Synthesis of poly(alkenoic acid) with L-leucine residue and methacrylate photopolymerizable groups useful in formulating dental restorative materials.

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Synthesis of poly(alkenoic acid) with L-leucine residue and methacrylate photopolymerizable groups useful in formulating dental restorative materials a

a

a

Tinca Buruiana , Marioara Nechifor , Violeta Melinte , Viorica a

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Podasca & Emil C. Buruiana a

Department of Polyaddition and Photochemistry, Petru Poni Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, Iasi 700487, Romania Published online: 07 Apr 2014.

To cite this article: Tinca Buruiana, Marioara Nechifor, Violeta Melinte, Viorica Podasca & Emil C. Buruiana (2014) Synthesis of poly(alkenoic acid) with L-leucine residue and methacrylate photopolymerizable groups useful in formulating dental restorative materials, Journal of Biomaterials Science, Polymer Edition, 25:8, 749-765, DOI: 10.1080/09205063.2014.905029 To link to this article: http://dx.doi.org/10.1080/09205063.2014.905029

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Journal of Biomaterials Science, Polymer Edition, 2014 Vol. 25, No. 8, 749–765, http://dx.doi.org/10.1080/09205063.2014.905029

Synthesis of poly(alkenoic acid) with L-leucine residue and methacrylate photopolymerizable groups useful in formulating dental restorative materials


Tinca Buruiana*, Marioara Nechifor, Violeta Melinte, Viorica Podasca and Emil C. Buruiana Department of Polyaddition and Photochemistry, Petru Poni Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, Iasi 700487, Romania (Received 23 October 2013; accepted 5 March 2014) To develop resin-modified glass ionomer materials, we synthesized methacrylatefunctionalized acrylic copolymer (PAlk-LeuM) derived from acrylic acid, itaconic acid and N-acryloyl-L-leucine using (N-methacryloyloxyethylcarbamoyl-N′-4-hydroxybutyl) urea as the modifying agent. The spectroscopic (proton/carbon nuclear magnetic resonance, Fourier transform infrared spectroscopy) characteristics, and the gel permeation chromatography/Brookfield viscosity measurements were analysed and compared with those of the non-modified copolymer (PAlk-Leu). The photocurable copolymer (PAlk-LeuM, ~14 mol% methacrylate groups) and its precursor (PAlk-Leu) were incorporated in dental ionomer compositions besides diglycidyl methacrylate of bisphenol A (Bis-GMA) or an analogue of Bis-GMA (Bis-GMA-1), triethylene glycol dimethacrylate and 2-hydroxyethyl methacrylate. The kinetic data obtained by photo-differential scanning calorimetry showed that both the degree of conversion (60.50–75.62%) and the polymerization rate (0.07–0.14 s−1) depend mainly on the amount of copolymer (40–50 wt.%), and conversions over 70% were attained in the formulations with 40 wt.% PAlk-LeuM. To formulate light-curable cements, each organic composition was mixed with filler (90 wt.% fluoroaluminosilicate/10 wt.% hydroxyapatite) into a 2.7:1 ratio (powder/liquid ratio). The lightcured specimens exhibited flexural strength (FS), compressive strength (CS) and diametral tensile strength (DTS) varying between 28.08 and 64.79 MPa (FS), 103.68–147.13 MPa (CS) and 16.89–31.87 MPa (DTS). The best values for FS, CS and DTS were found for the materials with the lowest amount of PAlk-LeuM. Other properties such as the surface hardness, water sorption/water solubility, surface morphology and fluorescence caused by adding the fluorescein monomer were also evaluated. Keywords: resin-modified glass ionomer; amino acid derivative; photopolymerization; mechanical properties

1. Introduction Glass polyalkenoates or glass-ionomer cements (GICs) are hybrid materials that exhibit attractive properties in restorative dentistry such as direct adhesion to tooth structure, minimized microleakage at the tooth–enamel interface, anti-cariogenic action, good biocompatibility and low cytotoxicity.[1–3] The simplest forms of GICs contain polymeric acids (e.g. poly(acrylic acid), poly(acrylic acid-co-itaconic acid), poly(acrylic *Corresponding author. Email: [email protected] © 2014 Taylor & Francis


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acid-co-maleic acid)) and a reactive glass powder (e.g. ion-leachable aluminosilicate), which are subsequently reacted through an acid–base setting reaction in the presence of water.[4,5] Lack of mechanical strength and toughness being a major problem of the GICs,[6] much effort has been focused over the years on improvements and developments. In this direction, significant progress has already been performed by the incorporation of N-vinylpyrrolidone [7–9] and various amino acids (glutamic acid, alanine, glycine and proline) [10–15] in the polymer backbone or through adding hydroxyapatite into GIC powders.[7] Furthermore, inclusion of the resin component in the conventional glass-ionomers allowed the production of resin-modified glass ionomer cements (RMGICs).[16–18] For instance, the adding of 2-hydroxyethyl methacrylate (HEMA) or diglycidyl methacrylate of bisphenol A (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) in the glass-ionomer liquid formulations gave rise to visible lightcured materials which proved to be stronger in tension, less brittle, less sensitive to moisture and less soluble.[2,19–21] Another approach has involved the grafting of photocurable (meth)acrylaytes, such as 2-isocyanatoethyl methacrylate (IEMA), glycidyl methacrylate and t-butylacrylate onto the parent polyacid backbone [22–24] and the obtaining of copolymers with methacrylate side groups that can be cross-linked by a light-curing mechanism in the presence of a photo-initiator. Their properties and clinical performance recommended them as good alternatives to other restorative materials.[17,25] Using the later strategy, our group reported data concerning functionalization of poly(acrylic acid-co-itaconic acid) and poly(acrylic acid-co-itaconic acid-co-N-vinylpyrrolidone) with a hydrophobic derivative of IEMA, (N-methacryloyloxyethylcarbamoyl-N′-2-hydroxyethyl) urea and their formulating in restorative materials.[26] In the light of these considerations, we decided to introduce a leucine derivate and a certain number of photopolymerizable groups in the composition of the acid copolymer, which will be reacted with other dental monomers, and active filler for generating hybrid networks useful in dental restorations. Besides, the use of a fluorescent methacrylate that undergoes polymerization can ensure aesthetic qualities to the restorations, and it may be employed in dental diagnosis.[27] Up to now, there has been no report in the literature on such amino acid-constructed GIC systems possessing fluorescent monomer units directly attached in the organic matrix. Herein, we describe the preparation of a novel ternary polyacid (PAlk-Leu) resulted from acrylic acid (AA), itaconic acid (IA) and N-acryloyl-L-leucine (ALeu), and its chemical modification with (N-methacryloyloxyethylcarbamoyl-N′-4-hydroxybutyl) urea (MCU) to obtain PAlk-LeuM. The effect of inclusion of such water-soluble copolymer in resin/ionomer formulations on some properties, such as polymerization rate, degree of conversion (DC), mechanical parameters, hardness, water sorption/solubility and surface morphology was investigated. To provide a dental restoration capable to exhibit a uniform fluorescence upon exposure to visible light simulating that of natural teeth, N-methacryloyloxyethylN′-fluoresceinyl (urea) was utilized as co-monomer in one of our formulations. 2. Materials and methods 2.1. Materials IA, 1,1′-azobis(cyclohexanecarbonitrile) (ABCN), 4-amino-1-butanol, N,N′-dicyclohexylcarbodiimide (DCC), IEMA, L-leucine, acryloyl chloride, HEMA, Bis-GMA, TEGDMA, sodium hydroxide and hydroxyapatite were purchased from Sigma Aldrich

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Chemical Co. (Taufkirchen, Germany) and used without further purification. AA was also purchased from Sigma Aldrich Chemical Co. (Taufkirchen, Germany) and was purified by vacuum distillation before use. The GC Fuji II LC Improved powder was purchased from GC Corporation, Tokyo, Japan, Batch number 1210161.


2.2. Synthesis of ALeu The synthesis of ALeu derivative was performed according to a procedure previously reported.[28] Firstly, L-leucine (10 g, 76 mmol) was dissolved in 30 mL NaOH solution (5 M) at room temperature.The solution was cooled to −5 °C in an ice bath, and then, acryloyl chloride (6.90 g, 76 mmol) was dropwise added under stirring for 1 h, maintaining temperature between 0 and −5 °C. The reaction mixture was stirred at room temperature for another 24 h, and then neutralized with HCl solution to an acidic pH. The solid monomer was collected by filtration and dried in vacuum at ambient temperature. Yield: 69% (9.70 g). Spectral and elemental analysis data for ALeu are given in Table 1. 2.3. Synthesis of PAlk-Leu copolymer AA (2.87 g), IA (1.3 g) and ALeu (1.85 g) were copolymerized into a molar ratio of 4:1:1, in 1,4-dioxane solution (30 mL). Radical polymerization was initiated by ABCN (0.2 wt.% with respect to the monomers) and performed at 80 °C for 72 h after degassing and purging the monomer solution (20 wt.%) with nitrogen in cylindrical glass tubes, according to a similar procedure previously reported.[29] The formed terpolymer (PAlk-Leu) was precipitated in a large excess of diethyl ether and a fine powder was obtained after drying at 60 °C for 12 h. Yield: 88% (5.3 g). Elem. anal. found: C, 51.42%; H, 6.11%, N, 2.27%. Calcd. for (C26H37O15 N)n: C, 51.74%; H, 6.18%; N, 2.32%. Table 1. Spectral characterization and elemental analysis data for the new monomers used in synthesis. 1

Sample ALeu

MCU

FT-IR (KBr) cm

−1

3339 (N–H), 2949 and 2882 (C–H), 1732 (COOH), 1663 (amide I), 1619 (C=C), 1535 (amide II), 1387 and 1369 (C–H of isopropyl group), 1212 (C–O) 3450–3250 (N–H, O–H), 2936 and 2887 (C–H), 1718 (C=O, ester), 1638 (C=C and NH–CO, amide I), 1569 (NH–CO–NH, amide II), 1452, 1378 (C–H of isopropyl group), 1173 (C–O), 888 (C=C)

H NMR (DMSO-d6), δ (ppm):

8.36 (1H, NH), 6.27 (1H, =CH–CO), 6.08 (1H, =CH trans), 5.62 (1H, =CH cis), 4.31 (1H, CH–N), 1.55– 1.62 (3H, CH–CH2), 0.85–0.88 (6H, CH3)

13

C NMR (DMSOd6), δ (ppm)

174.02 (COO), 164.53 (CO), 131.33 (=CH2), 125.59 (=CH), 50.29 (C–N), 40.10 (CH2), 24.36 (CH), 22.81 and 21.26 (–CH3) 1.57 (4H, CH2), 1.93 167.61 (CO (3H, CH3), 3.11–3.20 acrylate), 158.66 (CO urea), 136.00 and 3.44–3.49 (5H, (C=), 126.03 CH2–NH–CO and (=CH2), 64.19 OH), 3.63 (2H, CH2–OH), 4.19–4.21 (CH2–O), 62.19 (2H, CH2–O–CO), (C–OH), 40.08 5.26–5.34 (2H, NH), (C–N), 39.37 5.58 (1H, =CH trans), (C–N), 29.66 (CH2), 26.81(CH2), 6.11 (1H, =CH cis) 18.24 (CH3)

Elemental analysis (%) Calcd. for C9H15NO3: C, 58.36; H, 8.16; N, 7.56. Found: C, 58.97;H, 8.21; N, 7.73

Calcd. for C11H20N2O4: C, 54.08; H, 8.25; N, 11.47. Found: C, 53.97; H, 8.21; N, 11.61

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2.4. Synthesis of MCU


Methylene dichloride (5 mL) and 4-amino-1-butanol (2.95 g, 33 mmol) were added to a 50 mL three-necked flask equipped with a magnetic stirrer and an addition funnel. The flask was immersed in a cold water bath and IEMA (5.12 g, 33 mmol) was added dropwise. The resulting solution was kept under stirring at room temperature for 24 h. Complete conversion of the starting isocyanate was confirmed by Fourier transform-infrared (FT-IR) spectroscopy following the complete disappearance of the intensity band at 2265 cm−1 ascribed to NCO group. The solution was concentrated in vacuum to give MCU. Yield: 97% (7.82 g). Spectral and elemental analysis data are listed in Table 1. 2.5. Synthesis of PAlk-LeuM Initially, the desired amount of PAlk-Leu copolymer (typically 8 g) was dissolved in 30 mL 1,4-dioxane and cooled in an ice bath under stirring. Then, 2.86 g (13.9 mmol) DCC and 3.39 g (13.9 mmol) MCU dissolved individually in 1,4-dioxane were added, keeping the temperature around 0 °C for another 2 h. Further, the reaction mixture was heated to room temperature and stirred for 24 h under a dry nitrogen atmosphere. The formed dicyclohexylurea was filtered off and the polymer was precipitated into a large amount of diethyl ether. The crude product was separated by decantation and dried in vacuum for 12 h. Yield: 69% (7.5 g). 2.6. Synthesis of N-methacryloyloxyethyl-N′-fluoresceinyl urea N-methacryloyloxyethyl-N′-fluoresceinyl urea (MA-Fl) was prepared according to a previously described procedure.[30] 2.7. Measurements 2.7.1. General analysis The synthesized materials were characterized by FT-IR and nuclear magnetic resonance (NMR) spectrometry. The FT-IR spectra were recorded using a Bruker Vertex 70 Fourier transform infrared spectrometer, and 1H (13C) NMR spectra were recorded using a Bruker AC 400 instrument, at room temperature with deuterated chloroform (CDCl3), deuterated methyl sulfoxide (DMSO-d6) or deuterium oxide (D2O) as solvents, and tetramethylsilane as internal reference. The viscosity of the copolymers was determined at 25 °C using a programmable Brookfield cone/plate viscometer. The viscosities were measured by preparing 1:1 (wt/wt) mixture of polymer and distilled water at a spindle rotational speed of 30 rpm. The average molecular weight was determined in dimethylformamide by GPC analysis on a PL-EMD 950 apparatus equipped with two PL gel-mixed columns using polystyrene standards. Elemental analysis was done with a Perkin-Elmer 2400 Series II CHNS/O Elemental Analyzer (Perkin-Elmer, UK). 2.7.2. Measurement of double bond conversion (DC) The FT-IR photopolymerization experiments were performed on the organic formulations (detailed in Table 2; the polyacid was dissolved in ethanol) subjected to blue light using a dental-curing unit (LA 500, Model Blue-light, Apoza Enterprise Co. Taiwan), and


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recording the FT-IR absorption spectra after different irradiation times. The conversion degree was determined from the differences appeared in the C=C stretching vibration at 1637 cm−1 after various curing periods. For all samples, the average DC values of three specimens under each curing time were reported. Differential scanning photocalorimetry studies were achieved on a DuPont 930 apparatus with a double heat differential calorimeter 912. A standard high pressure mercury lamp with 4.5 mW/cm2 light intensity (belong to the calorimeter) was used for irradiating the sample (1.5 ± 0.5 mg) in the presence of Irgacure 819 (1 wt.%) as the initiator system, using standard pans. The measurements were performed in an isothermal mode under the ambient atmosphere, and irradiation started after 1 min of equilibration. A computer-controlled isothermal method was employed to determine the kinetic parameters (the time to reach the maximum polymerization heat (tmax), double bond conversion and rate of polymerization). 2.7.3. Fluorescence study The fluorescence intensity measurements were obtained on a Perkin-Elmer LS 55 spectrometer (Perkin-Elmer, UK) at room temperature. Monomer mixtures containing 1 wt.% MA-Fl were thoroughly homogenized and deposited in thin layers on glass support. The films were irradiated with the blue light using the same dental lamp and the fluorescence modification was monitored. 2.7.4. Mechanical characterization The resin-modified glass ionomer specimens for contact angle, water sorption/solubility and mechanical determinations were prepared by using a two-component system (liquid and powder) having a powder/liquid ratio (P/L, by weight) of 2.7/1. The composition of the organic liquid matrix is given in Table 2, each sample containing polyacid dissolved in distilled water (1:0.5 by weight), dental monomers and 1 wt.% Irgacure 819 as the initiator system, while the inorganic filler consists in 90 wt.% fluoroaluminosilicate glass Fuji II LC Improved and 10 wt.% hydroxyapatite. Flexural strength (FS), compressive strength (CS) and diametral tensile strength (DTS) were measured using a Shimadzu AGS-J testing machine, with a 5 kN load cell. Cylindrical specimens were prepared in glass tubing with dimensions of 4 mm diameter × 8 mm length for CS and of 4 mm diameter × 2 mm length for DTS investigation. A Teflon mould was used to fabricate the specimens for FS (3 mm in width × 3 mm in thickness × 25 mm in length). Specimens were blue-light irradiated for 30 s on each side, removed from the mould after 15 min in 100% humidity and conditioned in Table 2. Sample F1 F2 F3 F4 F5 F6 F7 F8 *

Composition (wt.%) of the experimental formulations (F1–F8) taken in study. PAlk-LeuM

PAlk-Leu

50 46.66 40 50 50 46.66 40 50

Water

Monomers*

25 23.33 20 25 25 23.33 20 25

25 30 40 25 25 30 40 25

Monomer mixture contains Bis-GMA (F1–F4) or Bis-GMA-1 (F5–F8), TEGDMA and HEMA (2/1/1 wt.%).


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distilled water at 37 °C for 24 h prior to testing. A crosshead speed of 1 mm/min was applied in these tests. The CS was calculated from the equation CS = P/πr2, where P is the load at fracture and r is the radius of the sample cylinder. DTS was estimated from the relationship DTS = 2P/πdt, where d is the diameter and t is the thickness, respectively, of the cylinder. The FS in three-point bending was obtained using the expression FS = 3Pl/2bd2, where l is the distance between the two supports, b the width and d the thickness of the specimen. The Vickers indentation method was utilized to provide controlled damage of the RMGICs. The samples were prepared as disc shapes with diameter of 10 mm and thickness of 2 mm, using Teflon moulds covered with microscope slides. Hardness was measured without polishing the surface of the specimen. A Shimadzu HMV-2 microhardness tester was employed to indent the specimens with a Vickers pyramid, using a force of 10 N and a loading time of 12 s. The 10 N load was chosen so that the indentations could be adequately measured. The indentations were randomly distributed on the surface of the specimens but they were spaced far enough so they do not affect the nearby indentations. Tests were repeated five times for each material. 2.7.5. Measurements of water sorption/solubility and contact angle The water sorption and water solubility were determined by preparing six disc specimens of reduced dimensions (6 ± 0.1 mm diameter and 2 ± 0.1 mm thickness) for each group of mixtures, using Teflon moulds covered with microscope slides. The specimens were hardened by exposing to blue light for 30 s on each side and then allowed to set for 15 min in 100% humidity at 37 °C before removing from the moulds. The samples were pre-conditioned over a desiccant containing calcium sulphate at 37 °C until their weight remain constant (initial weight m1). The specimens were placed in distilled water at 37 °C for seven days and then removed from the water, lightly blotted with a paper to eliminate the surface-adherent water and weighed (m2). Further, the specimens were dried into an oven at 50 °C until their weight was constant again (m3). The water sorption for each sample was determined using the Equation (1): m2 m3 (1) Water sorption ðlg mm3 Þ ¼ V The water solubility was calculated employing Equation (2): m1 m3 Water solubility ðlg mm3 Þ ¼ V

(2)

where m1 is the initial weight of the specimen, m2 is the weight after keeping in water, m3 is the weight of the sample dried at 50 °C and V is the specimen volume. 2.7.6. Scanning electron microscopy The microstructure of the cured samples in fracture was examined by using an environmental SEM QUANTA200 (FEI Company, Hillsboro, USA) coupled with an energy-dispersive X-ray equipment (ESEM/EDX). The composite was examined in low vacuum mode operating at 20 kV using an LFD detector. EDX analysis was utilized to collect the elemental distribution map of the cured samples in fracture. For the EDX analysis, an emission current of 115 μA, an accelerating voltage of 20 kV and spot size of 6 was used, with a collection time of 100 s.


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3. Results and discussion


3.1. Characterization of amino acid acrylate and its ternary copolymer It is well known that the existence of carboxylic acid groups close to the polymeric backbone of poly(acrylic acid) or poly(acrylic acid-co-itaconic acid) is responsible for the rigidity of the organic matrix. Into an attempt to provide improved GICs, ALeu was used as building block for preparing novel amino acid-containing ionomer resins. It is expected that the acid groups to be more available for the salt-bridge formation because leucine is a branched-chain α-amino acid with an aliphatic isobutyl chain. Subsequently, ALeu was synthesized via the Schotten-Baumann reaction of N-acylation of L-leucine with acryloyl chloride in alkaline solution, and the FT-IR and 1H (13C) NMR spectra of this monomer are consistent with the proposed structure (data given in the experimental part). In the first step, ALeu was polymerized with AA and IA monomers into a AA:AI:ALeu feed molar ratio of 4:1:1, to yield a novel polyalkenoic acid by a free-radical polymerization process. The synthesized copolymer (PAlk-Leu) was soluble in water and organic solvents, such as ethanol, N,N-dimethylformamide, dimethyl sulfoxide and 1,4-dioxane. The NMR and FT-IR data sustain the structure shown in Scheme 1, revealing the complete disappearance of the vinyl double bond. Thus, the 1 H NMR analysis of PAlk-Leu reflected the relative content of the three monomers used in synthesis and confirmed a good agreement between the experimental and calculated numbers of repeating units (AA, IA and ALeu). In the 1H NMR spectrum illustrated in Figure 1(a), there are some chemical shifts at 12.25, 7.71 and 0.87 ppm attributed to the protons of carboxyl, amide and methyl groups, respectively. The signals of the methylene/methine protons from IA, AA and ALeu can be identified in the region 2.4–1.2 ppm, and the signal at 2.58 ppm belongs to the methine protons of AA. The CH proton vicinal to NH in the pendant chain appears at 4.2 ppm, and the protons from CH–CH2 bridge at 1.62–1.64 ppm. The integration ratio of peaks ascribed to the protons from carboxyl function (12.25 ppm) and those of the methyl (0.87 ppm), and

Figure 1. 1H NMR spectra for PAlk-Leu (a) and PAlk-LeuM (b) copolymers recorded in DMSO-d6 (inset is given the signal for carboxylic protons from the copolymers).


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Figure 2. 13C NMR spectra for the unmodified PAlk-Leu (a) and modified PAlk-LeuM (b) copolymers recorded in DMSO-d6.

methine (2.58 ppm) groups corresponds to the experimental molar ratio (4:1:1) of the monomers (AA, IA and ALeu). The 13C NMR spectrum of PAlk-Leu (Figure 2(a)) showed different peaks of the three methylene groups in the backbone and another in the leucine side chain between 36 and 42 ppm, and four peaks in the range of 181.3– 176.97 ppm attributed to the four types of carboxyl acid groups from the copolymer structure. The presence of pendant leucine is sustained by distinctive peaks at 167.79 ppm (amide CO), 24.71 and 23.80 ppm (two methylene groups), and at 49.08 and 27.32 ppm (two methine linkages). The FT-IR spectrum for PAlk-Leu (Figure 3(a)) displayed three distinctive peaks at 1723, 1655 and 1546 cm−1 attributed to carbonyl, amide I and amide II bonds, respectively. In addition, a very wide band ranging from 3500 to 2500 cm−1 assigned to OH, NH, CH and CH3 vibration bands was identified. It is clear that carbon–carbon double bond absorption band (1625–1645 cm−1) is missing in the spectrum. The above copolymer was covalently functionalized with MCU, prior obtained by reacting IEMA with 4-amino-1-butanol in order to form via ester linkage a copolymer with photopolymerizable moieties in its structure. The resulting polymer (PAlk-LeuM) remains soluble in water, allowing solution-phase processing required at formulation of polymer/reactive glass/hydroxyapatite compositions with dental monomers. The chemical structure of PAlk-LeuM (Scheme 1) was confirmed by 1H (13C) NMR and FT-IR spectroscopy studies. In contrast to the spectrum of the parent copolymer, examination of the 1H NMR spectrum (Figure 1(b)) indicated the presence of two typical chemical shifts in the case of PAlk-Leu tethered with MCU, namely two singlets at 5.69 and 6.06 ppm associated to =CH trans and cis protons, respectively. The relative integration of methyl and olefinic protons signals indicated that about 14 mol% from the carboxylic moieties of the PAlk-Leu was functionalized with methacrylic moieties. Further proof for the molecular structure of the modified copolymer is provided by the investigation of the 13C NMR spectrum. Unlike the PAlk-Leu, the 13C NMR spectrum of the PAlk-LeuM (Figure 2(b)) presents the characteristic peaks of the butylene and ethylene spacers, methacrylate group, and carbonyl CO in the ester and urea groups. The four CH2 methylene groups in the butylene bridge are positioned at 67.19,



Figure 3.

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FT-IR spectra for PAlk-Leu (a) and PAlk-LeuM (b) copolymers.

41.56, 31.49 and 28.58 ppm, and the other two CH2 belonging to the ethylene linkage are located at 64.27 and 41.56 ppm. Additionally, there are characteristic peaks for the methacrylate group at 129.67 ppm (methylene=CH2), 138.70 ppm (quaternary=C–) and 20.17 ppm (CH3), and the ester CO and urea CO peaks can be identified at 172.49 ppm and 163.35 ppm, respectively. The FT-IR spectrum of PAlk-LeuM (Figure 3(b)) was similar to that for PAlk-Leu, excepting the appearance of new peaks at 1636 cm−1 (C=C stretching vibration) and 815 cm−1 (C=C out-of-plane bending) characteristic to the unsaturated side chains. Also, the intensity of the band at 1722 cm−1 increased due to the two C=O ester groups in the side chains. The GPC result shows that PAlk-LeuM had an average molecular weight (Mw) of about 22,600 mol g−1, which satisfies the molecular weight requirements encountered in the field of dental ionomers.[12,31] In connection, the Brookfield viscosity measurements of PAlk-LeuM dissolved in distilled water in a ratio of 1:1 (wt/wt) indicated a value of 910 cPs that allows achieving workable compositions, which are advantageous in dental practice.[32] 3.2. Photopolymerization study A facile way to evaluate the radical photopolymerization behaviour of photoreactive monomer mixtures consists in the appreciation of the conversion kinetics by FT-IR spectroscopy, taking into account that theoretically, both the stretching vibration of double bond at about 1637 cm−1 and the bending vibration in the 815 cm−1 region undergo changes upon exposure to visible light. In order to monitor the degree of the photocuring reaction, three experimental formulations (F1–F3) incorporating 40–50 wt.% PAlkLeuM (in ethanol that act as solvent to carry the polyacid) besides dental monomers, as Bis-GMA, TEGDMA and HEMA (compositions given in Table 2) were selected for further characterization of their properties, and to compare them with the PAlkLeu-based formulation (F4). By the replacement of Bis-GMA with an analogue Bis-GMA-1 (Scheme 2), which contains about 90% urethane methacrylate groups,[33]


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Figure 4. Evolution of the absorption band characteristic to the double bond from F5 formulation (a) and the conversion progress for all formulations (b) monitored as a function of the blue light irradiation time in the FT-IR spectra.

the F5–F7 formulations with a higher content of photopolymerizable double bonds were also investigated. The FT-IR spectra (Figure 4(a)) recorded for the F5 formulation before and after irradiation with a blue light-curing system in presence of Irgacure 819 (1 wt.%) indicated that the absorbance of the methacrylate double bond at 1637 cm−1 gradually decreased as the monomers photopolymerization progresses, reaching in this case a value of about 54.3% (after 180 s). As can be seen in Figure 4(a), the photopolymerization process of PAlk-LeuM (50 wt.%) and the monomer mixture (Bis-GMA: TEGDMA:HEMA, 2:1:1 gravimetric ratio) to give a highly cross-linked organic matrix runs faster in the first 20–30 s of irradiation. In the following stage of copolymerization, the viscosity of the system had a major impact on the evolution of the curing process. The DC for all formulations varied between 47.3 (F4) and 60% (F7) but surprisingly, the use of Bis-GMA-1 does not lead to significant differences. From the graphical representation (Figure 4(b)), it can remark that a decrease in the amount of PAlk-LeuM had as result an increase of the conversion degree, while for the formulations containing PAlk-Leu a lower DC was registered (DC, F4: 47.3%; F8: 50.4%). Such observations seem to suggest that the addition of increasing amounts of polyacid (modified/ non-modified) influences the photoreactivity of monomers in the system, thus reducing the extent of double bond conversion. In the literature, it is recognized that the photocrosslinking degree of the double bond depends on the polar nature of the polyacid and its hydrophilicity, which can generate opacity on the sample.[34] A similar approach has been carried out on these formulations using photo-DSC method that permits the determination of kinetic parameters of the photoprocess, the polymerization rate and the double bond conversion as a function of UV irradiation time. These findings are visible in Figure 5 where it is clear that the photoreactivity of F3 and F7 expressed through a maximum polymerization rate (Rp, 0.140 and 0.133 s−1, respectively) increased comparatively with the formulation F4 (Rp, 0.094 s−1) and F8 (Rp, 0.071 s−1). Such results obtained under UV irradiation of low intensity may be caused by the structural dissimilarity between PAlk-LeuM (F3, F7) and its unmodified form (F4, F8), as well as the high content of copolymer in the latter. The DC determined after 150 s of irradiation was higher in the case of F3 (DC: 75.62%) and F7 (DC: 73.90%) incorporating 40 wt.% PAlk-LeuM than in F1 (DC: 62.38%) and F8 (DC: 60.50%), where lower conversions were related to the viscosity change which limits the mobility of the double bonds.



Figure 5.

759

PhotoDSC profiles for F1–F8 formulations in the presence of 1 wt.% Irg819.

3.3. Fluorescence of fluorescein derivative Into an attempt to provide fluorescence to these polymeric products, an organic monomer, MA-Fl (structure given in Scheme 2) was added within the F2 formulation as a fluorescing agent (1 mmol%). Indeed, the use of this photopolymerizable additive resulted in an increase of the fluorescence intensity (λem: 419 nm) with irradiation time (up to 90 s), when the sample was excited at 340 nm (Figure 6). It appears that the formation of a hardened material upon light curing and the loss of structural mobility are accompanied by an increase of fluorescence signal up to four times. Finally, the fluorescence emission is blue shifted at 406 nm as photopolymerization occurs. For the hybrid composition (F2, see below) excited at 340 nm, the fluorescence intensity increased with only 10%, due to the physicochemical interaction between the organic matrix and the filler particles via their acidic structures and metal ions (Al and Ca), polymer–polymer (glass) ionic bonding and the polymer–polymer hydrogen bonding. A premise of this investigation is that improvements in the aesthetic quality of such materials can be made if we take into consideration that a fluorescence maximum at around 406 nm (λex: 354 nm) and at 472 nm (λex: 397 nm) was detected in dentin and in enamel, respectively.[27]

Figure 6. Fluorescence intensity modification for F2 formulation containing 1 mmol% MA-Fl with the irradiation time (λex = 340 nm).

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3.4. Formulation and evaluation of the hybrid specimens To formulate light-curable hybrid materials, it is needed to find a good balance between the indispensable components for this kind of products (resin ionomers), which includes the modified copolymer, water, co-monomer and reactive glass powder. The glass powder/liquid weight ratio being among the essential parameters, we prepared eight formulations by mixing the aqueous solution of PAlk-LeuM (F1–F3 and F5–F7) or PAlkLeu (F4 and F8) with Bis-GMA (Bis-GMA-1), TEGDMA, HEMA and filler (90 wt.% fluoroaluminosilicate/10 wt.% hydroxyapatite) using a powder/organic matrix ratio of 2.7:1. Since the most appropriate measure of the strength of GICs is the FS that offers simultaneous information on three types of stresses (tensile at one surface, compression at the other surface and shear strengths),[35] we selected this property to identify the better ionomer composition performed with the synthesized copolymers. The data collected in Table 3 tend to show that less PAlk-LeuM in formulation produced materials with the highest FS value (F3, 64.79 MPa), while more PAlk-LeuM contributed to lower FS values (F1, 36.40 MPa and F5, 28.08 MPa), probably due to fewer ionic/covalent cross-linking on (photo)curing during the delicate phase of the setting caused of some interferences between the both processes.[2] It was evidenced that the differences in composition and behaviour influenced the FS values in the systems incorporating PAlk-Leu, where these slightly increased at 40.77 MPa (F4) and 32.94 MPa (F8) compared to F1 and F5. At first glance, the FS value for F3 formulation seems to be consistent with investigations on the Fuji II LC, which presented a FS of 55.8 MPa, as reported by other authors.[36] On the same set of samples, the flexural modulus increased with decrease in the amount of copolymer (from 2.28 MPa in F1 to 4.12 MPa in F3, and from 2.15 MPa in F4 to 3.48 MPa in F6) determining thus a decrease in the elasticity of the samples. This attenuation of the elasticity can be attributed to the more elastic structure of the PAlk-LeuM included in lower quantities in F3 and F7, as well as to an increase in the cross-linking density of the samples with the increasing amount of photopolymerizable components. Also, the experimental resin ionomers were evaluated for CS and DTS. The CS and DTS values obtained for the specimens tested after storage in distilled water for 24 h are shown in Table 3. It is noticeable that the incorporation of 50 wt.% PAlk-LeuM in the mix did not modify the CS or DTS values (CSF1 = 103.68 MPa; DTSF1 = 17.58 MPa; CSF5 = 110.17 MPa; DTSF5 = 16.89 MPa), and this feature may be connected, at least in part, to a reduced conversion of the double bond in tandem with the slower Table 3. FS, flexural modulus, compressive and DTS, and Vickers microhardness (HV) for the filled materials.

Sample F1 F2 F3 F4 F5 F6 F7 F8

Flexural strength (MPa) 36.40 46.45 64.79 40.77 28.08 38.17 39.79 32.94

(±2.10) (±5.49) (±4.67) (±1.80) (±5.08) (±3.67) (±2.93) (±5.18)

Flexural modulus (GPa) 2.28 2.83 4.12 2.36 2.15 2.63 3.48 2.40

(±0.82) (±0.54) (±0.86) (±0.56) (±0.37) (±0.49) (±1.01) (±0.37)

Compressive strength (MPa) 103.68 (±8.94) 115.78 (±11.65) 139.56 (±9.86) 98.66 (±16.25) 110.17 (±9.70) 123.55 (±13.83) 147.13 (±10.37) 104.12 (±13.92)

Diametral tensile strength (MPa) 17.58 22.54 28.22 14.62 16.89 25.04 31.87 13.86

(±2.87) (±2.14) (±3.80) (±3.11) (±2.09) (±2.74) (±2.61) (±2.85)

HV 34.7 41.2 44.9 31.9 33.2 36.6 40.3 30.1

(±6.3) (±5.2) (±2.2) (±2.4) (±4.0) (±6.4) (±4.4) (±4.3)



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neutralization of the carboxylic groups on the polymer chain. The inclusion of increased amounts of photopolymerizable monomers favours the improvement of mechanical parameters; for instance, for F3 and F7, higher CS and DTS values were measured (CSF3 = 139.56 MPa; DTSF3 = 28.22 MPa; CSF7 = 147.13 MPa; and DTSF7 = 31.87 MPa). The lowest values for CS and DTS were obtained for F4 and F8 containing PAlk-Leu (CSF4 = 98.66 MPa; DTSF4 = 14.62 MPa; CSF8 = 104.12 MPa; DTSF8 = 13.86 MPa). These differences can be ascribed to the presence of photopolymerizable sequences in PAlk-LeuM, even if their contribution is not significant. Therefore, variations within our version of glass ionomer compositions, where water acts as plasticizer in the copolymer structure, have a direct impact on the hardened material properties. According to Rehman’results, the N-methacryloyl-proline-modified GICs [14] exhibited improved DTS (19–26 MPa) and FS (38–46 MPa) in comparison with the Fuji II commercial GICs (12–14 MPa for DTS and 13–18 MPa for FS). In connection, the surface hardness (Vickers Hardness Number HV) of the filled materials based on PAlk-LeuM had values between 33.2 and 44.9 (Table 3), slightly higher than those measured for the F4 and F8 formulations (Hν, F4: 31.9; F8: 30.1) incorporating the non-modified copolymer. The analysis of the data revealed that microhardness recorded for the RMGICs increased with the augmentation of photopolymerizable monomer percentage, and the replacement of Bis-GMA with Bis-GMA-1 does not induce relevant effects in complementary samples. It is important to point out that the microhardness of commercial RMGIC (Fuji II LC) is about 36.2.[37] Turning our attention towards the aqueous solutions of polyacid and the HEMA hydrophilic monomer used in formulations, water sorption and water solubility of the RMGICs were determined after seven days of storage in water at 37 °C, and the values plotted against time are given in Figure 7. Depending on the composition of the formulation, water sorption ranged from 35.18 μg/mm3 (F7) to 97.11 μg/mm3 (F5), whereas water was absorbed in higher amounts in the specimens with more PAlk-Leu (F4: 105.09 μg/mm3; F8: 112.92 μg/mm3). On the other hand, the rise in water solubility was more evident in the materials containing non-modified polyacid (F4: 63.22 μg/ mm3; F8: 73.69 μg/mm3). In the literature, it has been demonstrated that the RMGICs absorb more water than the resin composites,[38] for example water sorption for Fuji II LC Improved was found to be 152.37 μg/mm3.[39] It would be reasonable to note that water sorption leads to colour changes in fillings [40] and negatively affects the mechanical properties of them, in particular by decreasing FS, flexural modulus and hardness.[41]

Figure 7.

Evaluation of water sorption and water solubility for the prepared RMGICs.


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Figure 8. SEM image for RMGIC resulted from F2 formulation in fracture (a) and the corresponding EDX spectrum (b). Inset a SEM image and superposed EDX calcium-mapping distribution.

Scanning electron micrograph for F2 specimen was recorded after fracture (Figure 8(a)), and the presence of hydroxyapatite microcrystals intimately mixed with the fluoroaluminosilicate glass from Fuji II LC Improved linked together by the organic matrix was observed suggesting a good compatibility between the inorganic and organic constituents. The elemental analysis of the fractured sample was measured by registering the EDX spectrum (Figure 8(b)). As can be seen, the EDX spectrum confirms the appearance of carbon, oxygen, fluor, aluminium, silicon, phosphorus and calcium peaks from the main components of the inorganic and organic phase involved in the assembling of experimental resin ionomers. Also, the electron dot mapping image for calcium ions (Figure 8(b), inset) reveals a fairly well dispersion and a homogeneous distribution of these inside the composite, although in the HAP microparticles a cluster of calcium ions was remarked, sustaining that the well-organized long rods are indeed HAP crystals (SEM image). For the investigation of the release of fluoride and adhesion to the tooth structure of these materials, a series of experiments will be taken in study.

HOOC

HOOC *

* 4

1

* *

1

COOH COOH O

1

4

NH

O

O

COOH O

HOOC H3 C

PAlk-Leu

1

NH

HOOC CH3

H3C O

CH3

NH O HN O

CH2 CH3

PAlk-LeuM

Scheme 1. Structure of the ternary copolymer based on AA, IA and ALeu (PAlk-Leu) and its form modified with methacrylate groups (PAlk-LeuM).


763

CH3 O H 2C

O

CH3 O

O

CH3

OH

CH2

O

O

CH3

OH

Bis-GMA CH3

CH3 H N

O

H2C

CH3

O

O

2

O

O

O

H3 C

O

O CH3

H N

CH2 O

CH3

O

CH2

Bis-GMA-1

O H2 C

O 2

O

O

CH2


O

CH3

H N

O

H N O O O

HO

O

OH

N-methacryloyloxyethyl-N'-fluoresceinyl (urea)

Scheme 2.

Structures of the monomers used in our formulations.

4. Conclusions In this study, we prepared a copolymer comprising monomer units of AA, IA and ALeu, and partially modified with MCU to obtain the desired polyacid with photopolymerizable groups. The resulting copolymer was further formulated as aqueous solution in dental ionomer compositions together with dental monomers and filler (90 wt.% fluoroaluminosilicate/10 wt.% hydroxyapatite) using a ratio of 2.7:1 (powder/liquid ratio). However, the effect of modified copolymer addition on the photobehaviour, mechanical properties, surface hardness and water sorption/solubility was evaluated compared to parent copolymer-based formulations, Bis-GMA (Bis-GMA-1), TEGDMA and HEMA, the amount of copolymer affecting photoreactivity and the specific features of such systems. Through the appropriate selection of the fluorescent monomer, the RMGICs containing fluoresceine units (λem = 406 nm) chemically linked to the polymer backbone could find potential applications as novel functional dental materials (aesthetic quality and visual inspection for caries detection). Acknowledgement This work was supported by CNCSIS-UEFISCDI, project number PN-II-PT-PCCA-2011-3.2-1419.

References [1] Wilson AD, Kent BE. The glass-ionomer cement, a new translucent dental filling material. J. Appl. Chem. Biotechnol. 1971;21:313. [2] Culbertson BM. Glass-ionomer dental restoratives. Prog. Polym. Sci. 2001;26:577–604.


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[3] Kovarik RE, Haubenreich JE, Gore D. Glass ionomer cements: a review of composition, chemistry, and biocompatibility as a dental and medical implant material. J. Long Term. Eff. Med. Implants. 2005;15:655–671. [4] Culbertson BM. New polymeric materials for use in glass-ionomer cements. J. Dent. 2006;34:556–565. [5] Lohbauer U. Dental glass ionomer cements as permanent filling materials? – Properties, limitations and future trends. Materials. 2010;3:76–96. [6] Xie D, Brantley WA, Culbertson BM, Wang G. Mechanical properties and microstructures of glass-ionomer cements. Dent. Mater. 2000;16:129–138. [7] Moshaverinia A, Ansari S, Movasaghi Z, Billington RW, Darr JA, Rehman IU. Modification of conventional glass-ionomer cements with N-vinylpyrrolidone containing polyacids, nanohydroxy and fluoroapatite to improve mechanical properties. Dent. Mater. 2008;24:1381–1390. [8] Moshaverinia A, Roohpour N, Ansari S, Moshaverinia M, Schricker S, Darr JA, Rehman IU. Effects of N-vinylpyrrolidone (NVP) containing polyelectrolytes on surface properties of conventional glass-ionomer cements (GIC). Dent. Mater. 2009;25:1240–1247. [9] Yamazaki T, Brantley William, Culbertson BM, Seghi R, Schricker S. The measure of wear in N-vinyl pyrrolidinone (NVP) modifed glass-ionomer cements. Polym. Adv. Tech. 2005;16:113–116. [10] Kao EC, Culbertson BM, Xie D. Preparation of glass ionomer cement using N-acryloyl-substituted amino acid monomers – Evaluation of physical properties. Dent. Mater. 1996;12:44–51. [11] Culbertson BM, Dotrong MH. Preparation and evaluation of acrylic acid, itaconic acid, and N-methacryloylglutamic acid copolymers for use in glass-ionomer type dental restoratives. J. Macromol. Sci. Pure Appl. Chem. 2000;37:419–431. [12] Xie D, Park J-G, Faddah M, Zhao J, Khanijoun HK. Novel amino acid-constructed polyalkenoates for dental glass-ionomer restoratives. J. Biomater. Appl. 2006;21:147–165. [13] Wu W, Xie D, Puckett A, Mays JW. Synthesis of amino acid-containing polyacids and their application in self-cured glass-ionomer cement. Eur. Polym. J. 2003;39:959–968. [14] Moshaverinia A, Roohpour N, Rehman IU. Synthesis and characterization of a novel fastset proline-derivative-containing glass ionomer cement with enhanced mechanical properties. Acta Biomater. 2009;5:498–507. [15] Culbertson BM, Xie D, Thakur A. New matrix resins for glass polyalkenoates or glass-ionomers with pendant amino acid residues. J. Macromol. Sci. Pure Appl. Chem. 1999;36: 681–696. [16] Sidhu SK. Clinical evaluations of resin-modified glass-ionomer restorations. Dent. Mater. 2010;26:7–12. [17] Hse KMY, Leung SK, Wei SHY. Resin-ionomer restorative materials for children. Aust. Dent. J. 1999;44:1–11. [18] Antonucci JM, McKinney JE, Stansbury JW, inventors; US Dept. Health&Human Service, assignee. Resin-modified glass-ionomer dental cement - containing water-soluble or compatible vinyl resin and/or polymer to reduce catastrophic failure due to brittle fracture. United States Patent US 7,160,856-N. 1988 Jul 26. [19] Di Nicoló R, Shintome LK, Myaki SI, Nagayassu MP. Bond strength of resin modified glass ionomer cement to primary dentin after cutting with different bur types and dentin conditioning. J. Appl. Oral Sci. 2007;15:459–464. [20] Mathis RS, Ferracane JL. Properties of a glass-ionomer/resin-composite hybrid material. Dent. Mater. 1989;5:355–358. [21] Nicholson JW, Czarnecka B. The biocompatibility of resin-modified glass-ionomer cements for dentistry. Dent. Mater. 2008;24:1702–1708. [22] Xie D, Park J-G, Faddah M, Zhao J. Preparation, formulation and evaluation of novel photo-cured glass ionomers based on co-polymers of (meth)acrylated amino acids. J. Biomater. Sci. Polym. Ed. 2006;17:303–322. [23] Dotrong MH, Johnston WM, Culbertson BM. Visible light-curable N-methacryloyl glutamic acid modified polyelectrolytes for use in dental applications. J. Macromol. Sci. Pure Appl. Chem. 2000;37:911–926. [24] Zhao J, Xie D. A novel hyperbranched poly(acrylic acid) for improved resin-modified glassionomer restoratives. Dent. Mater. 2011;27:478–486. [25] Sidhu SK. Clinical evaluations of resin-modified glass-ionomer restorations. Dent. Mater. 2010;26:7–12.



765

[26] Buruiana T, Melinte V, Hitruc G, Buruiana EC. Novel polyalkenoates functionalized with photopolymerizable groups. Synthesis, characterization and testing in dental formulations. Rev. Roum. Chim. 2010;55:963–970. [27] Meller C, Klein C. Fluorescence properties of commercial composite resin restorative materials in dentistry. Dent. Mater. J. 2012;31:916–923. [28] Bignotti F, Penco M, Sartore L, Peroni I, Mendichi R, Casolaro M, D’Amore A. Synthesis, characterisation and solution behaviour of thermo- and pH-responsive polymers bearing L-leucine residues in the side chains. Polymer. 2000;41:8247–8256. [29] Casolaro M. Vinyl polymers containing L-valine and L-leucine residues: thermodynamic behavior of homopolymers and copolymers with N-isopropylacrylamide. Macromolecules. 1995;28:2351–2358. [30] Buruiana, EC, Podasca, VE, Buruiana, T. Preparation and characterization of novel p-acryloyloxybenzaldehyde copolymers bearing pyrene or fluorescein moieties. Interaction of fluorophore with some quenchers and silver nanoparticles. Des. Monomers Polym. 2014;17:89–100. [31] Kobayashi K, Kato S, inventors; GC Corporation, assignee. Paste type glass ionomer cement composition. United States Patent US 6,872,244 B2. 2005 Mar 29. [32] Xie D, Culbertson BM, Johnston WM. Improved flexural strength of N-vinyl-pyrrolidone modified acrylic acid copolymers for glass-ionomers. J. Macromol. Sci. Pure Appl. Chem. 1998;35:1615–1629. [33] Buruiana T, Melinte V, Costin G, Buruiana EC. Synthesis and properties of liquid crystalline urethane methacrylates for dental composite applications. J. Polymer. Sci. Polymer. Chem. 2011;49:2615–2626. [34] Berzins DW, Abey S, Costache MC, Wilkie CA, Roberts HW. Resin-modified glass-ionomer setting reaction competition. J. Dent. Res. 2010;89:82–86. [35] Prosser HJ, Powis DR, Wilson AD. Glass-ionomer cements of improved flexural strength. J. Dent. Res. 1986;65:146–148. [36] Xie D, Yang Y, Zhao J, Park JG, Zhang JT. A novel comonomer-free light-cured glass-ionomer cement for reduced cytotoxicity and enhanced mechanical strength. Dent. Mater. 2007;23:994–1003. [37] Attin T, Vataschki M, Hellwig E. Properties of resin-modified glass-ionomer restorative materials and two polyacid-modified resin composite materials. Quintessence Int. 1996;27:203–209. [38] Yap AUJ. Resin-modified glass ionomer cements: a comparison of water sorption characteristics. Biomaterials. 1996;17:1897–1900. [39] Cefaly DFG, Wang L, Mello LLCP, Santos JL, Santos JR, Lauris JRP. Water sorption of resin-modified glass-ionomer cements photoactivated with LED. Braz. Oral Res. 2006;20:342–346. [40] Iwami Y, Yamamoto H, Sato W, Kawaii K, Torii M, Ebisu S. Weight change of various light-cured restorative materials after water immersion. Oper. Dent. 1998;23:132–137. [41] Cattani-Lorente MA, Dupuis V, Payan J, Moya F, Meyer JM. Effect of water on the physical properties of resin-modified glass ionomer cements. Dent. Mater. 1999;15:71–78.

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