 1986;17:103-111. 53. Roulet JF, Salchow B, Wald M. Margin analysis of posterior composites in vivo. Dent mater 1991;7:44-49. 54. Ferracane JL, Condon JR. Rate of elution of leachable components from composite. Dent Mater 1990;6:282-287. 55. Øysæd H, Ruyter IE. Composites for use in posterior teeth: Mechanical properties tested under dry and wet conditions. J Biomed Mater Res 1986;20:261-271. 56. Söderholm KJM, Roberts MJ. Influence of water exposure on the tensile strength of composites. J Dent Res 1990;69:1312-1316. 57. Söderholm KJM. Degradation of glass filler in experimental composites. J Dent Res 1981;60:1867-1875. 58. Braga RR, Ferracane JL. Contraction stress related to degree of conversion and reaction kinetics. J Dent Res 2002;81:114-118. 59. Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: A systematic review. Dent Mater 2005;21:962-970. 223 60. Kemp-Scholte CM, Davidson CL. Complete marginal seal of class V resin composite restorations effected by increased flexibility. J Dent Res 1990;69:1243-1249. 61. Choi KK, Cndon JR, Ferracane JL. The effects of adhesive thickness on polymerization contraction stress of composite. J Dent Res 2000;79:812817. 62. Kanca J, 3rd, Suh BI. Pulse activation: reducing resin-based composite contraction stresses at the enamel cavosurface margins. Am J Dent 1999;12:107-112. 63. Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440-444. 64. Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without 'softstart-polymerization'. J Dent 1997;25:321-330. 65. Yap AU, Ng SC, Siow KS. Soft-start polymerization: influence on effectiveness of cure and post-gel shrinkage. Oper Dent 2001;26:260-266. 66. Price RB, Bannerman RA, Rizkalla AS, Hall GC. Effect of stepped vs. continuous light curing exposure on bond strengths to dentin. Am J Dent 2000;13:123-128. 67. Koran P, Kürschner R. Effect of sequential versus continuous irradiation of a light-cured resin composite on shrinkage, viscosity, adhesion, and degree of polymerization. Am J Dent 1998;10:17-22. 68. Yap AU, Soh MS, Siow KS. Effectiveness of composite cure with pulse activation and soft-start polymerization. Oper Dent 2002;27:44-49. 69. Stansbury JW. Synthesis and evaluation of new oxaspiro monomers for double ring-opening polymerization. J Dent Res 1992;7:1480-1512. 70. Stansbury JW. Ring-opening polymerization of a 2-methylene spiro orthocarbonate bearing a pendant methacrylate group. In: Shalaby SW, Ikada Y, Langer R, Williams J, editors. Polymers of Biological and Biomedical Significance: ACS Symposium series 540; 1993. 71. Byerley TJk, Eick JD, Chen GP, Chappelow CC, Millich F. Synthesis and polymerization of new expanding dental monomers. Dent Mater 1992;8:345-350. 72. Eick JD, Byerley TJK, Chappell RP, Chen GP, Bowles CQ, Chappelow CC. Properties of expanding SOC/epoxy copolymers for dental use in dental composites. Dent Mater 1993;9:123-137. 224 73. Miyazaki K, Takata T, Endo T, Inanaga A. Thermal and photopolymerization of methacrylates containing a spiro ortho ester moiety and the properties of polymethacrylates. Dent Mater 1994;13:9-18. 74. Tilbrook DA, Clarke RL, Howle NE, Braden M. Photocurable epoxypolyol matrices for use in dental composites I. Biomaterials 2000;21:17431753. 75. Kaisaki D, Mitra SB, Schultz W, Devoe R; Visible-light curable epoxy system with enhanced depth of cure. Int Pat Appl PCT WO 96/13538, 3M. 1996. 76. Ferracane JL. New polymer resins for dental restoratives. Oper Dent 2001;Suppl 6:199-209. 77. Nuyken O, Böhner R, Erdmann C. Oxetane photopolymerization - A system with low volume shrinkage. Macromol Symposia 1996;107:125-128. 78. Sasaki H, Kuriyma A, Kakuchi T. Fast photopolymerization of oxetanes in nitrogen atmosphere. J of Macromol Sci – Pure and Appl Chem 1994; A31(Suppl 5): 635-641. 79. Belfield KD, Abdelrazzaq FB. Photoinitiated cationic cross-linking of 4methylene-2-phenyl-1,3-dioxolane with 2,2'-(1,4-phenylene)bis(4methylene-1,3-dioxolane). Macromolecules 1997;30:6985-6988. 80. Moszner N, Völkel T, Zeuner F, Rheinberger V. Radical ring-opening monomers for dental composites. Polym Prepr 1997;38:86-87. 81. Evans RA, Rizzardo E. Free-radical ring-opening polymerization of cyclic allylic sulfides. Macromolecules 1996;29:6983-6989. 82. Moszner N, Zeuner F, Völkel T, Rheinberger V. Polymerization of cyclic monomers. VII. Synthesis and radical polymerization of 1,3-bis[(1alkoxycarbonyl-2-vinyl cyclopropane-1-yl)carboxyl] benzenes. J Appl Polym Sci 1999;72:1775-1782. 83. Moszner N, Zeuner F, Rheinberger V. Polymerization of cyclic monomers. 4. Synthesis and radical polymerization of bi-and trifunctional 2vinylcyclopropanes. Macromol Rapid Commun 1997;18:775-780. 84. Rawls HR, Wellinghoff VT, Norling BK, Leamo SH, Swynnerton NF, Wellinghoff ST. Low shrinkage resins from liquid crystal diacrylate monomers. Polym Prepr 1997;38:167-168. 85. Ritter H, Draheim G, Moszner N, Salz U. Dental materials on the basis of liquid-crystalline monomers. DE Patent Appl. 195,25,941 A1. 1997. 86. Ritter H, Draheim G, Moszner N, Salz U. Dental materials based on liquid crystalline monomers. US Patent 6,031,015. 2000. 225 87. Hölter D, Frey H, Mülhaupt R, Klee JE. Ambient-temperature liquidcrystalline bismethacrylates based on cholesterol: cholestric and smectic thermosets. Adv Mater 1998;10:864-868. 88. Hölter D, Frey H, Mülhaupt R, Klee JE. Liquid crystalline thermosets based on branched bismethacrylates. Macromolecules 1996;29:7003-7011. 89. Klee JE, Neidhart F, Flammersheim H-J. Monomers for low shrinking composites, 2. Synthesis of branched methacrylates and their application in dental composites. Macromol Chem Phys 1999;200:517-523. 90. Klee JE, Walz U, Hölter D, Frey H, Mülhaupt R. Branched macromonomers and their application in dental composites - Monomers for low-shrinking composites, 3. Angew Makromol Chem 1998;260:71-75. 91. Wan Q, Schricker SR, Culbertson BM. Studies on syntheses and characterization of hyperbranched multi-methacrylates and their potential applications. Polym Prepr 2000;41:101-102. 92. Wan Q, Schricker SR, Culbertson BM. Hyperbranched multi-methacrylates: their application in dental resin systems. Polym Prepr 2000;41:155-156. 93. Moszner N, Völkel T, Rheinberger V. Synthesis, characterization and polymerization of dendrimers with methacrylic end groups. Macromol Chem Phys 1997;197:621-631. 94. Rheinberger V, Moszner N, Völkel T, Burtscher P. Fine-grained polymerizable compositions flowable under pressure or shear stress. US Patent 5,886,064, Ivoclar AG. 1999. 95. Wolter H, Glaubitt W, Rose K. Multifuctional (meth)acrylate alkoxysilanes. A new type of reactive compounds. Mater Res Soc Symp Proc 1992;271:719-724. 96. Kirk RE, Othmer DF, Kroschwitz J, Howe-Grant M. Encyclopedia of chemical technology. New York: Wiley; 1991: 397. 97. Phillips RW. Skinner's science of dental materials. Philadelphia, PA: W.B. Saunders Co.; 1991: 215-248. 98. Drummond JL. Cyclic fatigue of composite restorative materials. J Oral Rehabil 1989;16:509-520. 99. Willems G, Lambrechts P, Braem M, Celis JP, Vanherle G. A classification of dental composites according to their morphological and mechanical characteristics. Dent Mater 1992;8:310-319. 100. Braem MJA, Davidson CL, Lambrechts P, Vanherle G. In vitro flexural fatigue limits of dental composites. J Biomed Mater Res 1994;28:13971402. 226 101. Reusens B, D'hoore W, Vreven J. In vivo comparison of a microfilled and a hybrid minifilled composite resin in Class III restorations: 2-year follow-up. Clin Oral Investig 1999;3:62-69. 102. Qvist V, Strom C. 11-year assessment of Class III resin restorations completed with two restorative procedures. Acta Odontol Scand 1993;51:253-262. 103. Ferracane JL. In vitro evaluation of composite resins. Structure-property relationships. Development of assessment criteria. Trans Acad Dent Mater 1989;2:6-35. 104. Denehy GE. A direct approach to restore anterior teeth. Am J Dent 2000;13:55-59. 105. Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials. J Am Dent Assoc 2003;134:1382-1390. 106. Noritake M, Yuasa S; Inorganic Composition. US Patent 5609675, Tokuyama Corp. 1997. 107. Pflug K; Dental materials having a nanoscale filler. WO 99/17716, Dentsply Corp. 1999. 108. Baseren M. Surface roughness of nanofill and nanohybrid composite resin and ormocer-based tooth-colored restorative materials after several finishing and polishing procedures. J Biomater Appl 2004;19:121-134. 109. Davis N. A nanotechnology composite. Compendium 2003;24:662-670. 110. Albers HF. Tooth-colored restoratives: Principles and techniques.Hamilton, Ont.;London: BC Decker: 2002. 111. Angeletakis C, Nguyen M-D. Dental composition containing discrete nanoparticles. US Patent 6593395. 2003. 112. Bauer F, Sauerland V, Ernst H, Glasel H-J, Naumov S, Mehnert R. Preparation of scratch-and abrasion-resistant polymeric nanocomposites by monomer grafting onto nanoparticles, 4-Applications of MALDI-TOF mass spectrometry to the characterization of surface modified nanoparticles. Macromol Chem Phys 2003;204:375-378. 113. Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interface Sci 1968;26:62-69. 114. Park SK, Kim KD, Kim HAT. Colloids and Surfaces A: Physicochem Eng Aspects 2002;197:7. 115. Giesche H. Synthesis of monodispersed silica powders, I. Particle properties and reaction kinetics. J Eur Ceram Soc 1994;14:189-204. 227 116. Giesche H. Synthesis of monodispersed silica powders, II. Controlled growth reaction and continuous production process. J Eur Ceram Soc 1994;14:205-214. 117. Pohl L, Marquard K, Waitl G, Reeh U, Wipfelder E; Inorganic fillers and organic matrix materials whose refractive index is adapted. WO 93/025611. 1993. 118. Furman BR, Wellinghoff ST, Rawls HR, Dixon H, Norling BK. Mechanically strong and transparent or translucent composites made using zirconium oxide nanoparticles, Bord of regents of the University of Texas System.US 6194481 B1, Southwest Research Institute. 2001. 119. Loy DA, Shea KJ. Bridged polysilsesquioxanes. Highly porous hybrid organic-inorganic materials. Chem Rev 1995;95:1431-1442. 120. Schubert U, Hüsing N, Lorenz A. Hybrid inorganic-organic materials by sol-gel processing of organofunctional metal alkoxides. Chem Mater 1995;7:2010-2027. 121. Hay JN, Raval HM. Synthesis of organic-inorganic hybrids via the nonhydrolytic sol-gel process. Chem Mater 2001;13:3396-3403. 122. Wei Y, Jin D. A new class of organic-inorganic hybrid dental materials. Polym Prep: Div Polym Chem, Amer Chem Soc 1997;38:122-123. 123. Moszner N, Völkel T, Stein S, Rheinberger V; Polysiloxanes Based Dental Materials. EP 1022013 A2. 2000. 124. Wei Y, Jin D, Yang C, Wie G. A fast convenient method to prepare hybrid sol-gel materials with low volume-shrinkage. J Sol-gel Sci Technol 1996;7:191-201. 125. Wei Y, Bakthavatchalam R, Whitecar CK. Synthesis of new organicinorganic hybrid glasses. Chem Mater 1990;2:337-339. 126. Wei Y, Yang D, Bakthavatchalam R. Thermal stability and hardness of new polyacrylate-SiO2 hybrid sol-gel materials. Mater Lett 1992;13:261-266. 127. Wei Y, Jin D, Xu J, Baran GR, Qui K-Y. Mechanical properties of interface-free polyacrylate-silica hybrid sol-gel materials for potential dental applications. Polym Adv Technol 2001;12:361-368. 128. Li G, Wang L, Ni H, Pittman Jr CU. Polyhedral oligomeric silsesquioxane (POSS) polymers and copolymers: A review. J Inorg and Organomet Polym 2001;11:123-154. 129. Haddad TS, Stapleton R, Jeon HG, Mather PT, Lichtenhan JD, Phillips S. Nanostructured hybrid organic/inorganic materials. Silsesquioxane modified plastics. Polym prepr 1999;40:496. 228 130. Gao F, Tong Y, Schricker SR, Culbertson BM. Evaluation of neat resins based on methacrylates modified with methacryl-POSS, as potential organic-inorganic hybrids for formulating dental restoratives. Polym Adv Technol 2001;12:355-360. 131. Fong H, Dickens SH, Flaim GM. Evaluation of dental restorative composites containing polyhedral oligomeric silsesquioxane methacrylate. Dent Mater 2004;21:520-529. 132. Sellinger A, Laine RM. Silsesquioxanes as synthetic platforms. 3. Photocurable, liquid epoxides as inorganic/organic hybrid precursors. Chem Mater 1996;8:1592-1593. 133. Bissinger P, Gasser O, Guggenberger R, Soglowek W, Eckhardt G. Cyclosiloxane-based cross-linker monomers, production thereof and use in polymerizable materials. WO 00/42092, ESPE Dental AG.2000. 134. Zech J, Lechner G; Hardenable mass with silane dendrimers. 2002. 135. Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:1564-1568. 136. ISO International Standard 4049. Resin-based International Standards Organization. 2000. filling materials. 137. Voronkov MG, Lavrent'yev VI. Polyhedral oligosilsesquioxanes and their HOMO derivatives. Top. Curr. Chem 1982;102:199. 138. Baney RH, Itoh M, Sakakibara A, Suzuki T. Silsesquioxanes. Chem Rev 1995;95:1409-1430. 139. Muller R, Dathe C, Heinrich L. A siloxane from siliconchloroform. J Physiolo 1959;9:71. 140. Agaskar PA. New synthetic route to hydridospherosiloxanes Oh-H8Si8O12 and D5h-H10Si10O15. Inorg Chem 1991;30:2707-2708. 141. Agaskar PA. Functionalized spherosilicates - soluble precursors of inorganic organic hybrid materials. Colloids and Surfaces 1992;63:131-138. 142. Hasegawa I, Motojima S. Dimethylvinylsilylation of Si8O208- silicate anion in methanol solutions of tetramethylammonium silicate. J organomet Chem 1992;441:373-380. 143. Weidner R, Zeller N, Deubzer B, Frey V. New Organooligosilsesquioxane German Patent DE 3837397 A1. 1991. 229 144. Yang Y, Taylor P, Bassindale A, Chen H, Mackinnon I. A novel method for synthesis of silsesquioxane cages. The Abstract of XIII International Symposium on Organosilicon Chemistry, Guanajato, Mexico 2002:17. 145. Laine RM, Zhang C, Sellinger A, Viculis L. Polyfunctional cubic silsesquioxane as building blocks for organic/inorganic hybrids. Appl. Organometal. Chem. 1998;12:715 - 723. 146. Laine RM, Choi J, Lee I. Organic-Inorganic Nanocomposites with completely defined interfacial interactions. Adv. Mater. 2001;13:800-803. 147. Sellinger A, Laine RM. Silsesquioxanes as synthetic platforms. Thermally curable and photocurable inorganic/organic hybrids. Macromolecules 1996;29:2327-2330. 148. Choi J, Kim SG, Laine RM. Organic/Inorganic Hybrid epoxy nanocomposites from aminophenylsilsesquioxanes. Macromolecules 2004;37:99-109. 149. Zhang C, Laine RM. Silsesquioxane-liquid crystalline (LC) composites: Potential precursors to dental material. Polym Prep: Div Polym Chem, Amer Chem Soc 1997;38:120-121. 150. Joshi M, Butola BS. Polymeric nanocomposites - Polyhedral oligomeric silsesquioxanes (POSS) as hybrid fillers. J Macromol Sci 2004;44:389 410. 151. Zheng L, Waddon AJ, Farris RJ, Coughlin E. Bryan X-ray characterizations of polyethylene polyhedral oligomeric silsesquioxane copolymers. Macromolecules 2002;35:2375-2379. 152. Bartsch M, Bornhauser P, Calzaferri G. H8Si8O12. A model for the vibrational structure of zeolite-A. J Phys Chem 1994;98:2817-2831. 153. Lok BM, Cannan TR, Messina CA. The role of organic-molecules in molecular-sieve synthesis. Zeolites 1983;3:282-291. 154. Hasegawa I, Kuroda K, Kato C. The effect of tetraalkylammonium ions on the distribution of the silicate anions in aqueous solutions. Bull Chem Soc Jpn 1986;59:2279 - 2283. 155. Hasegawa I, Sakka S, Sugahara Y, Kuroda K, Kato C. Silicate anions formed in tetramethylammonium silicate methanolic solutions as studied by 29 Si nuclear magnetic resonance. J chem soc chem commun 1989;4:208210. 156. Yap AU, Wang X, Wu X, Chung SM. Comparative hardness and modulus of tooth-colored restoratives: a depth-sensing microindentation study. Biomaterials 2004;25:2179-2185. 230 157. Sellinger A, Laine RM, Chu V, Viney C. Palladium- and platinumcatalyzed coupling reactions of allyloxy aromatics with hydridosilanes and hydridosiloxanes: Novel liquid crystalline/ organosilane materials. J Polym Sci Part A: Polym Chem 1994;32:3069-3089. 158. Lucas EF, Porterz RS. Thermal analyses of graft copolymers of poly (methyl methacrylate) main chain with poly (propylene oxide-B-ethylene oxide) graft chain. J Appl Polym Sci 1993;49:1211-1222. 159. Tate WH, Porter KH, Dosch RO. Successful photochruing: Don't restore with out it. Oper Dent 1999;24:109-114. 160. Yap AU. Effectiveness of polymerization in composite restoratives claiming bulk placement: impact of cavity depth and exposure time. Oper Dent 2000;25:113-20. 161. Penn RW. A recording dilatometer for measuring polymerization shrinkage. Dent Mater 1986;2:78-79. 162. de Gee AJ, Davidson CL, Smith A. A modified dilatometer for continuous recording of volumetric polymerization shrinkage of composite restorative materials. J Dent 1997;9:36-42. 163. Suliman AA, Boyer DB, Lakes RS. Polymerization shrinkage of composite resins: Comparison with tooth deformation. J Prosthet Dent 1994;71:7-12. 164. Puckett AD, Smith R. Method to measure the polymerization shrinkage of light-cured composites. J Prosthet Dent 1992;68:56-58. 165. Aw TC, Nicholls JI. Polymerization shrinkage of restorative resins using laser and visible light curing. J Clini Laser Med and Surg 1997;15:137-141. 166. Yap AU, Soh MS, Siow KS. Post-gel shrinkage with pulse activation and soft-start polymerization. Oper Dent 2002;27:81-87. 167. de Gee AJ, Feilzer AJ, Davidson CL. The linear polymerization shrinkage of unfilled resins and composites determined with a linometer. Dent Mater 1993;9:11-14. 168. Chung SM, Yap AU, Tsai KT, Yap FL. Elastic modulus of resin-based dental restorative materials: a microindentation approach. J Biomed Mater Res B Appl Biomater 2005;72:246-253. 169. Feilzer AJ, de Gee AJ, Davidson CL. Relaxation of polymerization contraction shear stress by hygroscopic expansion. J Dent Res 1990;69:3639. 170. Braem M, Davidson CL, Vanherle G, Van Doren V, Lambrechts P. The relationship between testy methodology and elastic behavior of composites. J Dent Res 1987;66:1036-1039. 231 171. Boots HRF, Pandey RB. Qualitative percolation study of free-radical crosslinking polymerization. Polym Bull 1984;11:415-420. 172. Zhang C, Babonneau F, Bonhomme C, Laine RM, Soles CL, Hristov HA, Yee AF. Highly porous polyhedral silsesquioxane polymers. Synthesis and Characterization. J Am Chem Soc 1998;120:8380-8391. 173. Heymann HO, Sturdevant JR, Bayne SC, Wilder AD, Sluder TB, Brunson WD. Examining tooth flexure effects on cervical restorations; a two-year study. J Am Dent Assoc 1991;122:41-47. 174. Anseth KS, Wang CN, Bowman CN. Reaction behaviour and kinetic constants for photopolymerizations of multi(meth)acrylate monomers. Polymer 1994;35:3243-3250. 175. Hofmann N, Hugo B, Klaiber B. Effect of irradiation type (LED or QTH) on photo-activated composite shrinkage strain kinetics, temperature rise, and hardness. Eur J Oral Sci 2002;110:471-479. 176. Pananakis D, Watts DC. Incorporation of the heating effect of the light source in a non-isothermal model of a visible-light-cured resin composite. J Mater Sci 2000;35:4589-4600. 177. Rueggeberg FA, Caughman WF, Curtis JW, Jr. Effect of light intensity and exposure duration on cure of resin composite. Oper Dent 1994;19:26-32. 178. Yap AU, Seneviratne C. Influence of light energy density on effectiveness of composite cure. Oper Dent 2001;26:460-466. 179. Versluis A, Tantbirojn D, Douglas WH. Do dental composites always shrink toward the light? J Dent Res 1998;77:1435-1445. 180. Kannurpatti AR, Anderson KJ, Anseth JW, Bowman CN. Use of living radical polymerizations to study the structural evolution and properties of highly crosslinked polymer networks. J Polym Sci Part B: Polym Phys 1997;35:2297-2307. 181. O'Brien WJ. Polymeric restorative materials in dental materials and their selection. Chicago: Quintessence Publishing Co.; 1997: 97-114. 182. Bayne SC, Thompson JY, Swift EJ, Jr., Stamatiades P, Wilkerson M. A characterization of first-generation flowable composites. J Am Dent Assoc 1998;129:567-577. 183. Anusavice KL. Phillip's science of dental materials. Philadelphia: WB Saunders Co.; 1996. 184. Sabbagh J, Vreven J, Leloup G. Dynamic and static moduli of elasticity of resin-based materials. Dent Mater 2002;18:64-71. 232 185. Vargas MA, Cobb DS, Schmit JL. Polymerization of composite resins: argon laser vs conventional light. Oper Dent 1998;23:87-93. 186. Venhoven BA, de Gee AJ, Davidson CL. Polymerization contraction and conversion of light-curing bisGMA-based methacrylate resins. Biomaterials 1993;14:871-875. 187. Shortall AC, Wilson HJ, Harrington E. Depth of cure of radiation-activated composite restoratives - Influence of shade and opacity. J Oral Rehabil 1995;22:337-342. 188. Pearson GJ, Longman CM. Water sorption and solubility of resin-based materials following inadequate polymerization by a visible-light curing system. J Oral Rehabil 1989;16:57-61. 189. Fan PL, Wozniak WT, Reyes WD, Stanford JW. Irradiance of visible lightcuring units and voltage variation effects. J Am Dent Assoc 1987;115:442425. 190. Ferracane JL. Correlation between hardness and degree of conversion during the setting reaction of unfilled dental restorative resins. Dent Mater 1985;1:11-14. 191. Sakaguchi RL, Berge HX. Reduced light energy density decreases post-gel contraction while maintaining the degree of conversion. J Dent 1998;26:695-700. 192. Ferracane JL, Greener EH. Fourier transform infrared analysis of degree of polymerization in unfilled resins-methods comparison. J Dent Res 1984;63:1093-1095. 193. Louden JD, Roberts TA. Cure profiles of light-cured dental composites by raman-spectroscopy. J Raman Spectroscopy 1983;14:356-366. 194. Pianelli C, Devaux J, Bebelman S, Leloup G. The micro-Raman spectroscopy, a useful tool to determine the degree of conversion of lightactivated composite resins. J Biomed Mater Res 1999;48:675-681. 195. Soh MS, Yap AU, Yu T, Shen ZX. Analysis of the degree of conversion of LED and halogen lights using micro-Raman spectroscopy. Oper Dent 2004;29:571-577. 196. Stansbury JW, Dickens SH. Determination of double bond conversion in dental resins by near infrared spectroscopy. Dent Mater 2001;17:71-79. 197. Eliades G, Vougiouklakis GJ, Caputo AA. Degree of double bond conversion in light-cured composites. Dent Mater 1987;3:19-25. 198. Imazato S, McCabe JF, Tarumi H, Ehara A, Ebisu S. Degree of conversion of composites measured by DTA and FTIR. Dent Mater 2001;17:178-183. 233 199. Tanimoto Y, Hayakawa T, Nemoto K. Analysis of photopolymerization behavior of UDMA/TEGDMA resin mixture and its composite by differential scanning calorimetry. J Biomed Mater Res B Appl Biomater 2005;72:310-315. 200. Loshaek S, Fox TG. Cross-linked polymers. I. Factors influencing the efficiency of crosslinking in copolymers of methyl methacrylate and glycol dimethacrylates. J Am Chem Soc 1953;75:3544-3550. 201. Yoon TH, Lee YK, Lim BS, Kim CW. Degree of polymerization of resin composites by different light sources. J Oral Rehabil 2002;29:1165-1173. 202. Rueggeberg FA, Caughman WF, Curtis JW, Davis HC. Factors affecting cure at depths within light-activated resin composites. Am J Dent 1993;6:91-95. 203. Baharav H, Abraham D, Cardash HS, Helft M. Effect of exposure time on the depth of polymerization of a visible light-cured composite resin. J Oral Rehabil 1988;15:167-172. 204. Ferracane JL. Dental composites: Present status and research directions. Second International Congress on Dental Materials 1993:45-53. 205. Soh MS, Yap AU, Siow KS. Comparative depths of cure among various curing light types and methods. Oper Dent 2004;29:9-15. 206. Hasegawa I, Sakka S, Sugahara Y, Kuroda K, Kato C. Silicate anions formed in tetramethylammonium silicate methanolic solutions as studied by 29 Si nuclear magnetic resonance. J chem soc chem commun 1989:208-210. 207. de Gee AJ, ten Harkel-Hagenaar E, Davidson CL. Color dye for identification of incompletely cured composite resins. J Prosthet Dent 1984;52:626-631. 208. Fowler CS, Swartz ML, Moore BK. Efficacy testing of visible light-curing units. Oper Dent 1994;19:47-52. 209. Lloyd CH, Scrimgeour SN, Chudek JA, Mackay RL, Hunter G, Pananakis D, Abel EW. Determination of the depth of cure for VLC composites by nuclear magnetic resonance microimaging. Dent Mater 1994;10:128-133. 210. Braden M, Clarke RL. Water absorption characteristics of dental microfine composite filling materials. Biomaterials 1984;5:369-372. 211. Mohsen NM, Craig RG. Hydrolytic stability of silanated zirconia-silicaurethane dimethacrylate composite resins. J Oral Rehabil 1995;22:213-220. 212. Ferracane JL. Hygroscopic and hydrolytic effects in dental polymer networks. Dent Mater 2005:1-12. 234 213. Venz S, Dickens B. NIR-spectroscopic investigation of water sorption characteristics of dental resins and composites. J Biomed Mater Res 1991;25:1231-1248. 214. Kalachandra S, Turner DT. Water sorption of polymethacrylate networks: bis-GMA/TEGDMA copolymers. Biomed Mater Res 1987;21:329-338. 215. Braden M, Davy KW. Water absorption characteristics of some unfilled resins. Biomaterials 1986;7:474-475. 216. Sideridou I, Tserki V, Papanastasiou G. Study of water sorption, solubility and modulus of elasticity of light-cured dimethacrylate-based dental resins. Biomaterials 2003;24:655-665. 217. Sideridou I, Achilias DS, Spyroudi C, Karabela M. Water sorption characteristics of light-cured dental resins and composites based on BisEMA/PCDMA. Biomaterials 2004;25:367-376. 218. Weinmann W, Thalacker C, Guggenberger R. Siloranes in dental composites. Dent Mater 2005;21:68-74. 219. Ishida H, Low HY. A study on the volumetric expansion of benzoxazinebased phenolic resin. 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-