Irini D. Sideridou

Effect of chemical structure on degree of conversion in light-cured dimethacrylate-based dental resins.

In this work the room-temperature photopolymerization of Bis-GMA, Bis-EMA, urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) induced by camphoroquinone/N,N-dimethylaminoethyl methacrylate, as photo-initiator system, was followed by FT-IR. The results obtained were then fitted by a non-linear least square method to a rational function, which permitted the accurate calculation of the limiting degree of conversion. The latter was found to increase in the order Bis-GMA < Bis-EMA < UDMA < TEGDMA. This trend is discussed in connection with the chemical structure of dimethacrylates. The photopolymerization of mixtures of Bis-GMA/TEGDMA, Bis-GMA/UDMA and Bis-GMA/Bis-EMA showed a good linear relationship of degree of conversion with the mole fraction of Bis-GMA and in the case of the first pair also with the Tg of the initial monomer mixture.

Study of water sorption, solubility and modulus of elasticity of light-cured dimethacrylate-based dental resins.

Polydimethacrylate resins were prepared by photopolymerization of Bis-GMA, TEGDMA, UDMA or Bis-EMA (4) monomer, initiated by camphoroquinone/N,N-dimethylaminoethyl methacrylate system. The study of physical properties of these resins showed that TEGDMA seems to create the most dense polymer network, which however is the most flexible (0.74GPa), absorbs the highest amount of water (6.33 wt%) and releases the lowest amount of unreacted monomer (2.41 microg/mm(3)). UDMA and Bis-EMA (4) create more rigid networks, which absorb lower water and release higher unreacted monomer than TEGDMA. Bis-EMA (4) absorbs the lowest water amount (1.79 wt%) and releases the highest amount of unreacted monomer (14.21 microg/mm(3)). Bis-GMA leads to the formation of the most rigid network (1.43 GPa), which absorbs lower water than the resin made by TEGDMA but higher than the resin made by UDMA and Bis-EMA (4). Copolymers of Bis-GMA with the other monomers were also prepared, using various monomer combinations and molar ratios. Copolymers Bis-GMA/TEGDMA (50/50 and 70/30 wt%) showed significantly higher values for Youngs modulus (1.83 and 1.78 GPa) than those predicted by the linear dependence of the values on the copolymer composition. Gradual replacement of TEGDMA with UDMA or/and Bis-EMA (4) in copolymerization with Bis-GMA resulted in more flexible resins with lower water sorption and higher solubility values, depending on the TEGDMA content.

Water sorption characteristics of light-cured dental resins and composites based on Bis-EMA/PCDMA

The water uptake characteristics of resins and composites based on an ethoxylated bisphenol A glycol dimethacrylate (Bis-EMA) and a polycarbonate dimethacrylate (PCDMA) were studied in detail. Polydimethacrylate resins were prepared by photopolymerization of the neat monomers and mixtures of them with various weight ratios, using the camphoroquinone/N,N-dimethylaminoethyl methacrylate system as initiator, while the composites were prepared from the light-curing of commercial samples (Sculpt-It and Alert). Water sorption/desorption was examined both in equilibrium and dynamic conditions in two adjacent sorption-desorption cycles. The equilibrium water uptake from all resins was very small with a trend to increase as the amount of PCDMA was increased. The inverse effect was observed in the solubility values. The composites studied exhibited also very low water uptake values in comparison to other composite materials reported in the literature. It was also observed that the equilibrium uptake decreased with increasing filler loading. Slightly larger equilibrium water uptake and much smaller solubility values were obtained during the second sorption-desorption cycle in comparison to the first one. Concerning the sorption rate data, it was observed that the resin materials followed Fickian diffusion during almost the whole sorption or desorption curve, while the composites showed this behavior until only M(t)/M( infinity ) congruent with 0.5. The diffusion coefficients calculated for the resins were larger than those of the composites and always higher during desorption compared to sorption. The values of the diffusion coefficients for both resins and composites were in the same order of magnitude with the values of the corresponding materials reported in the literature.

Physical properties of current dental nanohybrid and nanofill light-cured resin composites

OBJECTIVES The purpose of this work was the detailed study of sorption characteristics of water or artificial saliva, the determination of flexural strength and the flexural modulus, and the study of the thermal stability of some current commercial dental light-cured nanocomposites containing nano-sized filler particles. METHODS Three nanohydrid dental composites (Tetric EvoCeram (TEC), Grandio (GR) and Protofill-nano (PR)) and two nanofill composites (Filtek Supreme Body (FSB) and the Filtek Supreme Translucent (FST)) were used in this work. The volumetric shrinkage due to polymerization was first determined. Also the sorption, solubility and volumetric increase were measured after storage of composites in water or artificial saliva for 30 days. The flexural strength and flexural modulus were measured using a three-point bending set-up according to the ISO-4049 specification, after immersion of samples in water or artificial saliva for 1 day or 30 days. Thermal analysis technique TGA method was used to investigate the thermal stability of composites. RESULTS GR and TEC composites showed statistically no difference in volumetric shrinkage (%) which is lower than the other composites, which follow the order PR<FSB<FST. The amount of sorbed water and solubility is not statistically different than those in artificial saliva. In all the composites studied the amount of water, which is sorbed (% on composite) is not statistically different than the amount of water, which is desorbed and follows the order: GR<TEC<PR<FSB<FST. After immersion in water for 1 day the highest flexural strength showed the FSB and the lowest TEC. GR, PR and FST showed no statistically different flexural strength. The flexural modulus of composites after immersion for one day follows the order TEC<PR≤FST<FSB<GR. SIGNIFICANCE Among the composites studied, Grandio had the lowest polymer matrix content, consisting mainly of Bis-GMA. It showed the lowest polymerization shrinkage and water sorption and the highest flexural strength and flexural modulus after immersion in water or artificial saliva for 30 days. The water and artificial saliva generally showed the same effect on physical properties of the studied composites. Thermogravimetric analysis gave good information about the structure and the amount of organic polymer matrix of composites.

Effect of the structure of silane coupling agent on sorption characteristics of solvents by dental resin-nanocomposites

OBJECTIVE This work was aimed at the study of sorption characteristics of water or ethanol/water solution (75vol%/25vol%) by dental nanocomposites consisting of a Bis-GMA/TEGDMA (50/50, wt/wt) matrix and silica nanoparticles (Aerosil OX50) as filler, silanized with various silanes. The silanes used were 3-[(1,3(2)-dimethacryloyloxypropyl)-2(3)-oxycarbonylamido]propyltriethoxysilane, which is a urethane dimethacrylate silane (UDMS), 3-methacryloxypropyltrimethoxysilane (MPS), octyltrimethoxysilane (OTMS), blends UDMS/OTMS (50/50, wt/wt) or blends MPS/OTMS (50/50, wt/wt). METHODS The composite specimens (15mm diameterx1mm height) were immersed in water or ethanol/water solution at 37+/-1 degrees C for 30 days. At fixed time intervals they were removed, weighed and returned to the liquid for the study of sorption kinetics. Then desorption kinetics was studied by heating the samples in a drying oven maintained at 37 degrees C. The sorbed or desorbed liquid at equilibrium, diffusion coefficients, solubility, and volume increase after sorption were calculated. The sorption/desorption cycle was repeated for a second time. RESULTS The silane structure used for the silanization of nanosilica was found to affect the sorption behavior of water or ethanol/water solution (37 degrees C) by the composites. The composite containing the UDMS with the hydrophilic urethane group showed the highest amount of sorbed water. The composite with the OTMS, which does not contain a methacrylate moiety and cannot react with the dimethacrylate monomers showed the highest solubility both in water and ethanol/water. In all composites the amount of sorbed ethanol/water solution is much higher than that of water. OTMS-composite sorbed the highest amount of ethanol/water and the MPS-composite with the 3-methacryloxypropyltrimethoxysilane the lowest. SIGNIFICANCE The silane structure used for the silanization of nanosilica has an effect on the solvent sorption and solubility of composites.

Synthesis and study of properties of dental resin composites with different nanosilica particles size

OBJECTIVES The aim of this work was the synthesis of light-cured resin nanocomposites using nanosilica particles with different particle size and the study of some physical-mechanical properties of the composites. METHODS Various types of silica nanoparticles (Aerosil) with average particle size of 40, 20, 16, 14, and 7 nm, used as filler were silanized with the silane 3-methacryloxypropyl-trimethoxysilane (MPS). The total amount of silane used was kept constant at 10 wt% relative to the filler weight to ensure the complete silanization of nanoparticles. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (55 wt%) were mixed with a photoactivated Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by dynamic mechanical analyzer (DMA). Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution 75 vol% for 30 days. The TGA for composites was performed in nitrogen atmosphere from 30 to 700 °C. RESULTS As the average silica particle size decreases, the percentage amount of MPS attached on the silica surface increases. However, the number of MPS molecules attached on the silica surface area of 1 nm(2) is independent of filler particle size. As the average filler particles size decreases a progressive increase in the degree of conversion of composites and an increase in the amount of sorbed water is observed. SIGNIFICANCE The prepared composites containing different amount of silica filler, with different particle size, but with the same amount of silanized silica and organic matrix showed similar flexural strength and flexural modulus, except composite with the lowest filler particle size, which showed lower flexural modulus.

Volumetric dimensional changes of dental light-cured dimethacrylate resins after sorption of water or ethanol

OBJECTIVES This study evaluated the influence of water and ethanol sorption on the volumetric dimensional changes of resins prepared by light curing of Bis-GMA, Bis-EMA, UDMA, TEGDMA or D(3)MA. METHODS The resin specimens (15mm diameterx1mm height) were immersed in water or ethanol 37+/-1 degrees C for 30 days. Volumetric changes of specimens were obtained via accurate mass measurements using Archimedes principle. The specimens were reconditioned by dry storage in an oven at 37+/-1 degrees C until constant mass was obtained and then immersed in water or ethanol for 30 days. The volumetric changes of specimens were determined and compared to those obtained from the first sorption. RESULTS Resins showed similar volume increase during the first and second sorptions of water or ethanol. The volume increase due to water absorption is in the following order: poly-TEGDMA>poly-Bis-GMA>poly-UDMA>poly-Bis-EMA>poly-D(3)MA. On the contrary, the order in ethanol is poly-Bis-GMA>poly-UDMA>poly-TEGDMA>poly-Bis-EMA approximately poly-D(3)MA. The volume increase was found to depend linearly on the amount of water or ethanol absorbed. SIGNIFICANCE In the choice of monomers for preparation of composite resin matrix the volume increase in the resin after immersion in water or ethanol must be taken into account. Resins of Bis-EMA and D(3)MA showed the lowest values.

Thermal Behavior and Tensile Properties of Poly(ethylene terephthalate-co-ethylene isophthalate)

Poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI) homopolymers were synthesized by the two-step melt polycondensation process of ethylene glycol (EG) with dimethyl terephthalate (DMT) and/or dimethyl isophthalate (DMI), respectively. Nine copolymers of the above three monomers were also synthe- sized by varying the mole percent of DMI with respect to DMT in the initial monomer feed. The thermal behavior was investigated over the entire range of copolymer com- position by differential scanning calorimetry (DSC). The glass transition (Tg), cold crystallization (Tcc), melting (Tm), and crystallization (Tc) temperatures have been determined. Also, the gradually increasing proportion of ethyleno-isophthalate units in the virgin PET drastically differentiated the tensile mechanical properties, which were determined, and the results are discussed.

Cure Kinetics Study of Two Epoxy Systems with Fourier Tranform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC)

This work was aimed at the study of cure kinetics of two commercial thermosetting epoxy systems, Epikote resin 816 LV/Epikure F205 and Epikote resin 240/Epikure F205, by Fourier Tranform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). The studied systems consist of a resin (A), based on a diglycidyl ether of bisphenol A and a hardener (B) based on the Isophorodiamine (IPDA) a cycloaliphatic diamine. These systems are used for the building and civil engineering industries, e.g. flooring compounds, adhesives, mortars and grouts. FTIR spectroscopy was employed to investigate the isothermal curing kinetics at 30, 50 or 70°C and DSC analysis to study the non-isothermal curing kinetics at different heating rates 2.5, 5, 10 and 20°C/min, from 20 to 300°C. A kinetic model was employed to simulate the FTIR isothermal experimental data using two kinetic rate constants and incorporating also diffusion control at high degrees of conversion. Finally, the variation of the effective activation energy with the extent of curing was estimated using isoconversional analysis of non-isothermal DSC data.

STUDY OF THE EFFECT OF TWO BPO/AMINE INITIATION SYSTEMS ON THE FREE-RADICAL POLYMERIZATION OF MMA USED IN DENTAL RESINS AND BONE CEMENTS

ABSTRACT The kinetics of the free radical bulk polymerization of methyl methacrylate (MMA) was studied by DSC, using the benzoyl peroxide (BPO)/amine initiation system. N,N dimethyl-4-aminophenethyl alcohol (DMPOH), which is a newly synthesized and used amine in the preparation of acrylic dental resins and bone cements was examined, and the results compared to the most commonly used in these applications amine, the N,N dimethyl-p-toluidine (DMT). For both amines, the effect of the molar ratio of BPO/amine and of the reaction temperature, on the polymerization kinetics was investigated. The prepared polymers were characterized by determination of the average molecular weights (M¯ n and M¯ w ) and molecular weights distribution (M¯ w /M¯ n ) using Gel Permeation Chromatography. DMPOH was found to lead in slightly higher polymerization rates, lower gel times and lower molecular weights than DMT. The values of these parameters for both amines were influenced by the molar ratio of BPO to amine, when the product of the concentrations of these was kept constant. The highest polymerization rate occurred in the lowest gel time, resulting in polymers with the lowest molecular weight, and was observed when a molar ratio of about 1.5 BPO/amine was used. However, the final monomer conversion was found to be independent of the molar ratio and amine used. The activation energy of polymerization was found to be 51.8 kJ/mol K for BPO/DMPOH and 47.1 kJ/mol K for BPO/DMT.