Self-strengthening hybrid dental adhesive via visible-light irradiation triple polymerization

Abstract

A self-strengthening methacrylate-based dental adhesive system was developed by introducing an epoxy cyclohexyl trimethoxysilane (TS) which contains both epoxy and methoxysilyl functional groups. The experimental formulation, HEMA/BisGMA/TS (22.5/27.5/50, wt%), was polymerized by visible-light. Real-time Fourier transform infrared spectroscopy (FTIR) was used to investigate in situ the free radical polymerization of methacrylate, ring-opening cationic polymerization of epoxy, and photoacid-induced sol-gel reactions. Among the three simultaneous reactions, the reaction rate of the free radical polymerization was the highest and the hydrolysis/condensation rate was the lowest. With 40s-irradiation, the degrees of conversion of the double bond and epoxy groups at 600 s were 73.2±1.2%, 87.9±2.4%, respectively. Hydrolysis of the methoxysilyl group was initially <5%, and increased gradually to about 50% after 48 h dark storage. Photoacids generated through the visible-light-induced reaction were effective in catalyzing both epoxy ring-opening polymerization and methoxysilyl sol-gel reaction. The mechanical properties of copolymers made with TS concentrations from 5 to 35 wt% were obtained using dynamic mechanical analysis (DMA). In wet conditions, the storage moduli at 70 °C and glass transition temperature were significantly higher than that of the control (p<0.05); these properties increased with TS concentration and storage time. The post reaction of hydrolysis/condensation of alkoxysilane could provide persistent strengthening whether in a neutral or acidic environment and these characteristics could lead to enhanced mechanical properties in the oral environment. The cumulative amount of leached species decreased significantly in the TS-containing copolymers. These results provide valuable information for the development of dental adhesives with reduced leaching of methacrylate monomers and enhanced mechanical properties under the wet, oral environment.

Fabrication of hybrid crosslinked network with buffering capabilities and autonomous strengthening characteristics for dental adhesives

“…The FT-IR result indicates that none of ethoxy functional groups (SNE-containing samples) could be hydrolyzed. Meanwhile, the methoxy groups (SNM-containing specimens) could be hydrolyzed and the copolymers became more hydrophilic with the increase of silanol groups [46,47]. The complicated pathway for hydrolysis and condensation of SNM has inhibited our ability to simultaneously determine the kinetics of silanol formation and reaction.…”

“…Simultaneously, the methoxysilyl groups of SNM are hydrolyzed, which is catalyzed by the photoacid (Brønsted acid, ${\mathrm{H}}^{+}{\text{PF}}_6^{-}$) produced during the visible-light irradiation [67–69]. Our previous studies have shown that the degree of hydrolysis and condensation of methoxysilyl group is very limited (<5%) during 40 s visible-light irradiation, and the newly formed Si–O–Si bonds are limited even after 24 h post curing [46,47]. The increased rubbery moduli obtained from the DMA in dry condition (secondary cycle) supported this proposed mechanism.…”

“…Round disc samples (~4 mm diameter and ~1.2 mm thickness) were used for the leachable study [46,58]. Two disc samples were soaked in 2 mL ethanol (HPLC grade) at 23 ± 2 °C for 1 to 56 days.…”

“…However, MEMA-functionalized copolymers showed fast neutralization behavior when loosely crosslinked network was formed, which would potentially limit the long-term performance of these materials under clinical conditions. Recently, we have developed a self-strengthening dental adhesive by introducing photoacid-induced sol-gel reaction together with the free radical photo-polymerization of methacrylate [46,47]. The results indicated that whether in wet conditions (pH ~ 5.5 @25 °C) or in acidic conditions (pH ≤ 3.5 @25 °C), the sol-gel reaction is a suitable and novel approach to enhance the mechanical properties of the newly developed dental adhesive copolymers [46,47].…”

Polythiourethane microcapsules as novel self-healing systems for epoxy coatings

“…All spectra of composites before self-healing have N-H and O-H stretching vibration broad band with highest peak around 3360 cm -1 , while all spectra of composites after self-healing have this peak around 3390 cm -1 . This shift results from reduction of hydrogen bonding [40,41] during self-healing process, which provides better cross-linking and aforementioned reduction. The higher intensity of -1 peaks after self-healing compared to 3360 cm -1 peaks is also effect of better cross-linking, more precisely polyurethane secondary network forming, which results from the conversion of isocyanate groups into urethane bonding.…”