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
505 either in the reaction rate or in the final double bond conversion for the polymerization of formulations with and without PAA solution. 4. Discussion The polyacid aqueous solution may influence photo- polymerization by physical and chemical effects. The most important physical effect is an increase in viscosity of the formulation. Another possibility is ordering of the hydrophobic polymer backbone regions induced by water and acid groups, which act as physical crosslinking [10] or hydrogen bonding between mono- mer, polymer and water. An undesirable physical effect may be phase separation during polymerization, especially of a less hydrophilic monomer, like TEGDMA. Phase separation can lead to turbidity of the polymerizing system, which may result in worse light penetration and in a decreased efficiency of initiation. The chemical effect involves the presence of readily abstractable tertiary hydrogens from the polyacid backbone or a change in dielectric constant of the reaction medium, which may affect the initiation process by solvation effects. Usually dilution of a polymerizing system by an inert solvent leads to a delay of autoacceleration, reduction of R p max , and in the presence of larger amounts of the solvent, to complete suppression of autoacceleration. The final degree of conversion often increases. This has been observed for polymerization of HEMA in the presence of various amounts of polyethylene glycol 400 [4] . The low solubility of solutions of polyacids in HEMA precluded the investi- gation of polymerization kinetics with the use of a wide range of additive concentrations. However, this limited solubility suggests that during the setting of RMGIs, polymerization of HEMA proceeds in the presence of about 10% by its weight of polyacid solution; the rest of the polyacid forms a separate phase. When photopolymerization is initiated by DMPA, the addition of polyacid solution causes the autoacceleration to set in earlier and p f is slightly reduced. In this case initiating radicals are formed in a photofragmentation reaction (reaction (1)), so the addition of polyacids should not interfere with the initiation process (the only competing Fig. 6. The kinetic curves of CQ-initiated HEMA photopolymerization in air showing the effect of addition of 5 wt% of 45% PAA aqueous solution. Solid line represents polymerization without additive, dashed lines represent polymerization in the presence of the additive. (1) Coinitiator MBO, (2) coinitiator DMT: (a) polymerization rate vs. irradiation time curves, (b) double bond conversion vs. irradiation time curves. Fig. 5. The kinetic curves of CQ-initiated HEMA photopolymerization in Ar showing the effect of addition of 5 wt% of 45% PAA aqueous solution. Solid line represents polymerization without the additive, dashed lines represent polymerization in the presence of the additive. (1) Coinitiator MBO, (2) coinitiator DMT, (3) no coinitiator: (a) polymerization rate vs. irradiation time curves, (b) double bond conversion vs. irradiation time curves. E. Andrzejewska et al. / Dental Materials 19 (2003) 501–509 506 reaction of newly formed radicals to addition to monomer is hydrogen abstraction, mainly from monomer) Autoacceleration (an increase in polymerization rate despite monomer consumption) results from limited diffusion of macroradicals and suppression of termin- ation due to increase in viscosity of the polymerizing linear systems or network formation in crosslinking systems. The final degree of conversion depends on mobility of the system (or of the network) in the final reaction stages. Earlier onset of autoacceleration during HEMA polymerization after addition of polyacids may be a consequence of increased viscosity, but slight reduction of conversion in association with faster deceleration ( Figs. 1(c) and 2(c) ) may also suggest an influence of physical crosslinking induced by polyacid/ water mixture. Much lower reaction rates and conversions obtained for the polymerization in air ( Fig. 3 ) result from the very strong Fig. 8. The kinetic curves of TEGDMA photopolymerization in air initiated by CQ/MBO couple showing the effect of addition of 3 wt% of 45% PAA aqueous solution. Solid line represents polymerization without the additive, dashed lines represent polymerization in the presence of the additive. (1) Coinitiator MBO, (2) no coinitiator: (a) polymerization rate vs. irradiation time curves, (b) double bond conversion vs. irradiation time curves. Fig. 7. The kinetic curves of CQ-initiated TEGDMA photopolymerization in Ar showing the effect of addition of 3 wt% of 45% PAA aqueous solution. Solid line represents polymerization without the additive, dashed lines represent polymerization in the presence of the additive. (1) Coinitiator MBO, (2) no coinitiator: (a) polymerization rate vs. irradiation time curves, (b) double bond conversion vs. irradiation time curves. ð 1Þ Download 222.27 Kb. Do'stlaringiz bilan baham: |
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