Use of (meth)acrylamides as alternative monomers in dental adhesive systems

Abstract

Objectives: Methacrylamides are proposed as components for dental adhesive systems with enhanced resistance to hydrolytic and enzymatic degradation. The specific objective of this study was to evaluate the polymerization kinetics, water sorption and solubility, pH-derived degradation and microtensile bond strength of various monofunctional acrylamides and meth(acrylamides) when copolymerized with dimethacrylates.

Methods: Base monomers were added at 60 wt%, and included either BisGMA or UDMA. Monofunctional monomers were added at 40 wt%, including one (meth)acrylate as the control, two secondary methacrylamides and two tertiary acrylamides. DMPA (0.2 wt%) and DPI-PF6 (0.4 wt%)/BHT (0.1 wt%) were added as initiators/inhibitor. Polymerization kinetics wwere followed with near-IR spectroscopy in real time. Water sorption (WS) and solubility (SL) were measured following ISO 4049. Monomer degradation at different pH levels was assessed with ¹H NMR. Microtensile bond strength (MTBS) was assessed in caries-free human third molars 48 h and 3 weeks after restorations were placed using solvated BisGMA-based adhesives (40 vol% ethanol). Data were analyzed with one-way ANOVA/Tukey's test (α = 0.05).

Results: As expected, rate of polymerization and final degree of conversion (DC) were higher for the acryl versions of each monomer, and decreased with increasing steric hindrance around the vinyl group for each molecule. In general, UDMA copolymerizations were more rapid and extensive than for BisGMA, but this was dependent upon the specific monofunctional monomer added. WS/SL were in general higher for the (meth)acrylamides compared to the (meth)acrylates, except for the tertiary acrylamide, which showed the lowest values. One of the secondary methacrylamides was significantly more stable than the methacrylate control, but the alpha substitutions decreased stability to degradation in acid pH. MTBS in general was higher for the (meth)acrylates. While for all materials the MTBS values at 3 weeks decreased in relation to the 24 h results, the tertiary acrylamide showed no reduction in bond strength.

Significance: This study highlights the importance of considering steric and electronic factors when designing monomers for applications where rapid polymerizations are needed, especially when co-polymerizations with other base monomers are required to balance mechanical properties, as is the case with dental adhesives. The results of this investigation will be used to design fully formulated adhesives to be tested in clinically-relevant conditions.

Keywords: Copolymerization; Methacrylamides; Phase-separation; Polymer network; Polymerization kinetics; Steric hindrance.

Figure 1:

Polymerization rate (%·s⁻¹) as a function of conversion (%) for non-solvated BisGMA and UDMA-containing adhesives are shown in Figures A and B, respectively. Vinyl conversion was followed in real-time as the materials were photocured (630 mW/cm² for 300 seconds). (A1 and B1) Comparison of methacrylate, secondary methacrylamide and tertiary acrylamide. (A2 and B2) Comparison of the two tertiary acrylamides. (A3 and B3) Comparison of the secondary methacrylamides.

Figure 2.

Water sorption and Solubility results for all tested copolymerizations. Different uppercase letters indicate statistically significant differences among the monofunctional monomers copolymerized with BisGMA (p≤0.05). Different lowercase letters indicate statistically significant differences among the monofunctional monomers copolymerized with UDMA copolymerizations (p≤0.001). Significant differences between BisGMA and UDMA within the same monofunctional monomer are indicated by asterisks (p<0.05).

Figure 3.

Dentin Micro-tensile Bond Strength (MPa) for control groups and all mono-functional monomers copolymerized with BisGMA and UDMA after 24 hours (blue), 3 weeks (orange) and 6 months (grey) water storage. Different uppercase letters indicate statistically significant differences between the groups within the same storage time (p<0.05). Different lowercase letters indicate statistically significance difference between the storage times within the same group. Significant differences between BisGMA and UDMA within the same monofunctional monomer are indicated by asterisks (p<0.05). The BisGMA/HEMAM materials were not tested at 6 months.

Figure 4.

Percentage of remaining monomer after hydrolytic degradation at pH 1 and 2, as determined by the ¹H-NMR experiments (described in detail in the appendix). Different uppercase letters indicated statistical difference among neat monomers incubated at pH 1 and different lowercase letters indicated statistical difference among neat monomers incubated at pH 2 (p≤0.001). Note: the tertiary methacrylamide (HEMMA) was tested instead of the tertiary acrylamide (HEM). DMAM was not tested.

Figure 5:

(A) Kinetics polymerization curves of pure DMAM and combined with UDMA and BisGMA. (B) Comparison between UDMA and BisGMA when mixed with the secondary methacrylamides.