Observation of oxygen inhibited layer of organic dental resin by confocal Raman-microscopy

This study investigated degrees of conversion of oxygen inhibited layer (OIL) of organic dental resins for restoration using Confocal-Raman spectroscopy. The aim was to determine which laser is adapted to determine the degrees of conversion of OIL and to measure variations of thickness and degrees of conversion in OIL with respect to monomers proportions. Bis-GMA (bis-phenol A glycidyl dimethacrylate) and TEGDMA (triethylene glycol dimethacrylate) based resins with various ratio of an equimolecular mixture of camphorquinone/EDMAB (ethyl (4-dimethyl amino) benzoate) were studied with different lasers by confocalRaman spectroscopy. Results show that this technique is adapted for the non destructive measurement of OIL. The Thickness of OIL is not correlated with the proportions of Bis-GMA and TEGDMA in the resin and was close to 3-4μm. Thickness of OIL is very thin without inorganic fillers (3 or 4 μm). Inorganic fillers might be responsible of greater OIL in composite resins. Introduction Organic dental resins are commonly used in dentistry restoration. These materials are polymerized directly into the mouth under a light source. However, some problems remain like the existence of surface non polymerized layer. This phenomenon is due to oxygen which inhibits polymerization. A low polymerization led to poor mechanical properties, undesirable color and dimensional instabilities. In some cases, a low polymerization degree can appear as a useful characteristics and help linking together successive thicknesses of resin [1]. The biocompatibility of the final material can be affected [2] by release of formaldehyde for instance [3]. Typically, commercial dental composites are composed of a mixture of monomers like 2,2-bis [4-(2-hydroxy-3-methacryloxypropoxy) phenyl] propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) to adjust viscosity [4]. Initiators added to these dimethacrylate monomers mixtures initiate polymerization under a light source producing radicals. In the peculiar case of camphorquinone initiator, co-catalysts such as amines are needed to perform the polymerization [5-7]. Organic resins are more often filled (>80w.%) with different types of inorganic fillers like silica, borosilicate glasses, and strontium glasses to improve the resistance of the final composite (rigidity, hardness...). Authenticated | frederic.cuisinier@univ-montp1.fr author's copy Download Date | 9/8/13 10:17 AM 2 When dentists perform a filling with composites in the mouth of their patient, light curing occurs under atmospheric air and so, in the presence of oxygen. This gas modifies the efficiency of photoinitiators and lead to stable peroxides which cannot initiate the polymerization resulting in a non polymerized layer or in a partial conversion of monomers. The OIL (oxygen inhibited layer) thickness of composite materials containing inorganic fillers ranges from 4 to 40 μm and depends on the composition and nature of the monomers in the mixture, the morphology of fillers, the temperature, the concentration in radicals and the rate of oxygen consumption [8]. The aim of this study was to determine if Confocal Raman spectroscopy is adapted to point out this OIL layer and to determine experimental conditions to measure its thickness as a function of the composition of the resin. Nevertheless, this technique is not perfectly adapted to filled resins where scattering of the light can lead to a complete loss of confocality. Results and discussion At first, one sample (Bis-GMA/TEGDMA 60%/40% w/w) was studied with different laser types (Figure 1-4). Fig. 1. Raman spectra of the two groups (aromatic and methacrylate groups) with a 473 nm laser (LabRAM ARAMIS-IR2) on cured sample 4. Fig. 2. Raman spectra of the two groups (aromatic and methacrylate groups) with a 785 nm laser (LabRAM ARAMIS-IR2) on cured sample 4. Authenticated | frederic.cuisinier@univ-montp1.fr author's copy Download Date | 9/8/13 10:17 AM 3 The gradient of intensity close to the surface is only observed using 633 nm laser wavelength. For lasers operating at 473, 785 and 532 nm, surprisingly, no significant variations of peak intensities were observed between the Raman spectrum of the surface and the Raman spectrum in 50 μm depth. It is possible that these lasers photopolymerize the surface of the sample during the analysis resulting in any absence of differences on Raman spectra between the surface and 50 μm of depth. For the following part of the study, only a 633 nm laser was used. We can note a variation of the peak intensity of the methacrylate groups between cured and uncured samples (Fig.4.) but also, between the surface and the depth of the material (Fig.5.). Fig. 3. Raman spectra of the two groups (aromatic and methacrylate groups) with a 532 nm laser (Alpha 300R) on cured sample 4. Fig. 4. Raman spectra of the two groups (aromatic and methacrylate groups) with a 633 nm laser (LabRAM) on sample 4 cured and uncured. The evolution of the degrees of conversion as a function of the depth for samples 1 to 4 with different compositions was calculated as a function of the depth giving curves DP=f (Depth) (Fig.6). Authenticated | frederic.cuisinier@univ-montp1.fr author's copy Download Date | 9/8/13 10:17 AM 4 Neg: upper the surface of the resin Pos: downer than the surface of the resin\ Fig. 5. Example of spectra obtained as a function of the depth for sample 4 (LabRAM ARAMIS-IR2). Fig. 6. Evolution of the degree of polymerization as a function of the depth. Confocal Raman microscope explores samples from the top to the down at different depths. The window of observation is not perfectly bidimensional, in other words, the analyzer of the confocal microscope measures the signal of a given area but also, on a low thickness. So, it explains the fact that no conversion was observed at the starting point of analysis. We can observe that the composition in monomers of the resins has no impact on the degrees of conversion of the resins which reaches 65% in our conditions. The measured oxygen inhibited layer was close to 3-4 μm independently of the composition of the samples. This thickness is in agreement with values reported in literature and measured with different methodologies. In conclusion, the measure of the Oxygen Inhibited Layer appearing during UV crosslinking of dental resins has to be determined since it can be useful to help linking together successive thicknesses of resin. The determination of the depth of this layer is important also in order to improve the properties of the final material and to optimize the curing cycle. So, Confocal Raman Microscopy was proved to be a powerful non-destructive technique to observe the inhibited surface of organic composite materials for dental restoration. Using a 633 nm laser, the inhibited layer was close to 3-4 nm independently of the composition of the organic matrix. Authenticated | frederic.cuisinier@univ-montp1.fr author's copy Download Date | 9/8/13 10:17 AM 5 Experimental part Sample preparation: TEGDMA (Aldrich, France) and Bis-GMA (Aldrich, France) were mixed in a one neck round bottom flask in different proportions (Bis-GMA/TEGDMA w/w: 90%/10%, 80%/20%, 70%/30%, 60%/40%). Camphorquinon (Aldrich, France) (photoinitiator) and EDMAB (ethyl (4-dimethyl amino) benzoate) (Aldrich, France) (co-initiator) were used in the same molar proportions in all samples (camphorquinon 1w.% with respect to the resin mixture, [EDMAB]/[camphorquinon]=1). Finally, addition of dichloromethane (Aldrich, France) to the mixture permitted to mix the solution during eight hours to be sure that the composition was homogeneous. Later, dichloromethane was removed by evaporation under vacuum. The different samples (Tab. 1.) were stored in the dark at -18 °C until observation or photopolymerization. Tab.1. Composition of the samples studied. Bis-GMA TEGDMA Sample 1 90 % (w/w) 10 % (w/w) Sample 2 80 % (w/w) 20 % (w/w) Sample 3 70 % (w/w) 30 % (w/w) Sample 4 60 % (w/w) 40 % (w/w) These different samples were studied to evaluate: the effect of proportions of Bis-GMA or TEGDMA on the size of the OIL, the degrees of conversion of monomers in the OIL. The different samples were polymerized in steel moulds as cylindrical specimens using a light source ELIPAR® trilight. Power was set to 800 mW/cm2 during 40 seconds in the standard mode. The Raman spectroscopy measurements were achieved directly on the samples released from the mould without any specific preparation. Raman measurements: All the Raman measurements were carried out with a LabRAM ARAMIS-IR2 (HORIBA JOBIN YVON) Raman confocal spectrometer (objective x100, N.A. 0.90 and 3 lasers: 475 nm, 633 nm, 785 nm) and an Alpha 300R confocal Raman spectrometer (WITec) (objective x100, N.A. 0.95 and a laser of 532 nm). The thickness resolution was close to 2.5-3.5 microns. Calculation of the degree of conversion (DC) of monomers: The height of absorption peaks at 1638 cm (C=C aliphatic bond of methacrylate groups) varying during curing was measured with respect to the height of the constant absorption at 1610 cm (C=C of aromatic rings) (Fig.7.).