Effect of polyacid aqueous solutions on photocuring of polymerizable
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Effect of polyacid aqueous solutions on
E. Andrzejewska et al. / Dental Materials 19 (2003) 501–509
502 specially designed for photochemical measurements was applied. Throughout the experiments, the DSC unit was operated isothermally at 37 ^ 0.01 8C. The 10 mg samples were polymerized in open aluminum pans of 6.6 mm of diameter. The polymerizations were carried out under air or in a neutral atmosphere. In the second case the DSC chamber was purged with argon of special purity (,0.0005% of O 2 ) for 10 min prior to irradiation to remove oxygen from the chamber and that dissolved in the sample. DSC enables monitoring of the polymerization rate as a function of time by the measurement of heat evolved during polymerization. The DSC data obtained were analyzed for the corrected base line. The heat flux registered was recalculated into rate of polymerization by using the heat of reaction per methacrylate double bond 56 kJ/mol [6] . The polymerization rate R p was expressed as percent of double bonds reacted per second. After numerical integration of the rate versus time data, the plots of double bond conversion p versus time were obtained. Conversion at the time at which the polymerization rate decreased to 0 was taken as the final conversion. The measurement of each kinetic curve was repeated 3 – 5 times. The weight loss of the sample due to water and/or HEMA evaporation during a single measurement was about 5%. Due to this and because evaporation is an endothermic process, which overlaps on the exothermic effect of the polymerization, the reproducibility of the results was somewhat decreased. Polymerization reactions were initiated by the UV- initiator, DMPA, and by a two-component visible-light initiating system commonly used in photocurable dental compositions and based on CQ. The UV-initiator was used to exclude the possible effect of the additives investigated on the formation of initiating radicals. This enabled observation of the effect of the additives on the pure polymerization process only. For UV-initiation, the irradiation was performed with a medium pressure Hg lamp equipped with 310 – 400 nm filter (maximum transmittance at 366 nm). The initiating light intensity was 1.42 mW/cm 2 at the sample pan position. For visible-light initiation, the whole spectrum of a Philips 500 W GY 9.5 halogen lamp was used (total light intensity 415 mW cm 22 ). The first set of experiments involved studies of the photopolymerization of the two monomers induced by UV-light using DMPA at concentrations 0.02 M. Photo- polymerization was performed for the monomers alone and in the presence of additives. HEMA-based formulations containing 5 and 10 wt% of 45% aqueous solution of PAA and 5 and 10 wt% of AC were studied. The choice of the amounts of acid solutions, which were added to formu- lations, was imposed by limited miscibility of these solutions with HEMA. For TEGDMA photopolymerization only influence of 3% of PAA solution was studied due to its very limited solubility in the monomer. Photopolymerization was carried out in air (conditions as in practice) and in Ar atmosphere (to exclude the inhibiting effect of oxygen). The second set of experiments involved visible light initiated photopolymerization of TEGDMA and HEMA in the presence of 3% (TEGDMA) and 5% (HEMA) of PAA solution. Initiating radicals were formed in the reaction of excited CQ with the monomer and/or with two coinitiators: DMT, commonly used in dental applications [8] , and MBO. CQ and the coinitiators were added in equimolar amount (0.06 M). Because many kinetic curves were obtained in the course of the work and listing of the all statistical results would take too much space, for all the R p max values obtained, we calculated a range of standardized standard deviations, i.e. standard deviation divided by mean R p max value from the all measurements and multiplied by 100%. The mean value of such standard deviations was 0.57%, with the minimum value 0.11% and the maximum value 5.18%. 3. Results The results of HEMA photopolymerization initiated with 0.02 M DMPA in Ar in the absence and in the presence of AC and PAA solution are given in Figs. 1 (5% of the additives) and 2 (10% of the additives). The kinetic curves display the dependence of the polymerization rate R p expressed as a fraction of double bonds reacted per second ( Figs. 1(a) and 2(a) ) and the degree of conversion of double bonds p ( Figs. 1(b) and 2(b) ) on the irradiation time t. Figs. 1(c) and 2(c) show the polymerization rate as a function of degree of conversion of double bonds. The exemplary list of the parameters characterizing the polym- erization curves presented in Figs. 1 and 2 with confidence intervals for R p max is given in Table 1 . The polymerization of HEMA proceeds with the gel effect (autoacceleration), which sets in nearly 20% of double bond conversions ( Figs. 1(c) and 2(c) ). Maximum polymerization rate (R p max ) occurs at about 40% of conversion and the final degree of conversion, p f , reaches , 90% under the conditions used. As can be seen from Figs. 1 and 2 , addition of polyacid solutions causes earlier onset of autoacceleration and shortening of the time t Download 222.27 Kb. Do'stlaringiz bilan baham: |
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