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Influence of the Chemical Structure of Functional Monomers on Their Adhesive Performance
1 Leuven BIOMAT Research Cluster, Department of Conservative Dentistry, School of Dentistry, Oral Pathology and Maxillo-Facial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, BE-3000 Leuven, Belgium; Correspondence: * corresponding author, bart.vanmeerbeek{at}med.kuleuven.be
Functional monomers in adhesive systems can improve bonding by enhancing wetting and demineralization, and by chemical bonding to calcium. This study tested the hypothesis that small changes in the chemical structure of functional monomers may improve their bonding effectiveness. Three experimental phosphonate monomers (HAEPA, EAEPA, and MAEPA), with slightly different chemical structures, and 10-MDP (control) were evaluated. Adhesive performance was determined in terms of microtensile bond strength of 4 cements that differed only for the functional monomer. Based on the Adhesion-Decalcification concept, the chemical bonding potential was assessed by atomic absorption spectrophotometry of the dissolution rate of the calcium salt of the functional monomers. High bond strength of the adhesive cement corresponded to low dissolution rate of the calcium salt of the respective functional monomer. The latter is according to the Adhesion-Decalcification concept, suggestive of a high chemical bonding capacity. We conclude that the adhesive performance of an adhesive material depends on the chemical structure of the functional monomer.
Key Words: adhesion • adhesive • functional monomer • phosphonate • polarity • chemical bonding
The adhesion mechanism of dental adhesives to enamel and dentin is twofold. Micro-mechanical retention is still considered the principal mechanism (Nakabayashi et al., 1982). Recently, evidence for an additional chemical interaction has been provided (Yoshida et al., 2000, 2004; Fu et al., 2005). Such a chemical bond may be important in the prevention of nano-leakage, thus prolonging the intra-oral lifetime of an adhesive restoration (Inoue et al., 2005). Monomers in adhesives can be categorized into cross-linking and functional monomers (Van Landuyt et al., 2007). The latter are characterized by at least one polymerizable group and a functional group, which can serve different purposes, such as wetting and demineralizing the substrate. It has been shown that functional groups capable of releasing one or more protons, such as carboxyl, phosphate, and phosphonate groups, may also have the potential for chemical bonding to calcium in hydroxyapatite. The ‘Adhesion-Decalcification’ concept dictates that the functional monomer either decalcifies, or bonds to the tooth substrate (Yoshioka et al., 2002). According to this concept, the functional group first ionically interacts with calcium in hydroxyapatite. Depending on the stability of the resulting calcium-monomer complex in the adhesive suspension, this ionic bond may either decompose and demineralize the tooth surface, or remain stable and chemically bond with calcium. Some functional monomers have already been ranked based on their chemical bonding potential (Yoshida et al., 2004). Differences in adhesive performance between functional monomers must be explained from their chemical structure. The objective of this study was to investigate the influence of chemical structure on the adhesive performance of 3 functional monomers that differed from each other in only one chemical group in terms of their adhesive performance on enamel and dentin. The null hypothesis tested was that the chemical structure of functional monomers has no effect on their bonding effectiveness.
Four similar experimental cements were prepared by Ivoclar-Vivadent (Schaan, Liechtenstein), consisting of a two-component self-etching primer and a high-viscosity dual-curing cement (Table  ). The only difference was in the primer, which contained a different functional monomer, albeit in the same concentration (Table). As such, the adhesive performance of the following phosphonate monomers could be assessed: HAEPA or 2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate, EAEPA or ethyl 2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate, and MAEPA or 2,4,6 trimethylphenyl 2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate (Fig. 1) (Moszner et al., 1999, 2001). The carboxyl group in HEAPA was esterified in EAEPA and MAEPA with an ethyl- and a phenyl-group, respectively. The fourth experimental primer contained 10-MDP and served as a control.
The pH of the 4 primers before and after mixing of the 2 components was measured at ambient temperature (20–25°C) with a digital pH meter (Inolab pH Level 2, WTW, Weilheim, Germany).
Microtensile Bond-strength Testing (µTBS)
TEM Interface Characterization
Dissolution Rates of Ca-monomer Salts
When bonded to enamel, the 10-MDP-based primer showed significantly higher bond strengths than the other experimental primers (Fig. 1  ). The bond strengths of EAEPA and MAEPA to enamel did not differ significantly. In contrast, HAEPA exhibited several pre-testing failures, resulting in low bond strength to enamel. On dentin, all HAEPA and the majority of EAEPA samples failed before being tested. Conversely, MAEPA and 10-MDP attained bond strengths of 25 MPa without pre-testing failures. The following ranking for bond strength could be made: 10-MDP > MAEPA > EAEPA > HAEPA.
Failure analysis showed a predominantly mixed failure pattern (Fig. 2
TEM interface analysis showed that the interfaces of EAEPA, MAEPA, and 10-MDP were much alike. The hybrid layer (0.5 and 1.5 µm) was partially demineralized, and the dentinal tubules generally remained occluded by a smear plug. The interface of HEAPA could not be visualized by TEM, since all samples failed during processing.
AAS measurements (Fig. 3
One way to improve adhesive technology is to synthesize new functional monomers, tailored to exhibit specific functions. The phosphonate monomers in this study have been designed with the aim of good adhesive performance along with high hydrolytic stability (Moszner et al., 2005). It was expected that the addition of an aliphatic of aromatic group to the carboxyl group would influence, particularly, the polarity (and thus wetting) and the polymerizability of the monomer. To compare the performance of these experimental monomers, we selected 10-MDP, since its adhesive performance has been well-investigated, and it has been shown to have excellent bonding capacities. The amount of calcium in a solution can be measured by AAS, which has been shown to provide a good indication of the stability of Ca-monomer salts (Yoshida et al., 2004). According to the Adhesion-Decalcification concept, the stability of the salt determines whether an acidic monomer either demineralizes hydroxyapatite or remains attached to it. X-ray photoelectron spectroscopy studies confirmed that monomers that form a stable Ca-salt are also capable of chemically interacting with hydroxyapatite at the tooth surface (Yoshida et al., 2000). AAS can thus be regarded as a useful means to predict the potential of a monomer to chemically interact with the tooth surface. Interestingly, the stability of the Ca-monomer salts in this study measured by AAS corresponded to the bond strengths of the 4 cements. 10-MDP, the calcium salt of which exhibited the highest stability, also attained the highest bond strength to enamel and dentin. Considering the experimental monomers, the best overall bond-strength results were obtained by MAEPA. The low bond strength of EAEPA to dentin, partially due to many pre-testing failures, indicated that dentin remained the most difficult substrate to which to bond (Nuñes et al., 2005). Since the primers of the 4 cements differed only for the functional monomer, which was added in exactly the same concentration, the influence of the functional monomer alone could be assessed. Moreover, the 4 experimental monomers, HAEPA, EAEPA, and MAEPA, had an identical chemical structure, but for one group. The null hypothesis, stating that the chemical structure has no effect on the bonding effectiveness, must thus be rejected. Several factors may explain the effect of the chemical structure of the tested monomers on their adhesive performance. First, the differing chemical groups of the monomers greatly influence the monomer polarities. Whereas the phosphonate group exhibits hydrophilic properties, the trimethylphenyl group in MAEPA is most hydrophobic, and the carboxyl group in HAEPA is hydrophilic. The polarity of EAEPA would be in between. The polarity of functional monomers has been shown to be important for wetting behavior (Nakabayashi et al., 1982).The most polar molecule is HEAPA, having two polar groups. However, the adhesive capacity of this monomer was very poor. Second, since the varying group is attached to the polymerizable vinyl group, it may also influence polymerization through either substituent (electron-withdrawing or -releasing group) or steric effects. Third, AAS indicated that the group also influences the monomer’s potential for stable calcium bonding. The apolar phenyl group in MAEPA may keep water molecules at a distance from the ionic bond between calcium and the monomer, thereby hindering hydrolysis. Besides steric hindering, this group may also influence the phosphonate group through the substituent effect, modifying the electron distribution. 10-MDP was chosen as a control monomer, since reports in the literature have repeatedly indicated that adhesives containing this functional monomer are effective in laboratory as well as clinical studies (Van Meerbeek et al., 2005). Considering the chemical structure of 10-MDP, a similar reasoning to explain its good adhesive performance is plausible. 10-MDP also has an amphiphilic structure, with the phosphate group being the polar moiety and the spacer group consisting of a saturated carbon chain being the apolar moiety. A direct comparison between the experimental monomers and 10-MDP would be incorrect, since the latter has a phosphate group instead of a phosphonate group. To allow for a chemical interaction between hydroxyapatite in dentin and the monomer, it was imperative that the self-etching primers be mildly acidic. Even though the different primers B had various pHs, the buffering primer A rendered all mixed primers equally acidic, except for HAEPA. Since HAEPA consists of two acidic groups—i.e., a phosphonate group that may release 2 protons (depending on the pKa value of the monomer) and a carboxyl group—the primer with this monomer was more acidic. Nonetheless, all primers could still be categorized as mildly acidic (De Munck et al., 2005). Unfortunately, the interface of HAEPA with dentin could not be investigated, since the bond strengths were so low that TEM specimens failed during processing. Corresponding to their similar pHs, 10-MDP, MAEPA, and EAEPA indeed exhibited a partially demineralized hybrid layer, in which hydroxyapatite crystals were left available for chemical bonding. To conclude, changing only one parameter in the chemical structure of a monomer is an interesting model for investigating bonding mechanisms. This study showed that improvement of current adhesives may be achieved by synthesis of customized functional monomers with good chemical bonding potential.
KL Van Landuyt holds a Ph.D fellowship of the Research Foundation–Flanders (FWO). We thank Ivoclar-Vivadent for preparing the experimental cements and calcium-salts, and Dr. Salz for critically reviewing the manuscript. Received for publication August 28, 2007. Revision received April 3, 2008. Accepted for publication May 19, 2008.
Journal of Dental Research, Vol. 87, No. 8,
757-761 (2008)
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ABSTRACT
INTRODUCTION
= 0.05) was performed. The mode of failure was determined by stereomicroscopy. Representative fractured enamel/dentin and composite surfaces, exhibiting the most frequently observed failure mode and a µTBS close to the mean, were processed for Field-emission-gun Scanning Electron Microscopy (Feg-SEM; Philips XL30, Phillips, Eindhoven, Netherlands), by common specimen processing as previously described in detail (
