This paper investigates the photo-co-polymerization behavior of a blend of a diacrylamide (DEBAAP) with a phosphonylated acidic monomer using either bis(acyl)phosphine oxide or camphorquinone/amine as photo-initiator and studies the effect of variation of the structure of the phosphonylated acidic monomer on the shear bond strength to human dentin.
Photopolymerization kinetics has been assessed through the use of photo-DSC with either initiating system and with and without a phosphonic acid monomer, while the shear bond strengths (SBS) of dentin bonding agents formulated with several phosphonylated acidic monomers have been evaluated by macro SBS testing on human dentin.
Photo-DSC results show that bis(acyl)phosphine oxide initiates a faster polymerization than camphorquinone/amine and that both photopolymerizations are accelerated by the phosphonic acid monomer. Similar results were obtained between adhesives based on camphorquinone/amine and a commercial adhesive (AdheSE, Ivoclar-Vivadent, Schaan, Liechtenstein). The best performances were obtained when BAPO was used as the initiator, in many cases far better than the commercial adhesive. Adhesive SEA6 based on difluoromethylphosphonic acid C demonstrated the best adhesion results of this study.
Significance The bis(acyl)phosphine oxide photo-initiator causes faster photopolymerization of two-step self-etching dental adhesive, and its use could yield better bonding performance.
Photopolymerization is one of the most simple and efficient processes to form a polymer . It is widely used in restorative dentistry and particularly for dental adhesives. For the latter, two-step self-etching adhesive systems (SEA) require the use of two separate components: a self-etching primer (SEP) used to condition the dental substrate, followed by the application of a hydrophobic bonding resin . SEP are aqueous acidic solutions containing various vinyl monomers (acidic, hydrophilic and hydrophobic monomers) which can simultaneously etch and infiltrate dental tissues, then photopolymerize with the bonding resin, thus forming a bond between the dental substrate and the restorative material applied afterwards . Various monomers containing phosphate esters, carboxylic and phosphonic acids have been described in the literature for use in dental adhesives. In our previous studies, we have focused on the synthesis and polymerization kinetics of novel phosphonic acid derivatives because they interact chemically with hydroxyapatite but are not subject to hydrolysis in acidic aqueous solutions in contrast to their phosphate equivalents, therefore improving the SEA’s durability .
Acidic monomers and comonomers must be capable of rapid radical polymerization. While the kinetics of free radical polymerization are rather complex for mono-vinyl systems in solution , multivinyl polymerization in bulk is even more complex. Indeed, rate constant for radical termination starts to drop dramatically from the start of the polymerization due to radical diffusion effects , called the Trommsdorff or gel effect, resulting in a rapid increase in the rate of polymerization before monomer depletion starts to become important. In addition, increase in the glass transition temperature ( T g ) due to crosslinking process reduces the molecular mobility to such an extent that the material vitrifies leading to cessation of the propagation and termination reactions . In other cases where the T g of the polymerizing matrix is below the curing temperature, the polymerization may cease prior to full reaction because the radicals and/or the reactive vinyl groups are buried in the surrounding polymerized matrix and are not able to reach fresh monomer . Lastly the photoinitiation system will have a major influence on the polymerization kinetics.
In our previous studies on polymerization kinetics and bond strength of two-step SEAs, we employed the commonly used camphorquinone/amine photo redox initiator system which has the major advantage of absorbing in the blue region of the visible spectrum and thus can be activated without UV lamps . However, this system has some shortcomings since the camphorquinone has a yellowish effect , has demonstrated toxicity and has a low initiator efficiency . Moreover, camphorquinone requires the presence of a photo-reducer (or co-initiator) such as a tertiary amine. Because of these drawbacks, other visible light photo-initiators have emerged such as bis(acyl)phosphine oxides (BAPO) . These are classed as Norrish Type-I photo-initiators as they do not need any co-initiator since their homolytic α -cleavage is sufficient to produce two radical species both capable of initiating polymerization ( Fig. 1 ) . In terms of curing performances, resins formulated with BAPO photo-initiator usually reach a higher degree of conversion than the CQ-based ones . In addition, it is non-toxic at the concentration required for clinical use. For instance, concentrations of up to 0.5 wt% (as used in this work) and up to 1.0 wt% are well tolerated by osteoblast-like cells and MC3T3 pre-osteoblasts respectively . Most papers describe a lower toxicity of BAPO compared to CQ and as a consequence a higher cell viability which is an obvious asset for a dental restoration . However, BAPO has an absorption peak in the near UV region (UVA) which extends into the visible, thus requiring the use of specific violet (circa 390 nm) light-emitting diodes (LED) for the generation of free radicals .
In a previous paper , we observed an enhanced rate of polymerization of acrylamide monomers photoinitiated by CQ/amine, when acrylamide phosphonic acids were present in the reaction medium. We also showed that this rate of acceleration was not specific to the vinyl function of the phosphonic acid monomer, and was applicable to both linear polymerization and crosslinking reactions. In addition, it was also shown that this effect was also observed when a non-polymerizable organic phosphonic acid was present in the polymerizing medium. We therefore were able to conclude that the increase of the medium polarity brought by the presence of phosphonic acid moieties was responsible for the increased polymerization rate of acrylamide monomer.
One objective in this work was to investigate whether a rate enhancement through the use of phosphonic acid monomers is observed when a Norrish I type initiating system such as bis(acyl)phosphine oxide (BAPO) is used. Secondly the final extent of reaction or double bond conversion (DBC) is dependent on the monomer’s chemical structure but also possibly the photo-initiation system, and so this was also studied. The last objective of the work was to make a comparative study dealing with dentin adhesion strength tests using SEP made from a mixture of a dimethacrylamide (DEBAAP) with one of three phosphonic acid monomers (Monomers A – C ) or a phosphoric acid monomer ( D ) using either BAPO or CQ/EDAB as photo-initiating system ( Fig. 2 ).
Material and methods
Camphorquinone (CQ) and ethyl-4-(dimethylamino)benzoate (EDAB) were purchased from Sigma Aldrich (Sigma Aldrich Corporation, Saint Louis, MO, USA). BAPO was obtained as a gift (Irgacure 819, BASF Schweiz AG, formerly CIBA AG, Basel, Switzerland). These initiators were used as components of the free radical initiation systems without further purification. A difunctional acrylamide, N,N-diethyl-bisacrylamidopropane (DEBAAP), and monomers A – D were synthesized according to procedures described in a previous article . The structures of these monomers and initiators are given in Fig. 2 .
Photo-differential scanning calorimetry
The photo-initiating system camphorquinone (0.54 mol% or 0.97 wt%), ethyl-4-(dimethylamino)benzoate (0.54 mol% or 1.07 wt%) or BAPO (0.54 mol% or 2.26 wt%), were added to the DEBAAP and monomer A mixture for the photopolymerization studies. The concentration of 0.54 mol% for each component of the initiation system was used in order to be consistent with the previous work published by our team .
The pH of each primer was measured at room temperature (22 °C) immediately after formulation by placing a drop of the primer on a glass slide and bringing it into full contact with a flat contact electrode (PHC2441-8 Combined pH Electrode “Red Rod”) with a PHM210 Standard pH Meter (Radiometer Analytical, Villeurbanne, France).
Photopolymerization kinetics were monitored using a PerkinElmer DSC 7 differential scanning calorimeter (PerkinElmer Corporation, Waltham, MA, USA) calibrated with indium and zinc standards, which was modified to allow for irradiation of the sample and reference pans by use of a bifurcated fiber optic lead, thus minimizing the thermal heating effect of the photocuring source . Approximately 3 mg of material was spread as a 0.2 mm thin layer over the base of the 4.5 mm diameter, aluminum DSC pan. To minimize the effect of dissolved oxygen on the polymerization kinetics, all resin samples were equilibrated in the apparatus for at least 5 min under a 20 cm 3 min −1 flow of N 2 at 50 °C before commencing the experiment. All photo polymerizations were performed at 50 °C, using a Rofin Polilight PL400 source (Rofin Australia Pty Ltd., Dingly Victoria, Australia) with light intensities of 40 mW cm −2 near 470 nm for CQ and 40 mW cm −2 near 370 nm for BAPO, as measured with an Ocean Optics USB2000 ﬁber optic spectroradiometer (Ocean Optics Inc., Dunedin, FL, USA). An irradiation time of 600 s was used in the DSC studies for both photoinitators. The heat flow was monitored as a function of time with the DSC under isothermal conditions. All DSC experiments were performed in duplicate with good agreement. Double-bond conversion ( DBC ) was calculated as the quotient of the overall enthalpy evolved [Δ H p (J g −1 )] and the theoretical enthalpy obtained for 100% conversion of the mixtures [ ΔH 0 p (J g −1 )] (Eq. (1) ).
Δ H 0 p (in J g −1 ) was calculated according to the following formula (Eq. (2) ):
where x a and x b are the molar percentages of Monomers a and b in the mixture respectively, while M a and M b are their molecular weights. Δ H 0 a and Δ H 0 b are the theoretical enthalpies (in J mol −1 ) of reaction which are 120.6 kJ mol −1 for DEBAAP 60.3 kJ mol −1 for Monomer A . The rate of polymerization ( R p in units of fractional conversion per s) was calculated according to the following formula (Eq. (3) ):