J. Phys. IV France 125 (2005) 761-763  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005125175 In-situ photo-polymerization study of Si-(bis-GMA)/TEGDMA by correlations of PA signals F. Rivera 1 , M. Navarrete 2 , R. Vera-Graziano 1 and H. Sobral 3 1 Instituto de Investigación en Materiales, UNAM, AP 70-360, Coyoacán 04510, DF, Mexico 2 Instituto de Ingeniería, UNAM, AP 70-472, Coyoacán 04510, DF, Mexico 3 CCADET, UNAM, AP 70-186, Coyoacán 04510, DF, Mexico Abstract. The photo-polymerization reaction of Si-(bis-GMA)/TEGDMA (a bis-GMA modified with silyl groups and mixed with TEDGMA) has been studied by pulsed photoacoustic (PA) and FTIR techniques. The light from a pulsed laser is focused on the surface of the sample for both to activate the chemical reaction and generate PA signals. The in-situ acquisition of the PA signals, during photo- polymerization (PP), in consecutive way, permits to follow changes in its physical properties. The structural changes during polymer formation are recovered by a numerical procedure based on correlation coefficients r i . This numerical procedure, applied to digitally recorded PA signals, allows the construction of a PP profile dr i /dT i , and permits to detect the phase transitions during the whole process including the gel region. The obtained results are in agreement with those obtained from the FTIR analysis, under similar conditions. 1. INTRODUCTION Acrylic dental polymer networks are obtained by photo-polymerization, PP, of dimethacrylate monomers. During PP, two-phase transitions occur: Liquid to viscoelastic and viscoelastic to glass. 2,2-bis-[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (bis-GMA) is widely used in dental composites because of its good mechanical properties [1]. However, its high viscosity makes difficult resin handling 2. This problem is overcome by mixing bis-GMA with reactive diluents like TEGDMA: triethylene glycol dimethacrylate 3. The resulting copolymer absorbs water after implantation. However, TEGDMA affects the mechanical properties [4]. To surpass this problem, bis- GMA has been modified by substituting its hydroxyl groups with silyl groups, (H(CH 3 )2C(CH 3 )2Si-), as described elsewhere [5]. The resulting silylated monomer, Si-(bis-GMA), is used to prepare the photoactivated dental resin and to be able to study the PP reaction. A procedure to acquire the PA signals in real time conditions and to process the signals by using correlation coefficients is discussed. The pulsed PA technique has been used to find out the chemical and physical properties of the materials [6]. The pulsed laser light is used here for both to activate the chemical reaction and to follow in-situ the change in physical properties, measured through the variation of the time-of-flight from PA signals as the reaction moves forward. This numerical procedure, programmed in MathLab ® and applied to the digitally recorded PA signals, is used for the construction of a PP profile dr i /dT i and also to visualize in graphic form the phase transitions during the PP process, including the gel region. Equation 1 relates PA signal with the optic-thermo-elastic parameters [7]. The laser pulses activate the reaction and the acquired signal is related to the absorption coefficient of the photo-initiator. The detected PA signal results from the non-irradiative process that takes place after excitation of the photo-initiator. Dividing the PA signal by the laser pulse energy E 0 , ) 10 1 ( ) ( ) ( ) ( ) ( A i i i o i T Cp T T KE PA t X             , (1) Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005125175