Gustavo Arenas

Photobleaching of camphorquinone during polymerization of dimethacrylate-based resins

OBJECTIVE The aim of this study was to compare the photobleaching rate of CQ in different dental resins. METHODS The photodecomposition rate of CQ/amine system in bis-GMA/TEGDMA, bis-EMA and UDMA polymerizing monomers was evaluated at different light intensities. The photobleaching of the CQ was studied by monitoring the decrease in light absorption as a function of continuous irradiation time. The absorption changes were assessed by recording the transmitted light that passed through samples of monomers containing CQ/amine. RESULTS Complete photobleaching of CQ was observed in all the monomer tested and the rate constant for the photobleaching was proportional to the radiation intensity. Hydrogen abstraction from amines by the excited CQ state via electron transfer and direct hydrogen abstraction from monomer structures were involved in the CQ photoreduction. CQ was photobleached in the absence of coinitiator in a dimethacrylate monomer containing a carbamate functional group (UDMA). This behavior was attributed to the presence of labile hydrogen atoms in the UDMA monomer. The CQ photobleaching rate constant in UDMA containing CQ/amine was similar to that in UDMA in the absence of amine. Moreover, the efficiency of CQ to photoinitiate the polymerization of UDMA in the absence of amine demonstrated that the radicals derived from the UDMA monomer via hydrogen abstraction are highly reactive toward double bonds. SIGNIFICANCE CQ photoinitiates the polymerization of the UDMA monomer in the absence of amine and the efficiency of this process is comparable to that of traditional bis-GMA and bis-EMA monomers activated with CQ/amine.

Influence of thermal expansion on shrinkage during photopolymerization of dental resins based on bis-GMA/TEGDMA.

OBJECTIVE The aim of this study was to assess volume changes that occur during photopolymerization of unfilled dental resins based on bis-GMA-TEGDMA. METHODS The resins were activated for visible light polymerization by the addition of camphorquinone (CQ) in combination with dimethylamino ethylmethacrylate (DMAEMA) or ethyl-4-dimethyl aminobenzoate (EDMAB). A fibre-optic sensing method based on a Fizeau-type interferometric scheme was employed for monitoring contraction during photopolymerization. Measurements were carried out on 10mm diameter specimens of different thicknesses (1 and 2mm). RESULTS The high exothermic nature of the polymerization resulted in volume expansion during the heating, and this effect was more pronounced when the sample thickness increased. Two approaches to assess volume changes due to thermal effects are presented. Due to the difference in thermal expansion coefficients between the rubbery and glassy resins, the increase of volume due to thermal expansion was greater than the decrease in volume due to thermal contraction. As a result, the volume of the vitrified resins was greater than that calculated from polymerization contraction. The observed trends of shrinkage versus sample thickness are explained in terms of light attenuation across the path length during photopolymerization. SIGNIFICANCE Results obtained in this research highlight the inherent interlinking of non-isothermal photopolymerization and volumetric changes in bulk polymerizing systems.

Contraction Measurements of Dental Composite Material during Photopolymerization by a Fiber Optic Interferometric Method.

In order to monitor the shrinkage generated by new composites during photopolymerization, we have implemented a fiber optic sensing method based on a Fizeau‐type interferometric scheme. This simple, compact, non‐invasive and self‐calibrating system competes with both conventional and other high‐resolution bulk interferometric techniques. Several stages of the curing process were characterized from sample interferograms, including photopolymerization inhibition, the onset and degree of sample shrinkage, as well as typical kinetic behaviour of photopolymerization of dimethacrylates monomers. Some complementary studies (temperature and transmission measurements) were performed to complete the kinetic scheme. A simple polymerization model was generated in order to discuss the main results.

Vitrification of photo-curing resins by embedded cantilever and Fizeau interferometer

We present a method for estimating the relative viscosity and vitrification degree of photo-curing polymeric materials. Real-time knowing of solidification transition provides important information for many scientific areas. The technique involves the embedding of one end of a small aluminum cantilever subjected to oscillations of constant amplitude and frequency, into blends made of unfilled dental resins based on bis-GMA-TEGDMA. These resins were activated for visible light polymerization by the addition of camphorquinone (CQ) in combination with dimethylamino ethylmethacrylate (DMAEMA), or ethyl-4-dimethyl aminobenzoate (EDMAB). As the polymerization process progresses, the embedded end of the beam ceases to be free. Thus, changes in the oscillation amplitude at a given point near the lower end measured by a Fizeau fiber optic interferometer can be interpreted as a proportional indicator of solidification in the resin being photocured. Proper measurement of this phenomenon provides a more quantitative support in the early stages of photo-polymerization. Our results show that the resin passes from liquid to gel, and then to a vitreous state, in a short time compared with the total photo-curing evolution. As a consequence, after this transition, polymerization seems to develop mainly by diffusive processes.

Analysis of temporal and spectral response of an optical fiber Fizeau interferometer applied to the study of photocurable resins

In this paper an optical fiber Fizeau interferometer is used to obtain dimensional changes that occur during the solidification process of photocurable polymers, even if there is a reversal of the corresponding deformation. The simultaneous detection of both temporal and spectral evolutions of the interferometric signal allows recognizing the existence of thermal or mechanical processes by detecting any change in the direction of movement (expansion/contraction) or in the chemical reaction rate. As an example it is shown the analysis of the curing process of an experimental photocurable dental resin. The results obtained show a perfect agreement between temporal and spectral methods.