Linyong Songa,
Qiang Ye*a,
Xueping Gea,
Anil Misraab,
Candan Tamerlerac and
Paulette Spencer*ac
aUniversity of Kansas, Bioengineering Research Center, 1530 W. 15th Street, Lawrence, KS 66045-7609, USA. E-mail: yeq@ku.edu; pspencer@ku.edu
bUniversity of Kansas, Department of Civil Engineering, 1530 W. 15th Street, Lawrence, KS 66045-7609, USA
cUniversity of Kansas, Department of Mechanical Engineering, 1530 W. 15th Street, Lawrence, KS 66045-7609, USA
First published on 25th May 2016
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 40 s-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.
The degradation of dental adhesives has been an area of intense investigation and considerable attention has been directed towards reliable damage prediction and property degradation models.13–18 In spite of this effort, detecting adhesive damage in situ is difficult. It is even more difficult to repair the adhesive because the damage often occurs at sites within the restoration, e.g. at the interface with the dentin substrate or the composite material, that are largely inaccessible for external repair by a dentist.11
Scientists have proposed self-repair or self-healing resin as a mechanism for increasing the clinical lifetime of resin-based materials.19–22 The development of self-healing resins is considered breakthrough technology.23 Microcapsules within the self-healing resins rupture when a crack forms in the matrix. The ruptured microcapsule releases a healing agent that seals the crack to reduce the damage. Adapting this approach for dental adhesives faces numerous challenges including toxicity of the healing agents, the catalysts, limitations regarding the dimensions of the microcapsules, and maintaining the integrity of the interfacial bond in the presence of the ruptured microcapsules.24,25 An alternative strategy could be resins that provide intrinsic self-strengthening properties, i.e. resins that possess behavior reminiscent of living organisms.26
In 2005, Kowalewska described the formation of oxo-silica network formed by the photoacid catalyzed sol–gel reaction of an alkoxysilyl-modified disiloxane.27 This relationship between the UV-generated photoacids and the resulting inorganic network has been developed extensively. Versace et al. studied the relation between the sol–gel reaction and cationic polymerization by using epoxy cyclohexyltrimethoxysilane as a monomer.28,29 They investigated the formation of inorganic and organic polymers via the one-step simultaneous method by UV-light irradiation28 or two separate and consecutive steps (sol–gel reaction and cationic photopolymerization).29 Our group investigated the polymerization behavior and mechanical properties of dental adhesive copolymers prepared by dual polymerization via visible-light irradiation.26 A limited photoacid-induced sol–gel reaction was observed during visible-light irradiation and the prepared copolymers showed an autonomic self-strengthening characteristic in wet conditions.
Photoacids can catalyze the sol–gel reaction of alkoxysilyl groups and can also initiate the polymerization of oxirane groups. In contrast to free radical photopolymerization, cationic polymerization is not inhibited by oxygen, and unlike free radicals, the cationic centers are not reactive towards one another. Hence they have much longer lifetimes, which promotes curing in dark conditions. Silorane-based composite has been developed and the polyether structure exhibits strong mechanical and thermal properties, chemical resistance, and low shrinkage compared with methacrylate-based materials.30,31 Recently, we have studied the methacrylate/silorane hybrid adhesive systems.32 The results indicated that the crosslink density of dental adhesives was improved with the addition of silorane monomers, and the degree of conversion of epoxy groups was affected by the number of functionalities.33 Despite these developments, with the elongation of storage time in wet conditions, the mechanical properties of the copolymers were maintained or showed a gradual decrease.
Our research group recently incorporated γ-methacryloxypropyltrimethoxysilane (MPS) into the dental adhesive formulation and explored the visible-light induced photoacid-catalyzed sol–gel reaction.26 During photopolymerization, MPS was mainly incorporated into the polymer backbone and the methoxysilyl groups acted as pendent functions. When the copolymers were stored in wet conditions, the pendent groups would react with each other and new covalent bonds (Si–O–Si) were generated via hydrolysis and condensation reactions. The mechanical properties of the newly developed copolymer showed self-strengthening characteristics whether in neutral or acidic wet conditions. The self-strengthening dental adhesive system is still in its infancy and the relationship between the components and the repair process are not clear. The lack of understanding prompted us to more closely examine the visible-light induced triple polymerization behavior of the dental adhesive system containing CC double bond, epoxy, and trialkoxysilyl functional groups. In the first part of the present study, we focus on the triple polymerization kinetics using real-time Fourier transform infrared spectroscopy (FTIR). In our case, a competition is likely to occur between the sol–gel reactions involving the alkoxysilyl groups and the epoxy ring-opening cationic polymerization. In the second part of this investigation, attention is focused on understanding the effect of storage medium and time on the intrinsic self-strengthening and self-repair processes.
Run | HEMA/BisGMAa (wt%) | TS (wt%) | DC (%) | |
---|---|---|---|---|
a The resin was mixed HEMA/BisGMA in the ratio of 45/55 (w/w).b Significantly (p < 0.05) different from the control (C0). The value in the ( ) is the standard deviation. | ||||
C0 | 100 | 0 | 64.8 (0.2) | 21.2 (0.6) |
HBT-5 | 95 | 5 | 68.6b (0.8) | 21.9 (0.9) |
HBT-10 | 90 | 10 | 71.0b (0.4) | 17.6b (1.2) |
HBT-15 | 85 | 15 | 74.8b (0.6) | 15.2b (1.2) |
HBT-20 | 80 | 20 | 78.8b (0.9) | 12.4b (0.6) |
HBT-35 | 65 | 35 | 76.6b (0.5) | 7.3b (0.6) |
HBT-50 | 50 | 50 | 73.2b (1.4) | 6.0b (0.4) |
Static or monotonic tests were performed on dental adhesives in wet conditions to obtain the stress–strain curves. During the static tests, load was increased at a constant rate until the sample ruptured. A minimum of four specimens for each formulation were tested with 0.1 N min−1 loading rate at 37 °C.44 One group of the copolymer specimens, which were soaked in water at 37 °C for 1 week, was used as the control. The other two groups of copolymer samples were first soaked in a solution of ethanol and 1 M LA aqueous solution (1:
1, v/v) at 37 °C for 1 and 3 weeks, respectively. Then the specimens were transferred into greater volume of water at 37 °C for 3 days, to replace the absorbed ethanol and LA. The elastic modulus was calculated based on the maximum slope of the linear region of the stress–strain curve.
The analysis was made using high performance liquid chromatography (HPLC) on a system (Shimadzur® LC-2010C HT, software EZstart, version 7.4 SP2) equipped with a 250 × 4.6 mm column packed with 5 μm C-18 silica (Luna®, Phenomenex Inc., Torrance, CA). The mobile phase was acetonitrile/water (70/30, v/v). The system was operated under the following conditions: 0.5 mL min−1 flow rate; detection at 208 nm; 20 μL sampling loop; 40 °C temperature. The column was calibrated with known concentrations of the BisGMA and HEMA, at concentrations of 5, 10, 20, 50, 100 and 250 mg L−1 in ethanol. The calibration curves with the linear fittings of BisGMA (5–250 mg L−1, R2 = 0.999) and HEMA (5–500 mg L−1, R2 = 0.999) were used to calculate the concentration of these species in the extracts. The concentration was based on the intensity of the chromatographic peaks at the corresponding retention time. The HPLC analysis was performed using the extract of 3 samples for each formulation.
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Scheme 1 Chemical structures of monomers used in the formulations and illustration of the triple polymerization mechanism. |
Real-time photopolymerization kinetic behaviors of the control and experimental formulations are shown in Fig. 1 and the DC and maximum polymerization rate are summarized in Table 1. With the increase of TS concentration from 5 to 50 wt%, the DC (600 s) was significantly higher than that of the control (64.8 ± 0.2%) at the 0.05 level. The highest DC (78.8 ± 0.9%) was observed at a TS concentration of 20 wt%. The maximum polymerization rates (CC bond) of the experimental formulations were significantly lower than the control (p < 0.05), with the exception of the formulation at 5 wt% TS (see Table 1).
Fig. 2 shows the characteristic peaks of FTIR spectra of HBT-50 formulation before and after visible-light irradiation. The hydrolysis of methoxyl (2840 cm−1, –SiO3) group was observed through the disappearance of the CH3–O symmetric stretch band. Meanwhile, the intensity of the large band around 3400 cm−1 (hydrogen bond OH stretching mode) increased gradually and the maximum peaks moved from 3450 to 3350 cm−1 and the peak at 900 cm−1 (
–
H stretching vibration mode) was visible with time. These results indicated that the hydrolysis step of the sol–gel reaction occurred with storage time. In accordance with the condensation reaction, a broad peak with a maximum at ∼1025 cm−1, assigned to the Si–O–Si asymmetric stretching mode, was noticed, which is a characteristic of silica network formation. At the same time, the gradual broadening of peak at 1720 cm−1 and elevated at 1637 cm−1 were attributed to increasing water concentration in the polymers. These results are evidence that the condensation reaction between silanol–silanol and/or silanol–hydroxyl groups occurred gradually with storage time. In addition to the free radical promoted cationic ring-opening polymerization,32,33 a decrease of the band at 884 cm−1 attributed to the epoxy group, was visible in the FTIR spectrum (Fig. 2D). These results provide evidence that epoxy ring-opening occurred concomitantly with the free radical polymerization and sol–gel process. Photoacid is able to initiate the epoxy cationic photopolymerization, leading to the formation of polyether chains.
Fig. 3 shows the DMA data of the control and experimental adhesives in dry condition before and after soaking in water for 8 weeks. The HBT-50 formulation is not shown because the samples were too soft to proceed with the DMA test. With the increase in TS concentration from 5 to 35 wt%, the rubbery moduli of unsoaked copolymer specimens increased slightly. The Tg showed a decreasing trend and decreased from 146 ± 1.2 (C0) to 107.8 ± 1.4 °C (HBT-35). After the sample was soaked in water for 8 weeks and dried again, the rubbery modulus and Tg were significantly higher than the control (p < 0.05). Tg increased from 156.0 ± 0.7 (C0) to 178 ± 1.5 °C (HBT-35). The resulting increase in network density is assumed to have a negative effect on the mobility of side chains, which was also supported by the decreasing maximum intensity of the tanδ peaks in Fig. 3E and F. From the derivative storage modulus curves of un-soaked specimens (Fig. 3C), with the increase of TS concentration from 0 to 35 wt%, the phenomena of the first transition temperature remained similar (∼85 °C with HBT-35 the exception). The secondary transition peak gradually moved to lower temperature (from ∼128 to ∼80 °C), indirectly indicating that chains of experimental copolymers possessed higher mobility than that of the control. However, for the samples soaked in water for 8 weeks, the first transition peaks were similar at about 90 °C and the secondary transition peak moved from ∼138 to ∼165 °C.
Fig. 4 shows the mechanical properties of the control and experimental adhesive copolymers soaked in water at 37 °C for 1, 4, and 8 weeks. These data are summarized in Table 2. With the increase in storage time in water from 1 to 8 weeks, the storage modulus of the control at 70 °C increased from about 270 to 315 MPa, and the glass transition temperature increased about 3 °C, from 59.4 to 62.3 °C. Excluding the HBT-5 sample, the storage modulus of experimental specimens at 37 °C was significantly lower than that of the control (p < 0.05). The modulus at 70 °C was, however, significantly higher than the control (p < 0.05). After 8 weeks in water, the storage modulus at 70 °C of the sample with 20 wt% TS showed a maximum of nearly 1 GPa. Tg increased with TS concentration and storage time, and was higher than that of the control.
Time (week) | Run | Storage modulus (MPa) | Tg (°C) | ΔT* | |
---|---|---|---|---|---|
37 °C | 70 °C | ||||
a Same superscript letters in the same storage time group indicates no significant differences between each other (p < 0.05). * ΔT = Tg (experimental) − Tg (C0). | |||||
1 | C0 | 1924b (86) | 269b (29) | 59.4 (0.3) | — |
HBT-5 | 1832b,c (64) | 297b,c (7) | 63.3 (0.9) | 3.9 | |
HBT-10 | 1660c,d (63) | 282b,c (17) | 66.9 (0.8) | 7.5 | |
HBT-15 | 1638d (72) | 462d (37) | 69.5 (0.4) | 10.1 | |
HBT-20 | 1415 (50) | 415d (9) | 69.8 (0.8) | 10.4 | |
HBT-35 | 1085 (47) | 531 (15) | >75.0 | >15.6 | |
4 | C0 | 1995b (32) | 283 (7) | 61.4 (0.2) | — |
HBT-5 | 1920b (24) | 362 (20) | 66.2 (0.2) | 4.8 | |
HBT-10 | 1772c (37) | 573b (30) | 70.0 (0.6) | 8.6 | |
HBT-15 | 1688c,d (45) | 744c (9) | >75.0 | >13.6 | |
HBT-20 | 1636d (47) | 789c (35) | >75.0 | >13.6 | |
HBT-35 | 1085 (9) | 554b (25) | >75.0 | >13.6 | |
8 | C0 | 2066b (52) | 315b (20) | 62.3 (0.1) | — |
HBT-5 | 1953b,c (76) | 443b (39) | 67.3 (0.1) | 5.0 | |
HBT-10 | 1829c (28) | 708c (5) | 71.8 (0.2) | 9.5 | |
HBT-15 | 1784 (94) | 988 (86) | >75.0 | >10.7 | |
HBT-20 | 1584 (47) | 805c (22) | >75.0 | >10.7 | |
HBT-35 | 1191 (24) | 698c (38) | >75.0 | >10.7 |
Fig. 5 shows the mechanical properties of the control and experimental adhesive copolymers soaked in 0.1 M LA solution at 37 °C for 1, 4, and 8 weeks, respectively. These data are summarized in Table 3. With the increase of storage time in acidic solution from 1 to 8 weeks, the storage modulus of the control at 70 °C increased from about 226 to 264 MPa, and the glass transition temperature increased about 3 °C, from 59.4 to 62.8 °C. The storage moduli of the experimental formulations at 70 °C were significantly higher than that of the control (p < 0.05). With the increase in TS concentration from 15 to 35 wt%, the storage moduli at 70 °C were similar after storage in LA solution for 8 weeks.
Time (week) | Run | Storage modulus (MPa) | Tg (°C) | ΔT* | |
---|---|---|---|---|---|
37 °C | 70 °C | ||||
a Same superscript letters in the same storage time group indicates no significant differences between each other (p < 0.05). * ΔT = Tg (experimental) − Tg (C0). | |||||
1 | C0 | 1789b (55) | 226b (34) | 59.4 (0.7) | — |
HBT-5 | 1839b,c (38) | 286b (14) | 63.8 (0.6) | 4.4 | |
HBT-10 | 1713b,c (72) | 379 (15) | 67.4 (0.9) | 8.0 | |
HBT-15 | 1741b,c (49) | 606c (33) | 71.5 (0.2) | 12.1 | |
HBT-20 | 1542 (37) | 600c (15) | 74.6 (0.5) | 15.2 | |
HBT-35 | 1307 (37) | 655c (43) | >75.0 | >15.6 | |
4 | C0 | 1886b (8) | 255b (15) | 61.6 (0.3) | — |
HBT-5 | 1895b (20) | 329b (22) | 65.3 (0.3) | 3.7 | |
HBT-10 | 1758c (40) | 575 (39) | 70 (0.2) | 8.4 | |
HBT-15 | 1807b,c (59) | 848c (44) | >75.0 | >13.4 | |
HBT-20 | 1616 (5) | 808c,d (9) | >75.0 | >13.4 | |
HBT-35 | 1315 (36) | 727d (48) | >75.0 | >13.4 | |
8 | C0 | 2002b (19) | 264b (23) | 62.8 (0.3) | — |
HBT-5 | 1946b,c (55) | 353b (25) | 65.9 (0.3) | 3.1 | |
HBT-10 | 1870c,d (81) | 623 (93) | 70.2 (0.7) | 7.4 | |
HBT-15 | 1771d,e (64) | 885c (11) | >75.0 | >12.2 | |
HBT-20 | 1731e (37) | 858c (23) | >75.0 | >12.2 | |
HBT-35 | 1430 (51) | 840c (40) | >75.0 | >12.2 |
Fig. 6 shows the storage moduli of the control and experimental specimens at 70 °C measured in wet conditions. The values of the control in water were slightly higher than that in LA solution. However, the modulus values of the experimental at 70 °C in 0.1 M LA solution were comparable or higher than that in water with the same storage time. Whether in water or 0.1 M LA solution, the maximum values were observed with the optimal TS concentration in 10–20 wt% (stored in water) and 15–35 wt% (stored in LA solution), respectively.
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Fig. 6 Storage modulus at 70 °C of control and experimental samples stored in water (A) and 0.1 M LA solution (B) over 1, 4, and 8 weeks. |
Fig. 7 provides the stress–strain curves obtained from the static tests at a loading rate of 0.1 N min−1. The elastic modulus and stress at 4% strain are summarized in Table 4. The slopes of the linear region of the curves are defined as the modulus of elasticity (E). When the specimens were soaked in water for 1 week, the E of experimental was significantly lower than that of the C0 (p < 0.05). When the specimens were soaked in LA/EtOH solution for 1 or 3 weeks, the mean E of the experimental formulation was not significantly different from the C0 (p < 0.05), with the exception of the 35 wt% TS samples. The mean E of the C0 formulations was similar regardless of storage medium (water or LA/EtOH) or storage time (1 or 3 weeks). Meanwhile, the mean E of the experimental specimens, soaked in LA/EtOH, were significantly higher than that of samples stored in water (p < 0.05). When TS concentration was 5–20 wt%, the failure strain of experimental was comparable with that of the control (p < 0.05).
Run | ME (MPa) | Stress at 4% strain (MPa) | ||||
---|---|---|---|---|---|---|
Water 1w | LA/EtOH 1w | LA/EtOH 3w | Water 1w | LA/EtOH 1w | LA/EtOH 3w | |
a In the same column indicates the mean is significantly different compared with the C0 (p < 0.05).b In the same row indicates the mean is significantly different compared with the control (the value of water-1w used as the controls, p < 0.05), and with different superscript letter (b or c) indicates that the means difference in the same row is significant with each other (p < 0.05).c In the same row indicates the mean is significantly different compared with the control (the value of water-1w used as the controls, p < 0.05), and with different superscript letter (b or c) indicates that the means difference in the same row is significant with each other (p < 0.05). | ||||||
C0 | 962 (35) | 974 (21) | 997 (29) | 22.7 (1.4) | 23.7 (0.6) | 24.8b (0.8) |
HBT-5 | 889a (43) | 948b (23) | 1019c (25) | 22.4 (1.4) | 23.6 (0.5) | 25.1b (0.5) |
HBT-10 | 885a (32) | 955b (21) | 1008c (22) | 21.6 (1.1) | 23.4b (0.3) | 25.3c (0.7) |
HBT-15 | 887a (20) | 949b (33) | 993c (20) | 22.1 (0.3) | 23.2b (0.6) | 24.7c (0.4) |
HBT-20 | 780a (14) | 945b (21) | 959b (17) | 19.8a (0.4) | 23.9b (0.8) | 24.4b (0.6) |
HBT-35 | 543a (34) | 748a,b (22) | 754a,b (40) | 14.2a (0.6) | — | — |
Fig. 8 shows the results of cumulative leachate from the copolymers of control and experimental as a function of incubation time in ethanol at 23 ± 2 °C. With the increase of TS concentration from 0 to 50 wt%, the cumulative release of HEMA decreased from 1187 ± 19 to 30 ± 1 μg mL−1 and the cumulative release of BisGMA decreased from 680 ± 19 to 25 ± 2 μg mL−1. With the increase of TS concentration from 0 to 50 wt%, the percentage of leached HEMA decreased from 13.2 to 0.7 wt% and the percentage of leached BisGMA decreased from 6.2 to 0.1 wt%, respectively. No leached BisGMA was detected in the formulation with 35 wt% TS.
Real-time FTIR is not only useful for monitoring the absorbance band of the CC double bond but also for monitoring the progress of cationic polymerization and sol–gel reactions initiated by the visible-light generated photoacid. In the present study, when TS concentration was lower than 35 wt%, the signal of νsym(–SiO
3) was overlapping with that νsym(–CH2) of methacrylate in the FTIR spectra. To clearly show the variation of different functions before and after irradiation, 50 wt% TS (HBT-50) formulation was used to quantitatively characterize the triple polymerization reaction. The free radical photopolymerization behavior of methacrylate has been widely reported by several groups.50–56 The band ratio profile of 1637 cm−1 (C
C)/1608 cm−1 (phenyl) was used to monitor the conversion of C
C double bond. In the present study, the DC (double bond) of experimental was significantly higher than the control and reached a maximum value when TS was 20 wt%. This result indicated that the C
C double bond was efficiently polymerized in the presence of the TS monomer. At the same time, conversion of the methoxysilyl group (stretching band at 2840 cm−1) is the first indication of the hydrolysis. The evolution of the OH stretching intensity may give insight into the silanol (Si–OH) concentration after visible-light irradiation. From Fig. 2B, with the elongation of storage time to 24 h, the intensity of methoxy at 2840 cm−1 decreased with the intensity of OH around 3400 cm−1. It should be noted that this latter band is not totally selective of silanol functions as it was also affected by other hydroxylated molecules: HEMA, H2O or methanol (released during the photoacid-induced sol–gel reaction). From Fig. 2D, the intensity of the characteristic peak of epoxy function at 884 cm−1 decreased during 40 s irradiation and continued to decrease with time. Due to the formation of silanol, the intensity of its characteristic peak at 900 cm−1 gradually increased and overlapped with the epoxy peak, it was therefore hard to quantitatively determine the conversion of the epoxy group.
One interesting feature concerns the discrepancy between the free radical polymerization (CC bond), sol–gel (methoxysilyl) and cationic (epoxy ring-opening) reaction kinetics of HBT-50 formulation. After 40 s irradiation, the hydrolysis of methoxysilyl has barely started (<5%), whereas the C
C double bond conversion has reached about 60%, and the conversion of epoxy about 50%. Comparing the downward trend of 2840 and 884 cm−1 before and after light-irradiation, it can be inferred that the cationic polymerization rate was faster than the sol–gel reaction. This result indicated that the protons trapped in the polymethacrylate-based network can efficiently catalyze the epoxy ring-opening polymerization and sol–gel reaction after the visible light was turned off. At the same time, the photoacid-induced sol–gel reaction can be further affirmed by the mass change before and after storage in vacuum oven (due to the evaporation of generated volatile small molecules, such as methanol and water) and also by the DMA data (see ESI Fig. 2 and 3†).
DMA data provide information on the relaxation of molecular motions, which are sensitive to the polymer network structure. In this study, the DMA tests were carried out using both standard 3-point bending (for dry condition test) and 3-point bending submersion methods (for wet condition test). In dry conditions, these tests give the bulk mechanical properties of adhesive copolymer while the results acquired in wet conditions are more representative of the copolymer behavior in the oral environment.
Storage modulus defines the energy stored elastically in the materials.57 The storage moduli of commercial dental adhesives ranged from 2–6 GPa at 25 °C in dry conditions.58 In the present study, storage moduli of the control and experimental copolymers were 3.5–4.5 GPa except HBT-35. The storage moduli of the control and experimental copolymers were generally comparable with the storage moduli reported for commercial dental adhesives. With the increase in TS concentration from 5 to 35 wt%, the storage modulus at 37 °C and rubbery moduli (Fig. 3A) were comparable or slightly higher than that of the control. However, the Tg (Fig. 3C) is significantly lower than that of the control (p < 0.05). There are three main reasons: with the increase in TS concentration, (i) number of Si–O–Si bonds generated from photoacid-induced sol–gel reaction increases; (ii) the concentration of crosslinker BisGMA decreases from 55 to 27.5 wt%; and (iii) number of linear polyether chains formed from the epoxy ring-opening cationic polymerization increases. In dental adhesives, crosslinked copolymers have been shown to exhibit better physico-mechanical strength than linear copolymers.59,60 With the decrease in BisGMA concentration, the crosslinking density of the polymethacrylate-based network decreases. However, photoacids were able to initiate the hydrolysis and condensation reaction of methoxysilyl groups, leading to the formation of Si–O–Si linear or Si–O–C crosslinked structures. All in all, the slightly increased rubbery modulus suggested that a highly crosslinked network was obtained in the experimental copolymers.
In the early 1980s, Penczek and Kubisa discovered that the addition of alcohols during the cationic polymerization of oxirane monomers caused the occurrence of the “activated monomer” (AM) propagation mechanism.61,62 In the AM mechanism, reaction of the protonated cyclic ether with the hydroxyl group containing compounds leads to ring-opening that reforms the hydroxyl group, which can affect the network structure of the polymer.63–65 In the present study, in excess of hydroxyl-containing methacrylate monomers (HEMA and BisGMA) and one epoxy function in TS, the cationic polymerization is better viewed as a transfer reaction affording short and non-crosslinked polyether chains. With the increase in TS concentration, the crosslinking density of polymethacrylate-based network decreased while the number and length of polyether chains increased. The result was a concomitant decrease in the glass transition temperature.
Conversely, when all of the samples were soaked in water for 8 weeks and dried again, the storage modulus at 37 °C showed no significant difference from the control (p < 0.05). Also the rubbery modulus and Tg of experimental formulations were significantly higher than that of the control (p < 0.05). These differences may be attributed to the following: first, most of the leachates (such as unpolymerized monomers, hydrophilic oligomers, methanol, etc.) have been removed during the aqueous storage.14,66 Secondly, the pendant CC double bonds are further polymerized and finally, due to the continuance of the sol–gel reaction in wet conditions, a highly cross-linked network structure was obtained. From the full width at half maximum (FWHM) versus temperature plots (see ESI Fig. 4†), it can be observed that the experimental formulation with 10–15 wt% TS showed the lowest FWHM, which indicated the network became more homogeneous after storage in the wet environment.
The three-point bending water-submersion clamp method used in this work is expected to simulate the wet environment of the mouth. Increasing the storage time in aqueous media from 1 to 8 weeks provided the opportunity to gain insight regarding the autonomic self-strengthening process. With the increase in storage time, the storage modulus values of the control are similar about 270–315 MPa and the Tg increased by only 3 °C. When the samples were soaked in water, the water penetrates the network and the mobility of polymer chains is enhanced. The “trapped” free radical can further initiate the un-reacted CC double bond which leads to a slight improvement in the crosslink density, and the Tg value shifts to higher temperature.
With the exception of the samples containing 5 wt% TS, the storage modulus values of the experimental formulations at 70 °C were significantly higher than that of the control (p < 0.05). The storage modulus increased with storage time and reached maximum value (nearly 1 GPa) after 8 weeks when the TS concentration was between 15 and 20 wt%. With the addition of TS in the formulations, the “trapped” strong photo-generated Brønsted acid was efficient at driving the sol–gel reaction in wet conditions. The gradually formed Si–O–Si or Si–O–C bonds significantly improved the physico-mechanical strength.
To assess the resistance of the formulations to hydrolytic degradation, the properties were determined following specimen storage in low pH media.67,68 There is limited evidence to support significant degradation of dental adhesives at low pH.10,69 Results suggest that the degradation of resin-based dental materials occurs at similar rates in water, artificial saliva and in neutral to slightly low pH media.1 In our previous report, 1 mM lactic acid solution (LA, pH = 3.50/25 °C) was used to accelerate the degradation of HEMA/BisGMA copolymer. Under these conditions, the DMA results indicated that the mechanical properties of the control exhibited good stability.26
To accelerate the degradation of dental adhesive, 0.1 M LA solution (pH = 2.4/25 °C) was used as the storage solution. The storage modulus of the control in LA solution was slight lower than that in water (Tables 2 and 3). The storage modulus of the control at 70 °C decreased about 15%. This may be attributed to gradual degradation of the polymethacrylate-based network in acidic solution.
In an acidic environment (pH = 2.4/25 °C), the silanol species were likely protonated and the hydrolysis rate of methoxysilyl groups was fast. At the same time, the condensation rate was relatively slow when compared with the neutral conditions,70 which was prone to the formation of branched structure (lower crosslink density region). After 8 weeks storage in 0.1 M LA solution, the lower storage modulus at 70 °C with 5, 10, or 15 wt% TS, compared with that of samples stored in water, supported the formation of branched structure in acidic environment. However, the results appeared contradictory when the TS concentration was over 20 wt%, i.e. higher storage modulus at 70 °C in LA solution. In actuality, with the increase in TS concentration, the amount of unpolymerized epoxy function and the number of silanol groups increased. The higher concentration of silanol groups promote the sol–gel reaction and the epoxy ring-opening reaction, which contributed to the crosslink density. For the experimental formulations stored in acidic solution, the mechanical properties of polymethacrylate-based networks showed a slight decrease, but the newly formed Si–O–Si and/or Si–O–C bonds led to increased crosslink density and there was a gradual improvement in the mechanical properties of the hybrid copolymers.
To further investigate the effect of the self-strengthening process on the mechanical properties of the copolymers, we have performed monotonic stress–strain on the control and experimental formulations in wet conditions. When the samples were soaked in water for 1 week, the modulus of elasticity (E) of experimental formulation was significantly lower than that of the C0 (p < 0.05). After soaking in acidic solution for 1 week, the E values of the experimental were comparable with that of the C0. Due to pH dependence of hydrolysis, condensation, and depolymerization, at low pH (<3), the depolymerization rate decreased over 4 orders of magnitude, while the rate of condensation was low with respect to the rate of hydrolysis.70 Under these conditions, TS monomer may undergo essentially irreversible condensation, which improves the crosslink density of the network. It is apparent in Table 4, with the exception of samples containing TS concentration >20 wt%, the E of the experimental specimen soaked in LA/EtOH for 3 weeks was significantly higher than those samples soaked for 1 week (p < 0.05). When TS is 5–15 wt%, the failure strain is comparable to that of the control (data not published). However, by further increasing TS concentration to 20 or 35 wt%, the specimens became brittle after soaking in acidic solution (failure strain was less than 4% for sample with 35 wt% TS). These results indirectly support the formation of a highly crosslinked network structure.
It is conceivable that under clinical conditions with limited polymerization of the dental adhesive there could be significant leaching of monomers, such as HEMA, into the surrounding tissues. This is concerning since in vitro investigations report that HEMA can induce apoptosis, interfere with DNA synthesis and the production of reactive oxygen species (ROS).71–75 In addition, low concentrations of HEMA can interfere significantly with the expression of type I collagen by gingival fibroblasts.76 In the present study, HPLC results indicated considerable leaching of both HEMA (13.2 wt%) and BisGMA (6.2 wt%) from the control formulation. With the increase of TS concentration, the amount of both HEMA and BisGMA that was leached from the polymer decreased significantly, especially when the TS concentration was 35 wt%. Factors contributing to the differences in leachates include the higher DC of CC bond in the experimental formulations. The most important reason is related to the epoxy ring-opening polymerization and photoacid induced sol–gel reactions. The epoxy groups could react with the hydroxyl groups of HEMA or BisGMA and covalently bond the epoxide network to the polymethacrylate network. Meanwhile, the condensation reaction between the silanol/silanol or silanol/hydroxyl groups of HEMA/BisGMA further inhibited leaching of these species. In our previous study using MPS-containing copolymers, when the MPS concentration was 10 wt%, the leached percentages of HEMA and BisGMA were 3.8 and 2.3 wt%.26 In the present study, the leached percentages of HEMA and BisGMA from the copolymer with 10 wt% TS were 1.8 and 1.8 wt%, respectively. The molar concentration of MPS and TS in the formulation was similar (molecular weights of MPS and TS are 248.35 and 246.38 g mol−1). The lower leached percentage in TS-containing copolymer was mainly attributed to the epoxy ring-opening reaction with hydroxyl groups. Meanwhile, it can be observed that the release rates of HEMA/BisGMA in experimental samples were decreased according to the increase in the TS concentration. Both the epoxy ring-opening and the sol–gel reaction are beneficial in terms of enhancing the crosslink density and reducing the leaching of HEMA and BisGMA.
A schematic illustration of the hybrid network structure after light irradiation is given in Scheme 2. It is usually reported that the curing time is in the range of 30–60 seconds for dental resin polymerization.77–80 We used 40 s for this in vitro study to insure that the different resin formulations could polymerize well. Since the curing time, light source and energy were consistent for all of the formulations, the curing time should not be a major factor influencing the structure and properties of the cured polymers. The concentration of TS monomer in the formulation should be a major factor. Based on the FTIR results, the rate of free radical polymerization is the fastest and the photoacid-induced hydrolysis–condensation is the slowest. When the liquid resin is irradiated by visible-light, the polymethacrylate-based matrix network is formed first by free radical cross-linking polymerization of methacrylate monomers (HEMA and BisGMA). Simultaneously, the generated photoacids can catalyze the epoxy ring-opening polymerization and polyether chains are obtained. Due to the excess amount of hydroxyl groups (HEMA and BisGMA), most of the polyether chains are grafted onto the polymethacrylate chains via the AM mechanism.61,62 The degree of hydrolysis and condensation of methoxysilyl groups (<5%) is very limited during 40 s light-irradiation. After 24 h storage, the newly formed Si–O–Si covalent bonds are limited. The similar rubbery modulus obtained from the DMA in dry conditions (Fig. 3A) supported this proposed mechanism. When the specimens are soaked in water, the mobility of the backbone and side chains is improved due to the plasticizing effect of water. As a result of the increased mobility the opportunity for condensation between silanol groups or ring-opening reactions between the un-polymerized epoxy and silanol groups is enhanced. In summary, the copolymers containing TS showed higher crosslink density after storage in wet conditions.
The self-strengthening hybrid system developed here offers additional opportunities to integrate biological motifs into adhesive design. Biomolecular assisted design of hybrid interfaces that can be coupled into the polymerization scheme may further extend the capabilities of these promising systems. For example, biomolecules may be integrated with the polymeric networks to metabolize the volatile small molecules produced during the photoacid-induced sol–gel reaction. Biomolecular approaches could be further extended to bring bioactivity to the hybrid system to facilitate the integration of the restorative material–tissue interfaces.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09933e |
This journal is © The Royal Society of Chemistry 2016 |