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
507 inhibitory effect of oxygen (reaction (2)) R z þ O 2 ! ROO z ð 2Þ where R z is an initiator derived radical or macroradical. The polymerization process begins after an induction period associated with consumption of dissolved oxygen by initiator derived radicals. After a sufficient drop of oxygen concentration, polymerization develops unequally through the sample thickness, because it is further inhibited near the surface by diffusing oxygen. Thus, autoacceleration is observed only in deeper layers and for this reason is registered at so low conversion as 1% of the total amount of double bonds present in the whole sample ( Fig. 3(c) ). The R p max value is very low and decelerating processes set in at low conversions, but after a time a second autoacceleration peak appears, enabling the conversion to reach a higher value. The appearance of the two peaks was observed also in other works [7] ; the origin of the second peak will be the subject of another paper [11] . The substantial increase in the first peak of the reaction rate after addition of polyacid solution can result both from increased viscosity of the system, which makes oxygen diffusion more difficult, as well as from acceleration of oxygen consumption in a chain process (reactions (2) and (3)) ROO z þ DH ! ROOH þ D z ð 3Þ where DH is a hydrogen donor molecule (monomer, polymer or additive), due to the presence of readily abstractable hydrogens in the polyacrylate backbone. The results shown in Fig. 3 can suggest that AC is a better source of labile hydrogens than PAA (which is in accord with the structure of polymaleic component). The addition of polyacids to HEMA-based formulations exerts a strong accelerating effect on the polymerization initiated by CQ. Two-component initiating systems are usually much less efficient and the polymerization rates are significantly lower. Thus even slightly accelerating effects, as an increase in viscosity of the formulation caused by polymeric acids, can markedly influence the reaction. However, the system considered is much more complicated since polyacids may also affect the initiation process. It is well documented that in the presence of amines (hydrogen donors) initiating radicals are formed by photoreduction of CQ [12] The first stage of this reaction is electron transfer with the formation of the radical – ion pair followed by proton transfer ð 5Þ One of the characteristic features of electron transfer processes is their reversibility. The back electron transfer in radical – ion pairs leads to quenching. An increase in dielectric constant of the reaction medium prevents back electron transfer and promotes proton transfer increasing the efficiency of initiating radical formation. A similar mech- anism is expected to work when MBO is used as the hydrogen donor [13] . Thus, one may speculate that the addition of aqueous polyacid assists initiation by increasing the dielectric constant of the medium. It is also possible that polyacrylates increase the amount of readily abstractable hydrogens and participate in the formation of initiating radicals; this would lead to grafting of poly-HEMA on PAA. When the monomer itself serves as hydrogen donor (in the absence of any added coinitiator), polymerization is much less efficient ( Fig. 5 ) and the effect of polyacid solution much lower. Because hydrogen abstraction from ethers (oxyethylene unit in HEMA) by excited ketones occurs directly, without an electron transfer step, the presence of the polyacid solution should not influence radical formation. This is in agreement with the results obtained; the slight increase in rate observed in the presence of PAA solution may result from the increased viscosity and introduction of an additional source of labile hydrogens. In air the effect of PAA is even higher ( Fig. 6 ). PAA probably helps to overcome oxygen inhibition (reaction (2)) although coinitiators are more reactive in removing oxygen. Moreover, the increased viscosity of the system makes oxygen replenishment through the sample surface more difficult. It is worthy of notice that MBO is much more efficient than DMT in the formation of initiating radicals ð 4Þ Download 222.27 Kb. Do'stlaringiz bilan baham: |
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