Materials Science and Engineering C 43 (2014) 432–438
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Synthesis of iodine-containing cyclophosphazenes for using as radiopacifiers in dental composite resin Yuchen Zhao a, Jinle Lan a, Xiaoyan Wang c, Xuliang Deng c, Qing Cai a,b,⁎, Xiaoping Yang a,b a b c
State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, PR China Department of Geriatric Dentistry, Peking University School and Hospital of Stomatology, Beijing 100081, PR China
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Article history: Received 12 February 2014 Received in revised form 9 June 2014 Accepted 14 July 2014 Available online 19 July 2014 Keywords: Radiopaque Composite resin Cyclophosphazene Iodine
a b s t r a c t In this study, a strategy of using iodine-containing cyclophosphazenes as radiopacifiers for dental composite resin was evaluated. It was hypothesized that cyclophosphazenes bearing both iodine and acrylate group swere able to endow composite resins radiopacity without compromising mechanical properties. The cyclophosphazene compounds were synthesized by subsequently nucleophilic substitution of hexachlorocyclotriphosphazene with hydroxyethyl methacrylate (HEMA) and 4-iodoaniline. Cyclotriphosphazenes containing two different molar ratios of HEMA to 4-iodoaniline (1:5 and 2:4) were obtained, and were identified with 1H NMR, FT-IR, UV and mass spectroscopy. The iodine-containing cyclophosphazenes were able to dissolve well in bisphenol A glycidyl methacrylate (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) resin, and were added at two contents (10 or 15%wt. of the resin). The resins were photo-cured and post-thermal treated before characterizations. The resulting composite resins demonstrated the ability of blocking X-ray. And the addition of HEMA-co-iodoaniline substituted cyclotriphosphazenes caused minor adverse effect on the mechanical properties of the resins because the cyclotriphosphazenes could mix well and react with the resins. The presence of rigid phosphazene rings between resin backbones displayed an effective function of decreasing polymerization shrinkage. In summary, soluble and reactive iodine-containing cyclotriphosphazenes demonstrated advantages over traditional heavy metals or metal oxides in being used as additives for producing radiopaque dental resins. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Composite resins have been widely applied in dental clinic therapy as restorations, luting agents, base and core foundations, etc. [1]. To achieve efficient treatment, there is a growing interest to improve the properties of dental composite resins through years. The common challenges include improving mechanical properties, decreasing water sorption, reducing polymerization shrinkage, and increasing monomer conversion [2,3]. In addition, dental materials should be sufficiently radiopaque to be detected against a background of enamel and dentin for clinical diagnostic purposes. This property is quite useful not only to evaluate restoration, but also to monitor its long-term stability [2,4]. Acrylate-based resins such as bisphenol A glycidyl methacrylate (Bis-GMA) are the mostly used dental reparation materials, but they are radiolucent. Many approaches have been reported in attempting to give them some degree of radiopacity. Usually, radiopacifying agents such as heavy metal powders, inorganic salts of a heavy element, or organic compounds containing a heavy atom substituent were ⁎ Corresponding author at: Beijing Laboratory of Biomedical Materials, State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China. Tel./fax: +86 10 64412084. E-mail address: [email protected] (Q. Cai).
http://dx.doi.org/10.1016/j.msec.2014.07.050 0928-4931/© 2014 Elsevier B.V. All rights reserved.
physically blended with the polymer [5,6]. However, these attempts often suffered from mechanical and biocompatible problems that the resulted resins were clinically unacceptable [7,8]. Reactive metal salts of vinyl monomers such as barium and zinc acrylates, which were able to copolymerize with acrylate-based resins, could impart radiopacity to dental resins with a more homogeneous structure [9–11]. The ionic nature of these resins, however, led to a gradual loss of the opacifying atom due to water uptake and subsequent hydrolysis [12]. Halogen atoms such as iodine and bromine are quite radiopaque because of their high electronic densities. Therefore, vinyl monomers containing covalently bound iodine or bromine were chosen to copolymerize with other acrylic monomers to impart radiopacity [13,14]. They have been identified as good sources for radiopaque polymer matrixes [15–17]. The major issues of these iodine- or bromine-containing monomers are their reactivity and halogen content. The reported iodine- or bromine-containing monomers usually comprised an aromatic ring, halogen atoms and arcylate residues [18,19]. To achieve high stable radiopacity and fast reactions, cross-linkable monomers containing more than one acrylate group and one halogen atom are desired. Cyclophosphazenes have a ring-structure with alternative phosphorus and nitrogen single and double bonds. There are two groups attaching to each phosphorus atom, which are
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Fig. 1. 1H NMR spectra of two HEMA-co-iodoaniline substituted cyclotriphosphazenes: (a) CP01 with HEMA to 4-iodoaniline ratio of 1:5; (b) CP02 with HEMA to 4-iodoaniline ratio of 2:4.
connected via nucleophilic substitution starting from chlorocyclophosphazenes such as hexachlorocyclotriphosphazene (HCCP, P3N3Cl6) or octachlorocyclotetraphosphazene (OCCP, P4N4Cl8) [20,21]. Six or eight functional groups are able to be attached, which provide the potential to bear several acrylate groups and halogen atoms at the same time. Thus, it is hypothesized that incorporating iodine-containing cyclophosphazene monomers into composite resins is a promising way to achieve sufficient radiopacity at low addition amounts. And their organic feature and crosslinkability are envisioned advantageous over traditional radiopacifiers in modifying the performance of composite resins. To this end, hydroxyethyl methacrylate (HEMA) and 4-iodoaniline co-substituted cyclotriphosphazenes were designed and synthesized via nucleophilic substitution of HCCP. The resulting cyclotriphosphazenes were then incorporated into Bis-GMA resins and copolymerized. Characterizations including mechanical properties, conversion of vinyl group, polymerization shrinkage and radiopacity were carried out to evaluate the possibility using HEMAco-iodoaniline substituted cyclotriphosphazenes as radiopacifiers for dental composite resins. 2. Materials and methods 2.1. Materials HCCP (Boyuan New Material Ltd., Ningbo, China) was recrystallized by using anhydrous hexane and subsequently sublimed under vacuum
(55 °C, ~ 0.1 mm Hg). HEMA (Alfa Aesar) and triethylamine (Beijing Chemical Works, China) were dehydrated by distilling from CaH2. Tetrahydrofuran (THF, Beijing Chemical Works, China) was dehydrated with sodium and distilled. Light-curable dental restoratives including BisGMA and triethylene glycol dimethacrylate (TEGDMA), as well as camphorquinone (CQ), 2-(dimethylamino) ethyl methacrylate (DMAEMA), and 4-iodoaniline were supplied by Sigma-Aldrich and used directly. All other reagents and solvents were of analytical grade and supplied by Beijing Chemical Reagent Co., Ltd. (Beijing, China). 2.2. Synthesis of HEMA-co-iodoaniline substituted cyclotriphosphazenes The preparation of HEMA-co-iodoaniline substituted cyclotriphosphazenes was performed via subsequent nucleophilic substitution with HEMA and 4-iodoaniline. Briefly, the HCCP was dissolved in anhydrous THF with triethylamine (2:1 in molar ratio to P-Cl bond) being added. HEMA (1:6 or 2:6 in molar ratio to P\Cl bond) solution in THF was then added dropwise, and then the mixture was stirred continuously for 48 h at room temperature. Subsequently, iodoaniline/THF solution (5:6 or 4:6 in molar ratio to P\Cl bond) was added dropwise into the system. The reaction was continued for another 96 h at room temperature. The whole procedure was protected under dry nitrogen atmosphere to prevent oxidation. After the insoluble hydrochloride salts were removed by filtration, the resulting solution was concentrated by vacuum evaporation of THF. The remains were dissolved
Fig. 2. FT-IR (a) and UV (b) spectra of HEMA-co-iodoaniline substituted cyclotriphosphazenes and HCCP control.
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reached an optical density of between 1.5 and 2, and photos were taken. The exposure time was 0.3 s. 2.4. Fabrication of cyclophosphazene-containing dental resins
Fig. 3. A typical mass spectrum of HEMA-co-iodoaniline substituted cyclotriphosphazene (CP02).
in dichloromethane and extracted with water to remove any residual reagents and hydrochloride salts. Finally, the dichloromethane solution was dehydrated with magnesium sulfate and the purified product was obtained by drying out the dichloromethane. The products are viscous brown semisolid and the yield was ~65% based on HCCP. 2.3. Characterization of HEMA-co-iodoaniline substituted cyclotriphosphazenes The chemical structures of the substituted cyclotriphosphazenes were determined by hydrogen nucleation magnetic resonance (1H NMR, AV600, Bruker, Germany) analysis using CDCl3 as solvent. Fourier transform infrared spectrum (FT-IR) was obtained on a FT-IR spectrometer (Nicolet 6700, Thermo Scientific) by accumulating 10 scans with a resolution of 4 cm−1 in the range of 450–4000 cm−1. Ultraviolet–visible (UV) spectra were measured on an UV spectrophotometer (UV-3600, Shimadzu, Japan) in the range of 220–500 nm by compressing samples on a disc fulfilled with barium sulfate. Mass spectrum (MS) measurements were performed on an ESI-MS equipment (Xevo G2 Q-Tof, Waters) by dissolving samples in methanol. The X-ray photos of HEMA-co-iodoaniline substituted cyclotriphosphazenes were taken by a single-phase dental X-ray unit (Tube/Hv Generation Housing Assembly, Instrumentarium Imaging, Finland). The specimens and the aluminum step wedges were irradiated with X-rays at (65 ± 5) kV from a distance of 300–400 mm until the specimens and the aluminum
Firstly, certain amount (10 or 15%wt.) of HEMA-co-iodoaniline substituted cyclotriphosphazene was dissolved in TEGDMA to get a homogeneous liquid. Then the mixture was blended with Bis-GMA at a weight ratio of 1:1 to TEGDMA. Meanwhile, CQ (0.5%wt.) and DMAEMA (0.5%wt.) were added as photo initiator and co-initiator, respectively. Teflon mold was used to produce beam shape composite specimens for three-bending test and dynamic mechanical thermal analysis (DMTA), with the size of 25 mm × 2 mm × 2 mm (l × w × h) and 45 mm × 5 mm × 2 mm (l × w × h), respectively. The resin mixture was injected into molds and degassed under vacuum in the yellow-light room to avoid the premature curing. Subsequently, the resin was photocured 120 s for each side by using curing light (300 mW/cm2, Coltolux LED). The specimens were retrieved from the mold, post thermotreated (120 °C, 48 h) [22,23] and carefully polished before being subjected to three-point bending tests. The final dimensions of the specimens were measured by a Vernier caliper. 2.5. Characterization of cyclophosphazene-containing dental resins The photo polymerization kinetic was studied by using real time FTIR (MA 5700, Waltham, Thermo Electro Corporation) via putting liquid resin samples between two KBr pellets and initiated with a light-curing unit (300 mW/cm2, Coltolux LED). The degree of conversion (DC) of C_C bonds was calculated by normalizing the peak area of aliphatic C_C bond against that of aromatic C_C bond [14,22]. Polymerization shrinkage was determined by a single-view volumetric reconstruction mode for 2 min after cure. Briefly, 10 μl of resin was shaped into a semi-spherical container (ϕ = 5 mm) and then placed on an AcuVol's (Bisco, Inc., Schaumburg, IL, USA) pedestal. The sample was allowed to rest for 3 min before being light cured. Six specimens were tested for each group of resin samples. The flexural strength (Fs), flexural modulus (Ey) and work of fractrue (WOF) of resin specimens were measured by the three-point bending test on an Instron 1121 (Britain) according to the ISO10477:92 standard. With a span of 20 mm and a crosshead speed of 1.0 mm/min being set, the load–deflection curves were recorded (Composite Material V6.2). Fs, Ey and WOF were calculated from the following formula:
2
Fs ¼ 3Fl=2bh
Fig. 4. Comparison of FT-IR spectra for Bis-GMA/TEGDMA resins before and after photo-curing. The magnification of the 1500–1700 cm−1 region in (a) was presented as (b).
ð1Þ
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Fig. 5. Polymerization kinetics (a) and rate profile (b) of Bis-GMA/TEGDMA resins containing different amounts of CP01 or CP02.
3
3
Ey ¼ l F 1 =4fbh
ð2Þ
WOF ¼ A=bh
ð3Þ
where F is the applied load (N) at the highest point of load–deflection curve, l is the span length (20 mm), b is the width of the specimen, and h is the thickness, F1 is the load (N) at a convenient point in the straight line portion of the trace, and f is the deflection (mm) of the specimen at load F1. A in Joules (J) is the work done by the applied load to deflect and fracture the specimen, corresponding to the area under the load–deflection curve. WOF is the work of fracture in J/m2 or kJ/m2. Ten specimens were tested. Fracture surfaces were observed with a scanning electron microscope (SEM, Supra 55, Zeiss, German) at an accelerating voltage of 20 kV after being sputter-coated with platinum (30 mA, 60 s) using a sputter coater (Polaron E5600, USA). The iodine mapping illustration was performed as the same parameters to SEM observation and the exposure time was 180 s. Dynamic mechanical properties of all the samples were determined by a dynamic mechanical thermal analyzer (DMTA V, Rheometric Scientific Inc., Piscataway, US) in a three-point bending mode at a frequency of 5 Hz and a scan rate of 5 °C/min within a temperature range of 25 to 250 °C.
3. Results and discussion 3.1. Chemical structure of HEMA-co-iodoaniline substituted cyclotriphosphazenes In this study, two HEMA-co-iodoaniline substituted cyclotriphosphazenes were designed and prepared as shown in Fig. 1, including monofunctional (CP01) or bifunctional monomers (CP02). To identify the chemical structures of the obtained cyclotriphosphazenes, 1 H NMR, FT-IR, UV and MS were applied. From the 1H NMR spectra (Fig. 1), all the characteristic peaks corresponding to both HEMA and iodoaniline groups were detected. Peaks appearing at 6.18 and 5.58 ppm (peak a and b) were ascribed to the hydrogen atoms of\C_CH2 in HEMA group, and the peak at 7.49 ppm (peak f) resulted from the hydrogen atoms of symmetrically substituted benzene structure in iodoaniline group. According to the integral areas supplied by 1H NMR data, the molar ratios of the two functional groups were calculated. The molar ratios of HEMA to iodoaniline were indeed 1:5 and 2:4, respectively, for CP01 and CP02. Their FT-IR and UV spectra are shown in Fig. 2. Around 3400 cm−1, the absorption peaks of\NH\ group were observed, which was more visible in both CP01 and CP02 than that in the pristine HCCP (Fig. 2a). The signals at 1635 cm−1 and 648 cm−1 were assigned to the stretching vibration of C_C bond in
2.6. Statistical analysis All data were presented in mean ± standard deviation (SD). Statistical analysis was performed using ANOVA followed by Bonferroni comparison, and significant levels were considered at p ≤ 0.05.
Table 1 The degree of conversion (DC) of C_C bond and the polymerization shrinkage of different resin specimens after sequent photo- and thermo-curing. Resin specimens
Neat resin
CP01 resin
DC (%) Polymerization shrinkage (%)
90.2 ± 1.0 87.2 ± 0.9 91.6 ± 0.7 89.3 ± 1.3 83.8 ± 1.1 7.6 ± 0.5 7.9 ± 0.2 7.9 ± 0.2 6.4 ± 0.4 5.3 ± 0.3
10%wt.
CP02 resin 15%wt.
10%wt.
15%wt.
Fig. 6. The tanδ curves of the neat and cyclotriphosphazene-containing Bis-GMA/TEGDMA resins as a function of temperature.
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Fig. 7. Mechanical properties, including (a) flexural strength, (b) flexural modulus and work of fracture (c), of Bis-GMA/TEGDMA resins mixed with different iodine-containing cyclotriphosphazenes.
HEMA and of C\I bond in iodoaniline, respectively. The presence of P_N was also confirmed by the peak around 1200 cm−1. In UV spectra (Fig. 2b), both the characteristic absorption peaks of iodine and phenyl were clearly observed at 243 nm and 285 nm. In addition, as shown in Fig. 3, the fractions of HEMA, iodoaniline and phosphazene ring were also detected by MS. Based on all the above-mentioned results, HEMA-co-iodoaniline substituted cyclotriphosphazenes were successfully synthesized. It was worth noting that the ratios of the two functional groups could be controlled by adjusting their feeding doses.
3.2. Curing kinetics of the cyclotriphosphazene-containing Bis-GMA resins The prepared CP01 and CP02 contained both iodine and acrylate groups. And they were brown colored owing to the presence of phosphorous and nitrogen atoms. Therefore, we first verified if the incorporation of CP01 or CP02 would affect the photo-curing kinetics of acrylate resins. To this end, Bis-GMA/TEGDMA (1:1) resins containing 0, 10 and 15%wt. of CP01 or CP02 were subjected to the real time FTIR analysis of photo-curing procedure. From Fig. 4, it could be seen the strength of the signal around 1635 cm−1 decreasing significantly upon photo-curing. As aforementioned in the experimental section, the degrees of conversion of C_C bonds were obtained by normalizing the area of aliphatic C_C bond (1635 cm− 1) against that of aromatic C_C bond (1610 cm−1) for different specimens at different time points with photo-initiation. As shown in Fig. 5, it was clearly demonstrated that the photo-initiated polymerization rates were slowed down
remarkably with the incorporation of HEMA-co-iodoaniline substituted cyclotriphosphazenes. Initiated by the light, the neat Bis-GMA/TEGDMA resin polymerized promptly, and reached the highest polymerization rate around 25 s. As the reaction continues, the DC of the photo-cured neat Bis-GMA/ TEGDMA resin leveled off at ~ 50%. Nevertheless, the photo-curing rates of all the cyclotriphosphazene-containing Bis-GMA/TEGDMA resins decreased, which led their DCs of C_C bonds quite low. And higher content of cyclotriphosphazene or higher HEMA content in cyclotriphosphazene resulted in lower DC of C_C bond after photocuring. One possible cause was that the brown color of CP01 and CP02 might absorb some light and inhibit the polymerization. Moreover, the addition of CP01 or CP02 increased the density of C_C bond in the resin system, which might further decrease the DC. To promote the polymerization and obtain ideal composite resin matrixes, thermal post curing was necessary for all resin specimens. The thermo-post-treated specimens demonstrated high DCs of C_C bond (Table 1). To achieve high DC of C_C bond and high mechanical properties, thermopolymerization was commonly used for dimethacrylate-based dental resins recently in published works [23,24]. The thermally cured composite resins could be developed as composite resin blocks for computer-aided design/manufacturing applications. 3.3. Properties of the cyclotriphosphazenes-containing Bis-GMA resins From Table 1, it could be seen that the incorporation of the cyclotriphosphazene containing two acrylate groups (i.e. CP02) showed
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Fig. 8. SEM and iodine element mapping images of fracture surfaces of Bis-GMA/TEGDMA resins containing 10%wt. of HEMA-co-iodoaniline substituted cyclotriphosphazenes: (a, a′) CP01; (b, b′) CP02.
some effects on reducing the polymerization shrinkage, suggesting that the rigid phosphazene rings were bound to resin backbones via two acrylate groups, which restricted the polymerization shrinkage of the resin backbones. The tanδ curves of the neat and cyclotriphosphazene-containing BisGMA/TEGDMA resins as a function of temperature are illustrated in Fig. 6. A broad damping peak was observed for each specimen, and it shifted to lower temperature upon the incorporation of CP01 or CP02. The glass transition temperature (Tg) decreased more significantly when the content of the cyclotriphosphazene was 10 wt.% instead of 15%wt.. And the CP02-containing resin specimens exhibited higher Tg than the CP01-containing resin specimens. It was possible that the presence of rigid phosphazene rings between resin backbones might provide some free volume, which facilitated the movement of resin backbones and decreased the Tg. In case of increasing the content of cyclotriphosphazene, the Tg of the resulting Bis-GMA/TEGDMA resin slightly moved to higher temperature due to the increasing content of rigid phosphazene rings. Similarly, in case of increasing the density of C_C bond, the Tg of the resulting Bis-GMA/TEGDMA resin also slightly shifted to higher temperature due to the increasing crosslinking density. The Fs, Ey and WOF of neat and cyclotriphosphazene-containing BisGMA/TEGDMA resins were measured by three-point bending tests, as
listed in Fig. 7. Most specimens showed no significant difference in Fs, Ey and WOF between each other (p N 0.05). In case of CP02(15%wt.)containing resin specimens, the lower Fs, Ey and WOF values (p b 0.05) were the results of their lower DC of C_C bond after curing (Table 1). It was interesting that the incorporation of HEMA-co-iodoaniline substituted cyclotriphosphazene did not cause significantly adverse effect on the mechanical properties of Bis-GMA/TEGDMA resin. Fracture surfaces of cyclotriphosphazene-containing Bis-GMA/ TEGDMA resins were observed with SEM and presented in Fig. 8. The surfaces were homogeneous with no hint of HEMA-co-iodoaniline substituted cyclotriphosphazenes separating from the resin matrix. The iodine element mapping images clearly displayed the evenly distribution of CP01 or CP02 within Bis-GMA/TEGDMA resin, indicating the good miscibility between them. 3.4. Radiopacity of iodoaniline-containing cyclotriphosphazenes and corresponding Bis-GMA resins Radiopacity of HEMA-co-iodoaniline substituted cyclotriphosphazenes were evaluated as well. The cured Bis-GMA/TEGDMA resin specimens containing 10%wt. or 15%wt. of CP01 or CP02 were also detected by using X-ray, and were placed together with the original
Fig. 9. Radiographs of original iodine-containing cyclotriphosphazenes and the corresponding Bis-GMA/TEGDMA resin specimens taken with aluminum step wedges. (a) CP01; (b) CP02; (c) composite resin beam containing 10%wt. CP01; (d) composite resin beam containing 10%wt. CP02; (e) composite resin beam containing 15%wt. CP01; and (f) composite resin beam containing 15%wt. CP02.
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cyclotriphosphazenes on an occlusal radiographic film (Fig. 9). Both CP01 and CP02 demonstrated strong radiopacity themselves. Their shadows under X-ray were quite clear because of the presence of iodine atoms. As expected, all corresponding resin specimens also distinctly displayed radiopacity. By comparing to the reference, which is a standard aluminum step wedge with a step thickness of 0.5 mm, 2.0-mm-thick resin specimens containing 10%wt. of CP01 and CP02 showed the radiopacity values similar to that of 2 mm thickness aluminum density. When the content of CP01 or CP02 increased to 15%wt., the corresponding resin specimens showed increasing radiopacity values around 4 mm thickness aluminum density. With the same 2.0 mm thickness, CP01-contained resins demonstrated higher radiopacity values than CP02-contained samples due to the higher content of iodine atom in CP01. It was reported that the radiopacity values for human enamel and dentin were 4.3 and 2.3 mm Al/2.0 mm specimen, respectively [25]. A radiopacity value equaling to or slightly greater than that of enamel was desirable for dental materials. Therefore, composite resins with N10%wt. of iodoaniline-containing cyclotriphosphazenes could be effective to allow detection in dental restorations. 4. Conclusion HEMA-co-iodoaniline substituted cyclotriphosphazenes with two different ratios of HEMA to iodoaniline were synthesized. The substituted cyclotriphosphazenes could dissolve in Bis-GMA/TEGDMA resin and were involved in the polymerization. The resulting cyclotriphosphazene-containing Bis-GMA/TEGDMA demonstrated comparable mechanical properties to the neat resin, while the rigid phosphazene rings had contributions in reducing polymerization shrinkage. Benefiting from the evenly distributed iodine element in resin matrixes, the newly developed composite resins displayed significant radiopacity, which would enable potential detections in dental restorations. Acknowledgments The authors acknowledged the financial support from the National Natural Science Foundation of China (51073016 and 51373016), the National High Technology Research and Development Program of China (2012AA03A203) and the Program for New Century Excellent Talents in University (NCET-11-0556). We would also like to thank Dr. Zhiwei Xie from Pennsylvania State University in editing this paper. References [1] J.G. Leprince, W.M. Palin, M.A. Hadis, J. Devaux, G. Leloup, Progress in dimethacrylate-based dental composite technology and curing efficiency, Dent. Mater. 29 (2013) 139–156.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Synthesis of iodine-containing cyclophosphazenes for using as radiopacifiers in dental composite resin Yuchen Zhao a, Jinle Lan a, Xiaoyan Wang c, Xuliang Deng c, Qing Cai a,b,⁎, Xiaoping Yang a,b a b c
State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, PR China Department of Geriatric Dentistry, Peking University School and Hospital of Stomatology, Beijing 100081, PR China
a r t i c l e
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Article history: Received 12 February 2014 Received in revised form 9 June 2014 Accepted 14 July 2014 Available online 19 July 2014 Keywords: Radiopaque Composite resin Cyclophosphazene Iodine
a b s t r a c t In this study, a strategy of using iodine-containing cyclophosphazenes as radiopacifiers for dental composite resin was evaluated. It was hypothesized that cyclophosphazenes bearing both iodine and acrylate group swere able to endow composite resins radiopacity without compromising mechanical properties. The cyclophosphazene compounds were synthesized by subsequently nucleophilic substitution of hexachlorocyclotriphosphazene with hydroxyethyl methacrylate (HEMA) and 4-iodoaniline. Cyclotriphosphazenes containing two different molar ratios of HEMA to 4-iodoaniline (1:5 and 2:4) were obtained, and were identified with 1H NMR, FT-IR, UV and mass spectroscopy. The iodine-containing cyclophosphazenes were able to dissolve well in bisphenol A glycidyl methacrylate (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) resin, and were added at two contents (10 or 15%wt. of the resin). The resins were photo-cured and post-thermal treated before characterizations. The resulting composite resins demonstrated the ability of blocking X-ray. And the addition of HEMA-co-iodoaniline substituted cyclotriphosphazenes caused minor adverse effect on the mechanical properties of the resins because the cyclotriphosphazenes could mix well and react with the resins. The presence of rigid phosphazene rings between resin backbones displayed an effective function of decreasing polymerization shrinkage. In summary, soluble and reactive iodine-containing cyclotriphosphazenes demonstrated advantages over traditional heavy metals or metal oxides in being used as additives for producing radiopaque dental resins. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Composite resins have been widely applied in dental clinic therapy as restorations, luting agents, base and core foundations, etc. [1]. To achieve efficient treatment, there is a growing interest to improve the properties of dental composite resins through years. The common challenges include improving mechanical properties, decreasing water sorption, reducing polymerization shrinkage, and increasing monomer conversion [2,3]. In addition, dental materials should be sufficiently radiopaque to be detected against a background of enamel and dentin for clinical diagnostic purposes. This property is quite useful not only to evaluate restoration, but also to monitor its long-term stability [2,4]. Acrylate-based resins such as bisphenol A glycidyl methacrylate (Bis-GMA) are the mostly used dental reparation materials, but they are radiolucent. Many approaches have been reported in attempting to give them some degree of radiopacity. Usually, radiopacifying agents such as heavy metal powders, inorganic salts of a heavy element, or organic compounds containing a heavy atom substituent were ⁎ Corresponding author at: Beijing Laboratory of Biomedical Materials, State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China. Tel./fax: +86 10 64412084. E-mail address: [email protected] (Q. Cai).
http://dx.doi.org/10.1016/j.msec.2014.07.050 0928-4931/© 2014 Elsevier B.V. All rights reserved.
physically blended with the polymer [5,6]. However, these attempts often suffered from mechanical and biocompatible problems that the resulted resins were clinically unacceptable [7,8]. Reactive metal salts of vinyl monomers such as barium and zinc acrylates, which were able to copolymerize with acrylate-based resins, could impart radiopacity to dental resins with a more homogeneous structure [9–11]. The ionic nature of these resins, however, led to a gradual loss of the opacifying atom due to water uptake and subsequent hydrolysis [12]. Halogen atoms such as iodine and bromine are quite radiopaque because of their high electronic densities. Therefore, vinyl monomers containing covalently bound iodine or bromine were chosen to copolymerize with other acrylic monomers to impart radiopacity [13,14]. They have been identified as good sources for radiopaque polymer matrixes [15–17]. The major issues of these iodine- or bromine-containing monomers are their reactivity and halogen content. The reported iodine- or bromine-containing monomers usually comprised an aromatic ring, halogen atoms and arcylate residues [18,19]. To achieve high stable radiopacity and fast reactions, cross-linkable monomers containing more than one acrylate group and one halogen atom are desired. Cyclophosphazenes have a ring-structure with alternative phosphorus and nitrogen single and double bonds. There are two groups attaching to each phosphorus atom, which are
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Fig. 1. 1H NMR spectra of two HEMA-co-iodoaniline substituted cyclotriphosphazenes: (a) CP01 with HEMA to 4-iodoaniline ratio of 1:5; (b) CP02 with HEMA to 4-iodoaniline ratio of 2:4.
connected via nucleophilic substitution starting from chlorocyclophosphazenes such as hexachlorocyclotriphosphazene (HCCP, P3N3Cl6) or octachlorocyclotetraphosphazene (OCCP, P4N4Cl8) [20,21]. Six or eight functional groups are able to be attached, which provide the potential to bear several acrylate groups and halogen atoms at the same time. Thus, it is hypothesized that incorporating iodine-containing cyclophosphazene monomers into composite resins is a promising way to achieve sufficient radiopacity at low addition amounts. And their organic feature and crosslinkability are envisioned advantageous over traditional radiopacifiers in modifying the performance of composite resins. To this end, hydroxyethyl methacrylate (HEMA) and 4-iodoaniline co-substituted cyclotriphosphazenes were designed and synthesized via nucleophilic substitution of HCCP. The resulting cyclotriphosphazenes were then incorporated into Bis-GMA resins and copolymerized. Characterizations including mechanical properties, conversion of vinyl group, polymerization shrinkage and radiopacity were carried out to evaluate the possibility using HEMAco-iodoaniline substituted cyclotriphosphazenes as radiopacifiers for dental composite resins. 2. Materials and methods 2.1. Materials HCCP (Boyuan New Material Ltd., Ningbo, China) was recrystallized by using anhydrous hexane and subsequently sublimed under vacuum
(55 °C, ~ 0.1 mm Hg). HEMA (Alfa Aesar) and triethylamine (Beijing Chemical Works, China) were dehydrated by distilling from CaH2. Tetrahydrofuran (THF, Beijing Chemical Works, China) was dehydrated with sodium and distilled. Light-curable dental restoratives including BisGMA and triethylene glycol dimethacrylate (TEGDMA), as well as camphorquinone (CQ), 2-(dimethylamino) ethyl methacrylate (DMAEMA), and 4-iodoaniline were supplied by Sigma-Aldrich and used directly. All other reagents and solvents were of analytical grade and supplied by Beijing Chemical Reagent Co., Ltd. (Beijing, China). 2.2. Synthesis of HEMA-co-iodoaniline substituted cyclotriphosphazenes The preparation of HEMA-co-iodoaniline substituted cyclotriphosphazenes was performed via subsequent nucleophilic substitution with HEMA and 4-iodoaniline. Briefly, the HCCP was dissolved in anhydrous THF with triethylamine (2:1 in molar ratio to P-Cl bond) being added. HEMA (1:6 or 2:6 in molar ratio to P\Cl bond) solution in THF was then added dropwise, and then the mixture was stirred continuously for 48 h at room temperature. Subsequently, iodoaniline/THF solution (5:6 or 4:6 in molar ratio to P\Cl bond) was added dropwise into the system. The reaction was continued for another 96 h at room temperature. The whole procedure was protected under dry nitrogen atmosphere to prevent oxidation. After the insoluble hydrochloride salts were removed by filtration, the resulting solution was concentrated by vacuum evaporation of THF. The remains were dissolved
Fig. 2. FT-IR (a) and UV (b) spectra of HEMA-co-iodoaniline substituted cyclotriphosphazenes and HCCP control.
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reached an optical density of between 1.5 and 2, and photos were taken. The exposure time was 0.3 s. 2.4. Fabrication of cyclophosphazene-containing dental resins
Fig. 3. A typical mass spectrum of HEMA-co-iodoaniline substituted cyclotriphosphazene (CP02).
in dichloromethane and extracted with water to remove any residual reagents and hydrochloride salts. Finally, the dichloromethane solution was dehydrated with magnesium sulfate and the purified product was obtained by drying out the dichloromethane. The products are viscous brown semisolid and the yield was ~65% based on HCCP. 2.3. Characterization of HEMA-co-iodoaniline substituted cyclotriphosphazenes The chemical structures of the substituted cyclotriphosphazenes were determined by hydrogen nucleation magnetic resonance (1H NMR, AV600, Bruker, Germany) analysis using CDCl3 as solvent. Fourier transform infrared spectrum (FT-IR) was obtained on a FT-IR spectrometer (Nicolet 6700, Thermo Scientific) by accumulating 10 scans with a resolution of 4 cm−1 in the range of 450–4000 cm−1. Ultraviolet–visible (UV) spectra were measured on an UV spectrophotometer (UV-3600, Shimadzu, Japan) in the range of 220–500 nm by compressing samples on a disc fulfilled with barium sulfate. Mass spectrum (MS) measurements were performed on an ESI-MS equipment (Xevo G2 Q-Tof, Waters) by dissolving samples in methanol. The X-ray photos of HEMA-co-iodoaniline substituted cyclotriphosphazenes were taken by a single-phase dental X-ray unit (Tube/Hv Generation Housing Assembly, Instrumentarium Imaging, Finland). The specimens and the aluminum step wedges were irradiated with X-rays at (65 ± 5) kV from a distance of 300–400 mm until the specimens and the aluminum
Firstly, certain amount (10 or 15%wt.) of HEMA-co-iodoaniline substituted cyclotriphosphazene was dissolved in TEGDMA to get a homogeneous liquid. Then the mixture was blended with Bis-GMA at a weight ratio of 1:1 to TEGDMA. Meanwhile, CQ (0.5%wt.) and DMAEMA (0.5%wt.) were added as photo initiator and co-initiator, respectively. Teflon mold was used to produce beam shape composite specimens for three-bending test and dynamic mechanical thermal analysis (DMTA), with the size of 25 mm × 2 mm × 2 mm (l × w × h) and 45 mm × 5 mm × 2 mm (l × w × h), respectively. The resin mixture was injected into molds and degassed under vacuum in the yellow-light room to avoid the premature curing. Subsequently, the resin was photocured 120 s for each side by using curing light (300 mW/cm2, Coltolux LED). The specimens were retrieved from the mold, post thermotreated (120 °C, 48 h) [22,23] and carefully polished before being subjected to three-point bending tests. The final dimensions of the specimens were measured by a Vernier caliper. 2.5. Characterization of cyclophosphazene-containing dental resins The photo polymerization kinetic was studied by using real time FTIR (MA 5700, Waltham, Thermo Electro Corporation) via putting liquid resin samples between two KBr pellets and initiated with a light-curing unit (300 mW/cm2, Coltolux LED). The degree of conversion (DC) of C_C bonds was calculated by normalizing the peak area of aliphatic C_C bond against that of aromatic C_C bond [14,22]. Polymerization shrinkage was determined by a single-view volumetric reconstruction mode for 2 min after cure. Briefly, 10 μl of resin was shaped into a semi-spherical container (ϕ = 5 mm) and then placed on an AcuVol's (Bisco, Inc., Schaumburg, IL, USA) pedestal. The sample was allowed to rest for 3 min before being light cured. Six specimens were tested for each group of resin samples. The flexural strength (Fs), flexural modulus (Ey) and work of fractrue (WOF) of resin specimens were measured by the three-point bending test on an Instron 1121 (Britain) according to the ISO10477:92 standard. With a span of 20 mm and a crosshead speed of 1.0 mm/min being set, the load–deflection curves were recorded (Composite Material V6.2). Fs, Ey and WOF were calculated from the following formula:
2
Fs ¼ 3Fl=2bh
Fig. 4. Comparison of FT-IR spectra for Bis-GMA/TEGDMA resins before and after photo-curing. The magnification of the 1500–1700 cm−1 region in (a) was presented as (b).
ð1Þ
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Fig. 5. Polymerization kinetics (a) and rate profile (b) of Bis-GMA/TEGDMA resins containing different amounts of CP01 or CP02.
3
3
Ey ¼ l F 1 =4fbh
ð2Þ
WOF ¼ A=bh
ð3Þ
where F is the applied load (N) at the highest point of load–deflection curve, l is the span length (20 mm), b is the width of the specimen, and h is the thickness, F1 is the load (N) at a convenient point in the straight line portion of the trace, and f is the deflection (mm) of the specimen at load F1. A in Joules (J) is the work done by the applied load to deflect and fracture the specimen, corresponding to the area under the load–deflection curve. WOF is the work of fracture in J/m2 or kJ/m2. Ten specimens were tested. Fracture surfaces were observed with a scanning electron microscope (SEM, Supra 55, Zeiss, German) at an accelerating voltage of 20 kV after being sputter-coated with platinum (30 mA, 60 s) using a sputter coater (Polaron E5600, USA). The iodine mapping illustration was performed as the same parameters to SEM observation and the exposure time was 180 s. Dynamic mechanical properties of all the samples were determined by a dynamic mechanical thermal analyzer (DMTA V, Rheometric Scientific Inc., Piscataway, US) in a three-point bending mode at a frequency of 5 Hz and a scan rate of 5 °C/min within a temperature range of 25 to 250 °C.
3. Results and discussion 3.1. Chemical structure of HEMA-co-iodoaniline substituted cyclotriphosphazenes In this study, two HEMA-co-iodoaniline substituted cyclotriphosphazenes were designed and prepared as shown in Fig. 1, including monofunctional (CP01) or bifunctional monomers (CP02). To identify the chemical structures of the obtained cyclotriphosphazenes, 1 H NMR, FT-IR, UV and MS were applied. From the 1H NMR spectra (Fig. 1), all the characteristic peaks corresponding to both HEMA and iodoaniline groups were detected. Peaks appearing at 6.18 and 5.58 ppm (peak a and b) were ascribed to the hydrogen atoms of\C_CH2 in HEMA group, and the peak at 7.49 ppm (peak f) resulted from the hydrogen atoms of symmetrically substituted benzene structure in iodoaniline group. According to the integral areas supplied by 1H NMR data, the molar ratios of the two functional groups were calculated. The molar ratios of HEMA to iodoaniline were indeed 1:5 and 2:4, respectively, for CP01 and CP02. Their FT-IR and UV spectra are shown in Fig. 2. Around 3400 cm−1, the absorption peaks of\NH\ group were observed, which was more visible in both CP01 and CP02 than that in the pristine HCCP (Fig. 2a). The signals at 1635 cm−1 and 648 cm−1 were assigned to the stretching vibration of C_C bond in
2.6. Statistical analysis All data were presented in mean ± standard deviation (SD). Statistical analysis was performed using ANOVA followed by Bonferroni comparison, and significant levels were considered at p ≤ 0.05.
Table 1 The degree of conversion (DC) of C_C bond and the polymerization shrinkage of different resin specimens after sequent photo- and thermo-curing. Resin specimens
Neat resin
CP01 resin
DC (%) Polymerization shrinkage (%)
90.2 ± 1.0 87.2 ± 0.9 91.6 ± 0.7 89.3 ± 1.3 83.8 ± 1.1 7.6 ± 0.5 7.9 ± 0.2 7.9 ± 0.2 6.4 ± 0.4 5.3 ± 0.3
10%wt.
CP02 resin 15%wt.
10%wt.
15%wt.
Fig. 6. The tanδ curves of the neat and cyclotriphosphazene-containing Bis-GMA/TEGDMA resins as a function of temperature.
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Fig. 7. Mechanical properties, including (a) flexural strength, (b) flexural modulus and work of fracture (c), of Bis-GMA/TEGDMA resins mixed with different iodine-containing cyclotriphosphazenes.
HEMA and of C\I bond in iodoaniline, respectively. The presence of P_N was also confirmed by the peak around 1200 cm−1. In UV spectra (Fig. 2b), both the characteristic absorption peaks of iodine and phenyl were clearly observed at 243 nm and 285 nm. In addition, as shown in Fig. 3, the fractions of HEMA, iodoaniline and phosphazene ring were also detected by MS. Based on all the above-mentioned results, HEMA-co-iodoaniline substituted cyclotriphosphazenes were successfully synthesized. It was worth noting that the ratios of the two functional groups could be controlled by adjusting their feeding doses.
3.2. Curing kinetics of the cyclotriphosphazene-containing Bis-GMA resins The prepared CP01 and CP02 contained both iodine and acrylate groups. And they were brown colored owing to the presence of phosphorous and nitrogen atoms. Therefore, we first verified if the incorporation of CP01 or CP02 would affect the photo-curing kinetics of acrylate resins. To this end, Bis-GMA/TEGDMA (1:1) resins containing 0, 10 and 15%wt. of CP01 or CP02 were subjected to the real time FTIR analysis of photo-curing procedure. From Fig. 4, it could be seen the strength of the signal around 1635 cm−1 decreasing significantly upon photo-curing. As aforementioned in the experimental section, the degrees of conversion of C_C bonds were obtained by normalizing the area of aliphatic C_C bond (1635 cm− 1) against that of aromatic C_C bond (1610 cm−1) for different specimens at different time points with photo-initiation. As shown in Fig. 5, it was clearly demonstrated that the photo-initiated polymerization rates were slowed down
remarkably with the incorporation of HEMA-co-iodoaniline substituted cyclotriphosphazenes. Initiated by the light, the neat Bis-GMA/TEGDMA resin polymerized promptly, and reached the highest polymerization rate around 25 s. As the reaction continues, the DC of the photo-cured neat Bis-GMA/ TEGDMA resin leveled off at ~ 50%. Nevertheless, the photo-curing rates of all the cyclotriphosphazene-containing Bis-GMA/TEGDMA resins decreased, which led their DCs of C_C bonds quite low. And higher content of cyclotriphosphazene or higher HEMA content in cyclotriphosphazene resulted in lower DC of C_C bond after photocuring. One possible cause was that the brown color of CP01 and CP02 might absorb some light and inhibit the polymerization. Moreover, the addition of CP01 or CP02 increased the density of C_C bond in the resin system, which might further decrease the DC. To promote the polymerization and obtain ideal composite resin matrixes, thermal post curing was necessary for all resin specimens. The thermo-post-treated specimens demonstrated high DCs of C_C bond (Table 1). To achieve high DC of C_C bond and high mechanical properties, thermopolymerization was commonly used for dimethacrylate-based dental resins recently in published works [23,24]. The thermally cured composite resins could be developed as composite resin blocks for computer-aided design/manufacturing applications. 3.3. Properties of the cyclotriphosphazenes-containing Bis-GMA resins From Table 1, it could be seen that the incorporation of the cyclotriphosphazene containing two acrylate groups (i.e. CP02) showed
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Fig. 8. SEM and iodine element mapping images of fracture surfaces of Bis-GMA/TEGDMA resins containing 10%wt. of HEMA-co-iodoaniline substituted cyclotriphosphazenes: (a, a′) CP01; (b, b′) CP02.
some effects on reducing the polymerization shrinkage, suggesting that the rigid phosphazene rings were bound to resin backbones via two acrylate groups, which restricted the polymerization shrinkage of the resin backbones. The tanδ curves of the neat and cyclotriphosphazene-containing BisGMA/TEGDMA resins as a function of temperature are illustrated in Fig. 6. A broad damping peak was observed for each specimen, and it shifted to lower temperature upon the incorporation of CP01 or CP02. The glass transition temperature (Tg) decreased more significantly when the content of the cyclotriphosphazene was 10 wt.% instead of 15%wt.. And the CP02-containing resin specimens exhibited higher Tg than the CP01-containing resin specimens. It was possible that the presence of rigid phosphazene rings between resin backbones might provide some free volume, which facilitated the movement of resin backbones and decreased the Tg. In case of increasing the content of cyclotriphosphazene, the Tg of the resulting Bis-GMA/TEGDMA resin slightly moved to higher temperature due to the increasing content of rigid phosphazene rings. Similarly, in case of increasing the density of C_C bond, the Tg of the resulting Bis-GMA/TEGDMA resin also slightly shifted to higher temperature due to the increasing crosslinking density. The Fs, Ey and WOF of neat and cyclotriphosphazene-containing BisGMA/TEGDMA resins were measured by three-point bending tests, as
listed in Fig. 7. Most specimens showed no significant difference in Fs, Ey and WOF between each other (p N 0.05). In case of CP02(15%wt.)containing resin specimens, the lower Fs, Ey and WOF values (p b 0.05) were the results of their lower DC of C_C bond after curing (Table 1). It was interesting that the incorporation of HEMA-co-iodoaniline substituted cyclotriphosphazene did not cause significantly adverse effect on the mechanical properties of Bis-GMA/TEGDMA resin. Fracture surfaces of cyclotriphosphazene-containing Bis-GMA/ TEGDMA resins were observed with SEM and presented in Fig. 8. The surfaces were homogeneous with no hint of HEMA-co-iodoaniline substituted cyclotriphosphazenes separating from the resin matrix. The iodine element mapping images clearly displayed the evenly distribution of CP01 or CP02 within Bis-GMA/TEGDMA resin, indicating the good miscibility between them. 3.4. Radiopacity of iodoaniline-containing cyclotriphosphazenes and corresponding Bis-GMA resins Radiopacity of HEMA-co-iodoaniline substituted cyclotriphosphazenes were evaluated as well. The cured Bis-GMA/TEGDMA resin specimens containing 10%wt. or 15%wt. of CP01 or CP02 were also detected by using X-ray, and were placed together with the original
Fig. 9. Radiographs of original iodine-containing cyclotriphosphazenes and the corresponding Bis-GMA/TEGDMA resin specimens taken with aluminum step wedges. (a) CP01; (b) CP02; (c) composite resin beam containing 10%wt. CP01; (d) composite resin beam containing 10%wt. CP02; (e) composite resin beam containing 15%wt. CP01; and (f) composite resin beam containing 15%wt. CP02.
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cyclotriphosphazenes on an occlusal radiographic film (Fig. 9). Both CP01 and CP02 demonstrated strong radiopacity themselves. Their shadows under X-ray were quite clear because of the presence of iodine atoms. As expected, all corresponding resin specimens also distinctly displayed radiopacity. By comparing to the reference, which is a standard aluminum step wedge with a step thickness of 0.5 mm, 2.0-mm-thick resin specimens containing 10%wt. of CP01 and CP02 showed the radiopacity values similar to that of 2 mm thickness aluminum density. When the content of CP01 or CP02 increased to 15%wt., the corresponding resin specimens showed increasing radiopacity values around 4 mm thickness aluminum density. With the same 2.0 mm thickness, CP01-contained resins demonstrated higher radiopacity values than CP02-contained samples due to the higher content of iodine atom in CP01. It was reported that the radiopacity values for human enamel and dentin were 4.3 and 2.3 mm Al/2.0 mm specimen, respectively [25]. A radiopacity value equaling to or slightly greater than that of enamel was desirable for dental materials. Therefore, composite resins with N10%wt. of iodoaniline-containing cyclotriphosphazenes could be effective to allow detection in dental restorations. 4. Conclusion HEMA-co-iodoaniline substituted cyclotriphosphazenes with two different ratios of HEMA to iodoaniline were synthesized. The substituted cyclotriphosphazenes could dissolve in Bis-GMA/TEGDMA resin and were involved in the polymerization. The resulting cyclotriphosphazene-containing Bis-GMA/TEGDMA demonstrated comparable mechanical properties to the neat resin, while the rigid phosphazene rings had contributions in reducing polymerization shrinkage. Benefiting from the evenly distributed iodine element in resin matrixes, the newly developed composite resins displayed significant radiopacity, which would enable potential detections in dental restorations. Acknowledgments The authors acknowledged the financial support from the National Natural Science Foundation of China (51073016 and 51373016), the National High Technology Research and Development Program of China (2012AA03A203) and the Program for New Century Excellent Talents in University (NCET-11-0556). We would also like to thank Dr. Zhiwei Xie from Pennsylvania State University in editing this paper. References [1] J.G. Leprince, W.M. Palin, M.A. Hadis, J. Devaux, G. Leloup, Progress in dimethacrylate-based dental composite technology and curing efficiency, Dent. Mater. 29 (2013) 139–156.
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