J Mater Sci: Mater Med DOI 10.1007/s10856-014-5141-4

New urethane oligodimethacrylates with quaternary alkylammonium for formulating dental composites Tinca Buruiana • Violeta Melinte • Ionela D. Popa Emil C. Buruiana

•

Received: 10 October 2013 / Accepted: 2 January 2014 Ó Springer Science+Business Media New York 2014

Abstract The aim of this study was to prepare urethane dimethacrylates containing quaternary alkyl (C16, C12) ammonium and polyethylene glycol short sequences (Mn, 400 g/mol) and to investigate their behaviour in some experimental formulations in order to evaluate their potential applicability in the dental composites field. The structure of urethane dimethacrylates has been confirmed by 1H (13C) NMR and FTIR spectra, as well as by electrospray ionization tandem mass spectroscopy, and gel permeation chromatography measurements. The effects of the cationic macromers on the properties of the filled/non-filled composites were examined through FTIR, photoDSC, and specific measurements as volumetric polymerization shrinkage, water sorption/solubility, contact angle, mechanical parameters, and morphology. The monomer compositions based on cationic dimethacrylate (6.88–27.52 wt%), BisGMA-analogue (48.18–68.82 wt%) and TEGDMA (23.3 wt%) showed a good photoreactivity in terms of double bond conversion (DC, 50.07–68.81 %) and polymerization rate (Rp, 0.099–0.141 s-1) measured by photoDSC compared to a control sample (BisGMA-1/TEGDMA: DC, 45.91 %; Rp, 0.162 s-1), while the polymerization shrinkage increased in acceptable limits (5.37–7.74 vol%). The mechanical properties (compressive, flexural and diametral tensile strength) of the composite resin incorporating 70 wt% silanized

T. Buruiana (&)  V. Melinte  E. C. Buruiana Department of Polyaddition and Photochemistry, Petru Poni Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, 700487 Iasi, Romania e-mail: [email protected] I. D. Popa Gr. T. Popa Medicine and Pharmacy University, 16 University, 700115 Iasi, Romania

zirconium silicate micro/nanopowder can be modulated by the initial co-monomer concentrations.

1 Introduction Restorative composites based mainly on a mixture of photopolymerizable monomers/oligomers, photoinitiator systems, additives and different fillers remain a continuing field of study in dental materials science, where the composite viscosity, the filler nature and the resin type often make the difference giving them a distinctive note [1, 2]. Among the conventional mono(di)methacrylates, the most extensively investigated was the highly viscous monomer, diglycidyl methacrylate of bisphenol A (BisGMA) and its diluent co-monomer such as triethylene glycol dimethacrylate (TEGDMA), ethylene glycol dimethacrylate (EGDMA) or decanediol dimethacrylate (D3MA) [3, 4]. Practically, the properties of a successful dental composite depend strongly on the selection of the appropriate monomers involved in the formation of the polymer network [2] as well as on the inorganic filler nature (loading, particle size, shape, distribution, and surface modification) embedded within the organic matrix, the latter strategy revolutionizing the panel of dental materials over the last decade [5–7]. Besides their advantages (aesthetic superiority, strong bonding ability to tooth structure) they have some drawbacks such as vulnerability to water sorption, incomplete conversion of double bonds, and the inherent volumetric shrinkage that accompanies polymerization of the organic phase with major impact on the physicochemical and mechanical properties of the final composites [8–11]. Therefore, significant improvements have been made through the development and structural analysis of novel monomeric systems such as modified BisGMA

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[12, 13], liquid crystalline (LC) monomers [14, 15], dendritic methacrylates [16, 17], bis-acrylamides [18] or ormocers [19, 20] to name a few, and lately, siloranes [21, 22] that use a new resin chemistry (Silorane technology). A practicable alternative to BisGMA has been offered by the urethane di(meth)acrylates (UDMA) whose high reactivity in the free radical polymerization was demonstrated [23–26]. A good example is the urethane product Kalore/Venus Diamond, which has been characterized by very small volumetric shrinkage [27]. Recently, we have reported results about the synthesis of LC monomers [28], Bis-GMA analogous [28, 29], and reactive acid/non-acid functionalized oligourethane dimethacrylates [30–32], and studies concerning the properties of photo-cured networks produced from these monomers used in combination with acrylate-type comonomers. It should be noted that the choice of a longer spacer between the methacrylate groups was encouraged by the fact that the poly(ethylene glycol) derivatives could be an option to 2-hydroxyethyl methacrylate in dental composites [33], and the poly(propylene glycol) chain is a component of the FotofilÒ product used in dental practice [34]. Moreover, the reactivity of such macromers increases with the increase of the length and flexibility of the spacer [35]. On the other hand, within the family of dental monomers there is a major interest in developing chemically bound structures that contain charged sites, such as quaternary ammonium compounds capable to impart long term antibacterial activity to resin-based dental materials. To date, a number of cationic monomethacrylates including 12-methacryloyloxydodecylpyridinium bromide [36], alkylammonium methacrylates [37–39] or methacrylamide monomers bearing quaternary ammonium fluoride [40] have been synthesized and used in dental materials to inhibit bacterial growth by contact mechanism. In one creative approach focussed on improving the properties, the bifunctional monomers have drawn a lot of attention and consideration, but unfortunately, relatively few antibacterial dimethacrylates [41, 42] and macromers [43, 44] were incorporated into conventional resins and composites for achieving dental materials with therapeutic effects. In the light of these limited reports, especially concerning dental macromers, we decided to employ a flexible building block to prepare novel urethane dimethacrylates of oligomer type that contain two photopolymerizable end groups and a quaternary alkylammonium group which will be further reacted with other conventional monomers to create polymeric networks with potential antimicrobial functions of interest for dental composite resin, and not only. It is expected that our dimethacrylates will exert similar effects on photopolymerized surfaces of the dental materials, where a structural complexity is required to

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satisfy a certain set of properties (high conversion, good processability, biocompatibility, etc). Herein we report the synthesis of two macromers containing poly (ethylene oxide) sequences and quaternary alkyl (C12, C16) ammonium, and a first study of the influence of the cationic oligomers on the photopolymerization kinetics, polymerization shrinkage, surface and mechanical properties of the cured resins.

2 Materials and methods 2.1 Materials and instruments Polyethylene glycol (PEG, Mn–400), isophorone diisocyanate (IPDI), 2-hydroxyethyl methacrylate, TEGDMA, dibutyltin dilaurate and phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (Irgacure 819) were purchased from Sigma Aldrich Chemical Co and used without further purification. The synthesis and characterization of BisGMA functionalized with urethane methacrylate groups (BisGMA-1) were described in a previous paper [28], as well as the synthesis of quaternary ammonium diols (C12–OH and C16–OH) employed into the polyaddition reactions [45]. The filler zirconium silicate nanopowder was purchased from SigmaAldrich Chemical and has an average particle diameter \100 nm as specified by the producer. The filler was silanized by using the silane coupling agent 3-methacryloyloxypropyltrimethoxysilane from Sigma Aldrich Chemical, according to a method reported in the literature [19]. 1H NMR, 13C NMR and FTIR spectra were measured on a Bruker 400 MHz spectrometer and a Bruker Vertex 70 spectrometer, respectively. Viscosity measurements were determined with a Brookfield cone and plate viscometer at room temperature. The test was run at spindle speeds of 6 and 12 rpm and the viscosity readings obtained were recorded and expressed as pascal second (Pa s). The Q-TOF LC/MS conditions were set as follows: electrospray ionization in negative ion mode, drying gas (N2) flow rate 7.0 L/min; drying gas temperature 325 °C; nebulizer pressure 15 psig, capillary voltage 4,200 V; fragmentation voltage 400 V; the full-scan mass spectra of the investigated compounds were acquired in the range m/z 50–3,000. The average molecular weight was determined in chloroform by GPC analysis on a PL-EMD 950 apparatus equipped with two PL gel mixed columns using polystyrene standards. For FTIR photopolymerization experiments we used visible light coming from a dental-curing unit (LA 500, Model blue-light, Apoza Enterprise Co, Taiwan), and differential scanning photocalorimetry studies were performed on a DuPont 930 apparatus with a double heat differential calorimeter 912, calibrated with indium metal standard. The microstructure of the cured samples in fracture was

J Mater Sci: Mater Med

examined by using an environmental scanning electron microscope QUANTA200 operating in low vacuum mode. 2.2 Synthesis The quaternary ammonium urethane dimethacrylates (UDMA-Q1 and UDMA-Q2) were prepared following the same synthetic procedure, for which reason we only describe the obtaining of the UDMA-Q1 derivative. Thus, 2.84 g (7.1 mmol) PEG and 3.01 g (7.1 mmol) C16–OH were degassed under vacuum for 2 h and then 4.6 mL (21.3 mmol) IPDI were added, the mixture being stirred at 60 °C for 6 h in the presence of a catalytic amount of dibutyltin dilaurate. To decrease the viscosity of the reaction medium, tetrahydrofuran (THF) was added. After a decrease of temperature at 40 °C, 1.8 mL (14.2 mmol) HEMA were added and the reaction continued for 10 h. The course of the reaction was pursued through FTIR spectroscopy following the absorption of the isocyanate stretching band at 2,260 cm-1, the reaction being considered complete after the disappearance of this band from the spectrum. After separation of the organic phase and evaporation of the solvent, each macromer was again dissolved in methylene chloride and the solution was stirred with 2 % HCl solution (4:1 v/v) at room temperature to remove the catalyst. Finally, after the removal of the solvent, the urethane dimethacrylates UDMA-Q1 and UDMAQ2 (based on C12–OH) were collected as colourless viscous liquids. 2.2.1 UDMA-Q1 Yield: 9.8 g (78.8 %). FTIR (KBr, cm-1): 3,359 (NH); 2,855–2,953 (C–H); 1,714 (C=O); 1,638 and 816 (CH2=C); 1,538 (amide II); 1,245, 1,142 and 1,051 (C–O–C). g = 65.8 Pa s. 2.2.2 UDMA-Q2 Yield: 9.65 g (75.15 %).1H NMR (CDCl3, d ppm): 6.15 (d, 2H, CH2=C in transposition relative to CH3 unit from HEMA); 5.60 (s, 2H, CH2=C in cis position relative to CH3 unit from HEMA); 4.32–4.18 (m, 12H, –CH2–CH2–O– CO–NH); 3.84–3.92 (m, 4H, COO–CH2); 3.65 (m, 27H, –O–CH2-CH2–O– from PEO); 3.45 (m, 5H, CH2–N?– CH3); 2.90 (m, 6H, O–CO–NH–CH2); 1.96 (s, 6 H, CH3 from HEMA); 1.73–0.86 (m, 68 H, protons from isophorone unit and aliphatic dodecyl chain). 13C NMR (CDCl3, d ppm): 167.23, 156.41 and 155.76 (C=O); 136.14 and 125.27 (CH2=C); 70.16 (CH2–O); 67.26 (CH2–O–CO); 60.14 (CH2–O–CO–NH); 56.82 (CH2–N?); 48.6 (CH3– N?); 48.07–39.72 (C from isophorone ring); 37.08–21.03 (CH2 from alkyl chain linked to quaternary nitrogen and

CH3 linked to isophorone); 18.07 (CH2=C(CH3)); 14.13 (CH3 from dodecyl). FTIR (KBr, cm-1): 3,342 (NH); 2,859–2,953 (C–H); 1,714 (C=O); 1,639 and 817 (CH2=C); 1,542 (amide II); 1,245, 1,109 and 1,052 (C–O–C). g = 52.3 Pa s. For the synthesis of BisGMA functionalized with urethane methacrylate groups (BisGMA-1), 5 g (10 mmol) BisGMA were dissolved in 20 mL THF and 2.53 mL (18 mmol) 2-isocyanatoethyl methacrylate were added, and the reaction conducted at 40 °C for 24 h. The consumption of isocyanate groups was determined by FTIR spectroscopy, while the removal of the catalyst was performed similarly to that described for UDMA-Q macromers. BisGMA-1 was collected by solvent evaporation as a colourless viscous liquid. 2.3 Measurement of double bond conversion (DC) In order to determine the conversion degree by FTIR spectroscopy, mixtures of photopolymerizable dimethacrylates (compositions given in Table 1) and Irgacure 819 photoinitiator (1 wt%) were homogenized and manually coated on KBr pellets. Subsequently, the decrease of the C=C stretching vibration at 1,637 cm-1 after various curing periods was monitored comparatively to the unchanging C=O peak of the ester group at 1,720 cm-1, used as reference peak since its intensity remains constant before and after polymerization. Additionally, the degree of conversion was measured by photoDSC, using a standard high pressure mercury lamp incorporated in the device with 4.5 mW/cm2 light intensity to irradiate the samples (1.5 ± 0.5 mg) in the presence of Irgacure 819 as the initiator (1 wt%). The measurements were performed in an isothermal mode under 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). 2.4 Determination of volumetric polymerization shrinkage The polymerization shrinkage of the cured specimens tested in the present study was determined by the following equation: Polymerization shrinkage =

dcured  duncured 100 dcured

where d is the density of the formulation. Photocurable pastes for the volumetric shrinkage measurement were formulated using the weight ratios of urethane oligomer: BisGMA-1:TEGDMA and Irgacure 819

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J Mater Sci: Mater Med Table 1 Composition (wt%) of the experimental formulations (S), polymerization shrinkage and contact angle for the S1–S7 samples (percent standard deviation is in parentheses) Sample*

UDMA-Q1

UDMA-Q2

BisGMA-1

TEGDMA

Polymerization shrinkage (%)

Contact angle (8)

S1

6.88

–

68.82

23.3

5.83 (0.28)

66.39 (3.59)

S2

13.76

–

61.94

23.3

6.65 (0.33)

62.34 (5.19)

S3

27.52

–

48.18

23.3

7.74 (0.32)

59.62 (4.86)

S4

–

6.88

68.82

23.3

5.37 (0.39)

74.98 (3.14)

S5

–

13.76

61.94

23.3

6.79 (0.52)

68.42 (6.72)

S6

–

27.52

48.18

23.3

7.34 (0.34)

60.72 (5.32)

S7

–

75.7

23.3

5.18 (0.47)

81.09 (5.37)

–

*

Each formulation contains 1 wt% Irgacure 819. In resin composites, the organic matrix has the same composition but is mixed with silanized zirconium silicate filler in 30/70 wt/wt% ratio

given in Table 1. The monomer and polymer densities, respectively, were measured gravimetrically by density bottle method, using a Partner balance (accuracy = ± 0.000001 g). Three successive measurements for all tested composites were performed. 2.5 Measurements of water sorption/solubility and contact angle The filled resin composites for water sorption and water solubility determinations were prepared by using 30 wt% organic matrix and 70 wt% inorganic filler (silanized zirconium silicate micro/nanopowder). The water sorption and water solubility values were determined by preparing five disk specimens of reduced dimensions (15 ± 0.1 mm diameter, 1 ± 0.1 mm thickness) for each group of mixtures, using a Teflon split ring mould between two glass plates covered with polyethylene film. The resins were preconditioned over a desiccant containing calcium sulfate at 37 °C until their weight remained constant (initial weight m1). The specimens were further placed in distilled water at 37 °C for 7 days and then removed from the water, lightly blotted with a paper to eliminate the surface-adherent water, and weighed (m2). After that, the specimens were placed into a desiccator with calcium sulphate and dried at 37 °C until their weight was constant again (m3). The thickness and diameter of the specimens were measured using a digital calliper, rounded to the nearest 0.01 mm, and these measurements were used to calculate the volume (V) of each specimen (in mm3). The water sorption for each sample was determined using the equation. Water sorption ¼ ðm2  m3 Þ=V whereas, the water solubility was calculated employing the equation. Water solubility ¼ ðm1  m3 Þ=V on

The static water contact angle measurements were made disk-shaped specimens (15 ± 0.1 mm diameter,

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1 ± 0.1 mm thickness) using a goniometer KSV Cam 200. 2 lL droplets of double-distilled water were placed on the resin composites surface, the average contact angle being calculated starting from at least ten separate measurements. 2.6 Mechanical characterization Compressive (CS), flexural (FS) and diametral tensile strengths (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 9 6 mm length for compressive strength (CS) and of 4 mm diameter 9 2 mm length for diametral tensile strength (DTS) investigation. A Teflon mold was used to fabricate the specimens for flexural strength (3 mm in width 9 3 mm in thickness 9 25 mm in length). The specimens were blue-light irradiated for 30 s on each side, removed from the mold and conditioned in distilled water at 37 °C for 24 h prior to testing. A crosshead speed of 1 mm/min was applied in these tests. The compressive strength was calculated from the equation CS = P/pr2, where P is the load at fracture and r the radius of the sample cylinder, while the relationship DTS = 2P/pdt, where d is the diameter and t the thickness of the cylinder, respectively was employed for DTS measurements. The flexural strength 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.

3 Results and discussion 3.1 Synthesis of cationic macromers Two novel urethane dimethacrylates carrying alkylammonium groups and an organic spacer between the photopolymerizable moieties were prepared by a classical twostep addition reaction starting from poly(ethylene oxide)

J Mater Sci: Mater Med Scheme 1 Structures of the urethane oligodimethacrylates with quaternary alkylammonium groups (UDMA-Q1, UDMA-Q2), BisGMA modified with urethane methacrylic groups (BisGMA-1) and TEGDMA used in resin formulations

n

O

CH2

O

O

CH3

NH

O

O

CH3

CH3

CH3

CH3

NH

O

O

O

O O

NH CH3

CH3 CH3

CH3

O

CH3

BrN+ CH3

O

NH

CH3

O

NH

O

NH

O O

6.68

O O

CH2 CH3

n = 11 for UDMA-Q1 n = 7 for UDMA-Q2 CH3 CH3 O

CH2

NH O

CH3

O

O

CH2

2

O

O

O

NH

O

O

O

2

O

CH3

CH3

O

O

CH3

O

CH2

CH2

BisGMA-1

O CH2

CH3 O

O

O

CH3

O

CH2 O

TEGDMA

(PEO, Mn: 400 g/mol), isophorone diisocyanate (IPDI) and di(2-hydroxyethyl)-hexadecylmethyl ammonium bromide or di(2-hydroxyethyl)-dodecylmethyl ammonium bromide, followed by a coupling of each NCO-terminated precursor with 2-hydroxyethyl methacrylate (HEMA) under proper conditions to prevent the formation of undesired products. The structures presented in Scheme 1 are typical to macromers that result predominantly from the mentioned process, by applying the reactivity difference between the more reactive secondary NCO group of isophorone diisocyanate and the primary NCO group in the presence of dibutyltin dilaurate [46]. Finally, the resultant macromers (UDMA-Q1, UDMA-Q2) differ by the length of the alkyl chain linked through the quaternary nitrogen (C16, C12), which may have an effect on its antibacterial activity in agreement with literature data [38, 47]. PEO400 was preferred as suitable flexible core because the use of a longer spacer might compromise the strength of the restorative material after its storage into a wet environment. Both urethane dimethacrylates are clear, colourless, and viscous liquids (UDMA-Q1, g: 65.8 Pa s; UDMA-Q2, g: 52.3 Pa s) at room temperature, and are soluble in usual organic solvents such as tetrahydrofuran (THF), chloroform, methylene chloride, and dimethylacetamide (DMAc). Also, they are completely miscible with other common hydrophilic and hydrophobic dental monomers, like TEGDMA, HEMA, etc. The structure and purity of the cationic dimethacrylates were clearly evidenced from 1H NMR, 13C NMR and FTIR

spectra, the results being in harmony with the proposed structure (see the Sect. 2). For instance, the 1H NMR spectrum of UDMA-Q1 (Fig. 1a) confirmed the presence of signals corresponding to the resonances of the olefinic protons (trans/cis: 6.15/5.60 ppm), methylene protons of the urethane-ester (4.35–4.20 ppm) and ester (3.86–3.93 ppm) groups, and the methylene protons from PEO (3.65 ppm). Other signals are associated with the methyne protons close to urethane moieties (3.50 ppm), methylene protons linked to quaternary nitrogen (3.40 ppm), methylene/methyl protons near the urethane groups and those connecting quaternary nitrogen (2.92 ppm). The signals of the methyl protons from HEMA, methyl/methylene protons from isophorone, and the hexadecyl chain appeared at around 1.96 and 1.78–0.86 ppm, respectively. Besides, the structural analysis was complemented by 13C NMR spectroscopy, a 13C NMR spectrum of the UDMA-Q1 being depicted in Fig. 1b. Carbon signals at 167.43 ppm and in the 155.62–156.62 ppm range are given by the carbonyl in ester and ester-urethane functions, respectively, whereas the resonances at 136.03 ppm (quaternary carbon) and 125.65 ppm (double bond carbon) are derived from methacrylate-carbon atoms. Distinctive signals coming from carbon atoms of PEO sequence (70.04 ppm), urethane-ester linked to quaternary nitrogen (69.46 ppm) and ester-urethane (60.27 ppm) groups, alkyl chain linked to quaternary nitrogen (41.56–47.46 ppm), isophorone ring (36.30–23.31 ppm), methyl of methacrylate (18.18 ppm) and those of hexadecyl chain (13.98 ppm) can be observed, all providing evidence

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PEO, as suggests the most probably chemical structure given in Scheme 1, and these results seem to be in good agreement with the molar ratio used in the synthesis of urethane oligodimethacrylates. The formation of welldefined oligomers was also confirmed by GPC measurements, which gave a molecular weight (Mn: 1,652 g/mol for UDMA-Q1 and 1,732 g/mol for UDMA-Q2) close to the calculated values. 3.2 Photopolymerization kinetics

Fig. 1 1H NMR and CDCl3

13

C NMR spectra of UDMA-Q1 oligomer in

of the macromer structure. In connection, the FTIR spectra (not shown here) of urethane dimethacrylates sustained the expected structures (urethane NH at 3,359 cm-1, carbonyl CO at 1,714 cm-1 and carbon-carbon double bond from the methacrylate function at 1,638 and 816 cm-1). For an unequivocal characterization of the alkylammonium dimethacrylates of oligomer type, their structure was verified by means of electrospray ionization tandem mass spectroscopy (ESI-MS). In our case, the appearance of characteristic mass signals was detected in the ESI-MS spectra of the macromers in THF. As exemplified in Fig. 2, the single Gaussian curve recorded for UDMA-Q1 contains more signals, all separated by 44 Da that correspond to ethylene oxide units from the monomer structure. The highest peak identified at m/z = 1,750 in the ESI-MS spectrum of UDMA-Q1 corresponds to the parent ion identified as the major species, whereas the peaks with lower intensities are given by the ions resulted during the fragmentation. Therefore, both dimethacrylates contain

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The photocuring behaviour of the proposed experimental formulations was firstly evaluated by FTIR spectroscopy following the changes in the absorbance of the methacrylate double bond before photopolymerization and after exposure to visible irradiation in the presence of Irgacure 819 (1 wt%). These experiments were carried out on monomer combinations incorporating UDMA-Q1 or UDMA-Q2, TEGDMA and a base monomer (structure given in Scheme 1) BisGMA-1 (g: 6.9 Pa s), which was synthesized in our group by the replacement of about 90 % hydroxyl groups from commercial BisGMA with urethane methacrylic functions [28]. It is of interest to note that the addition of UDMA-Q1 or UDMA-Q2 in amounts ranging between 6.88 and 27.51 wt% has been achieved to control the concentration of quaternary ammonium groups (0.5–2 %) in each formulation (S1–S6, Table 1). The FTIR spectra collected after different exposure times (0–180 s) indicated that the absorbance at 1,638 cm-1 decreased in situ as the monomer photopolymerization proceeded, reaching a value of about 70 % decay in the case of S3 formulation containing 27.51 wt% UDMA-Q1 (Fig. 3a). Excepting the S4 formulation (DC: 43 %), the values of DC (Fig. 3b) were increased by the addition of UDMA-Q1 (S1–S3, DC: 55–70 %) or UDMA-Q2 (S5–S6, DC: 53–65 %) in each sample and these are higher than the value obtained for the S7 formulation based on BisGMA-1/ TEGDMA (DC: 50 %). Such observations point out the conclusion that the reactivity of both alkylammonium oligodimethacrylates is similar to that of other multifunctional comonomers capable to generate denser polymeric networks by copolymerization that reduce the extent of double bond conversion [48]. Changing the irradiation source, we investigated the reactivity of the cationic dimethacrylates under conditions of free radical polymerization by using photoDSC method, which monitors the main parameters of the polymeric network forming process (the rate of polymerization, Rp, the degree of conversion of the methacrylate C=C bonds, DC and the time to attain the maximum heat of polymerization, tmax). All polymerizations were initiated with a low UV light intensity lamp (4.5 mW/cm2, integrated in device) in the presence of 1 wt% Irgacure 819 that acts as

J Mater Sci: Mater Med Fig. 2 ESI-MS spectrum for UDMA-Q1 urethane dimethacrylate oligomer

Fig. 4 PhotoDSC rate profiles for the experimental formulations S4–S7 (a) and double bond conversion for the investigated materials after 120 s of UV irradiation (b)

Fig. 3 Changes in the double bond absorption from the FTIR spectrum of S3 formulation exposed to visible irradiation (a) and the conversion augment with the irradiation time for all monomer mixtures (b)

an efficient photoinitiator for UV curable systems including dental restorative materials [49]. In Fig. 4 the photopolymerization rate profiles and the double bond conversions as a function of time for S1–S6 formulations

are given. It could be seen that the photoreactivity of monomers reflected through the maximum polymerization rate increased in the presence of increasing amount of UDMA-Q1 (S1–S3, Rp : 0.099–0.132 s-1) or UDMA-Q2 (S4–S6, Rp : 0.108–0.141 s-1), but remained lower than that of the formulation S7 based on BisGMA-1/TEGDMA (Rp : 0.162 s-1). Accordingly, the degree of conversion (after 120 s) increased from S1 (DC: 50 %) to S6 (DC: 68.81 %) compared to that of the formulation S7 (DC: 45.91 %) without cationic macromer, and this result might

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J Mater Sci: Mater Med Table 2 Photo DSC data of the formulations employed in the study Samples

tmax (s)

RPmax (s-1)

Conversion (%)*

S1

2.46

0.099

50.07

S2

2.82

0.112

57.35

S3

2.7

0.132

61.27

S4

3

0.108

57.08

S5

3.11

0.118

62.12

S6

3

0.141

68.81

S7

3.3

0.162

45.91

*

After 120 s of UV irradiation

be explained by the contribution of the flexible spacer and the ammonium groups capable to absorb water acting like a plasticizer. This observation reveals that both urethane macromers can be appropriately photocured, but despite the structural similarity, UDMA-Q1 was less reactive than UDMA-Q2. The fact that tmax varied between 2.46 (S1) and 3.11 s (S5) is a good indicator of the photoinitiator activity (Table 2) used in our study. The experimental data measured by these two photopolymerization methods are coherent with the observation that the organic phase is not completely polymerized in the case of light cured dental materials [50]. 3.3 Volumetric polymerization shrinkage A consequence of the urethane oligodimethacrylates structure involved in photopolymerization is that the volumetric shrinkage could decrease [51]. Although both macromers and BisGMA-1 have a lower viscosity than BisGMA (gBisGMA = 574 Pa s, [52]), they were diluted with TEGDMA, which usually affects the polymerization shrinkage and water uptake [3]. When UDMA-Q1 or UDMA-Q2 were mixed with BisGMA-1 and TEGDMA, the volumetric polymerization shrinkage (PS: 5.83–7.74 vol%) increased with the increase of the quantity of cationic macromer within the reaction mixture (Table 1), a lower value being obtained for the formulation S4 incorporating 6.88 wt% UDMA-Q2 (PS: 5.37 vol%). Therefore, a higher degree of conversion of the macromer containing resins led to higher polymerization shrinkage values [12]. From this point of view, the role of UDMA-Q1(Q2) seems to be important especially for its potential bioactivity, because in its absence a polymerization shrinkage of 5.18 vol% was found for the S7 formulation (BisGMA-1/TEGDMA: 75.7/23.3 wt%). Once again, the quality of BisGMA analogue to induce a lower polymerization shrinkage than the conventional ones (e.g. BisGMA/TEGDMA: 70/30 wt%, PS: 7.0 vol%) [52] has been evidenced, and consequently it may be selected for preparing filled dental resins.

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Fig. 5 Water sorption and water solubility characteristics for the prepared resin composites

3.4 Water sorption and solubility To investigate from macroscopic point of view the further performance of photocured dental resins filled with 70 wt% silanized Zr/Sr glass, we studied their stability in a wet environment because it is significant in connection with the viability of such materials. In fact, the absorption of water is generally a diffusion-controlled process and depends on the hydrophilicity [53] and crosslinking degree of the cured resins [54]. The literature data concerning the water sorption and solubility behaviour of some dental restorative resins recommended for such materials water sorption (WS) values lower than 40 lg/mm3, and solubility in water below the limit 7.5 lg/mm3 [55]. As shown in Fig. 5, with the addition of UDMA-Q1(Q2) into the organic matrix the WS values of the resin composites S1–S3 and S4–S6, respectively increased from 30.62 to 43.75 lg/mm3, while the solubility in water decreased from S1 (*9.43 lg/mm3) to S6 (*5.72 lg/mm3). Consequently, the presence of a higher concentration of hydrophilic groups within a dental formulation had as effect an increase of the water sorption values compared to the specimen derived from BisGMA-1 and TEGDMA (S7, WS: 30.91 lg/mm3), which absorbs a water amount comparable with the resins enclosing lower proportion of cationic macromers (S1 and S4). Although the combination of BisGMA-1/TEGDMA (water solubility *7.61 lg/mm3) generated a denser network due to the different chemical structure, its water solubility was higher than that of S3 (6.19 lg/mm3) and S6 (5.72 lg/mm3) that did not significantly differ in hydrophobicity of the cured polymer. Surprisingly, a higher quantity of hydrophilic component from the oligodimethacrylates containing PEO400 spacer and long hydrophobic side chains did not increase the solubility in water of the final composites. We

J Mater Sci: Mater Med Fig. 6 SEM micrographs for S2 (a) and S5 (b) composite resins in fracture, the EDX spectrum registered for S5 composite (c) and the morphology of the zirconium silicate filler before silanization (d). Inset, a SEM image and the EDX mapping distribution for zirconium atoms

presume that these differences may be mainly caused by the structural heterogeneity observed in the polymer networks prepared by free-radical polymerization of the common dimethacrylate monomers [56] without neglecting the cumulative effect of the alkylammonium chain (hydrophilic/hydrophobic). 3.5 Contact angle Another significant feature characterizing the hybrid resin composites is the contact angle (h), a measure of the wetting characteristics for this type of materials that can act as an indicator of their hydrophilicity/hydrophobicity. Accordingly, the relative hydrophilicity of the S1–S7 hybrid composites containing various amounts of cationic urethane oligodimethacrylates was evaluated by comparing the contact angle values achieved with distilled water on the surface of the photocured specimens. The contact angle measurements (Table 1) indicated that the resin composites are mainly hydrophilic, the increased wettability being caused by the amount of cationic urethane dimethacrylates. Thus, the contact angle values were found to vary between 59.62° and 74.988 for the resin composites containing UDMA-Q, while the highest hidrophobicity was measured for the S7 sample (81.09o) without cationic macromer.

3.6 Surface morphology Scanning electron micrographs of the specimens S2 and S5 were recorded after fracture (Fig. 6a, b), where significant differences in the microstructure of the fractured surfaces having different composition were not observed. Moreover, in both cases there is a homogeneous distribution of the filler particles which are completely embedded within the organic matrix of the cured resins, suggesting a good compatibility between the inorganic zirconium silicate and the organic constituents. The elemental analysis of the S5 fractured sample was measured by registering the energy dispersive X-ray spectrum (Fig. 6c). As can be seen, the EDX spectrum confirms the appearance of carbon, oxygen, silicon and zirconium atoms as main constituents of the inorganic and organic phases involved in the assembling of experimental composite. Also, the electron dot-mapping image obtained from the EDX analysis for the zirconium element (Fig. 6c, inset) reveals the high density and the uniform distribution of the zirconium atoms inside the composite. Furthermore, the SEM image registered for the as-received zirconium silicate nanopowder was included (Fig. 6d) in order to provide supplementary information regarding the morphology of the inorganic filler used in our study.

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J Mater Sci: Mater Med Table 3 Mechanical properties of the experimental resin composites (standard deviations in parentheses)

Sample

CS (MPa)

DTS (MPa)

FS (MPa)

FM (GPa)

S1

219.93 (21.95)

47.48 (3.55)

132.61 (16.40)

6.34 (0.51)

S2

177.52 (15.76)

42.72 (8.43)

123.88 (3.59)

5.89 (0.61)

S3

100.47 (13.15)

26.56 (3.15)

S4

243.50 (14.84)

51.12 (2.77)

138.86 (8.36)

7.84 (1.47)

S5

192.51 (28.97)

45.75 (3.58)

120.18 (6.76)

5.65 (0.53)

S6

106.29 (18.51)

32.34 (4.25)

91.79 (7.51)

4.95 (1.23)

S7

258.36 (32.16)

48.55 (3.91)

130.73 (6.45)

7.66 (2.73)

3.7 Mechanical properties Considering the differences in base monomer structure and implicitly the conversion of the double bonds, it was important to examine if the alkylammonium groups within the urethane macromer could improve the mechanical properties of composites, due to their propensity to promote the formation of ionic bonds including hydrogen bonding. Thus, parameters such as compressive (CS), flexural (FS) and diametral tensile strength (DTS) were determined for all resin composites investigated in our study, the obtained results being listed in Table 3. The analysis of these data showed that the CS and DTS values for the resin composites containing UDMA-Q1 or UDMA-O2 are lower in the case of S2–S6 (CS) and S3, S6 (DTS) or are comparable (CS: S1, S4; DTS: S1, S2, S4, S5) with those recorded for the S7 sample (CS: 258.3 MPa; DTS: 48.5 MPa). On the other hand, the flexural strength reaches higher values in the hybrid composites incorporating 6.88 wt% UDMA-Q (FSS1 = 132.6 MPa; FSS4 = 138.8 MPa) in comparison with the S7 composite (FSS7 = 130.7 MPa). In essence, all parameters decreased from S1 (CS: 219.9; DTS: 47.4 MPa; FS: 132.6 MPa) to S3 (CS: 100.4; DTS: 26.5 MPa; FS: 89.4 MPa), as well as in the resin composites with the latter macromer, where CS varied between 243.5 MPa (S4) and 106.2 MPa (S6), DTS from 51.1 MPa (S4) to 32.3 MPa (S6), and FS from 138.8 MPa (S4) to 91.8 MPa (S6). On the contrary, the flexural modulus (FM) is decreasing with the enhancement in the ammonium quaternary derivatives (from 6.3 MPa in S1 to 4.6 MPa in S3 and from 7.8 MPa in S4 to 4.9 MPa in S6) resulting thus an increase in elasticity. This increase of elasticity may be attributed to the flexible structure of the quaternary macromers included in higher quantities from S1 to S3 and from S4 to S6, respectively. Overall, it is suggestive that the more hydrophilic specimens, which have superior relaxation ability presented lower CS, DTS and FS values and this is a major determinant of the mechanical characteristics of the above materials. At the same time, the favourable mechanical strength properties were achieved in the case of S1 and S4 and these seem to be in reasonable agreement with data obtained on dental materials based on BisGMA/TEGDMA (70:30) [52].

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89.45 (11.26)

4.68 (2.26)

The combination of these properties with the potential biocidal activity of both cationic charge density and alkyl chain length belonging to the chemically bonded entity into a polymer backbone needs a specialized study and the first results will be presented in the near future. 4 Conclusions In summary, reactive novel urethane dimethacrylates containing PEO400 spacer and quaternary alkylammonium groups (UDMA-Q) were prepared and evaluated in dental composite systems as co-monomer besides an analogue of BisGMA (BisGMA-1) and TEGDMA. Each composite resin was compared to a control BisGMA-1/TEGDMA system for polymerization kinetics, polymerization shrinkage, water sorption/solubility, hydrophilicity, mechanical parameters, and morphology. In general, all these properties were significantly dependent on the amount of cationic co-monomer incorporated in dental formulations, the obtained values being in a reasonable range. Such macromers would be helpful for the obtaining of biocide dental composites and especially adhesives, which are attractive candidates for future studies. Acknowledgements The authors acknowledge the financial support of the CNCS-UEFISCDI through a project from the National Research Program (PN-II-ID-PCE 2011-3-0164; No. 40/5.10.2011).

References 1. Moszner N, Hirt T. New polymer-chemical developments in clinical dental polymer materials: enamel-dentin adhesives and restorative composites. J Polym Sci Polym Chem. 2012;50: 4360–402. 2. Stansbury JW. Dimethacrylate network formation and polymer property evolution as determined by the selection of monomers and curing conditions. Dent Mater. 2012;28:13–22. 3. Podgo´rski M. Structure-property relationship in new photo-cured dimethacrylate-based dental resins. Dent Mater. 2012;28: 398–409. 4. Froes-Salgado NRG, Boaro LC, Pick B, Pfeifer CS, Francci CE, Meier MM, Braga RR. Influence of the base and diluent methacrylate monomers on the polymerization stress and its determinants. J Appl Polym Sci. 2012;123:2985–91.

J Mater Sci: Mater Med 5. Moszner N, Salz U. Recent developments of new components for dental adhesives and composites. Macromol Mater Eng. 2007; 292:245–71. 6. Wang Y, Lee JJ, Lloyd IK, Wilson OC, Rosenblum M, Thompson V. High modulus nanopowder reinforced dimethacrylate matrix composites for dental cement applications. J Biomed Mater Res. 2007;82A:651–7. 7. Gao Y, Sagi S, Zhang L, Liao Y, Cowles DM, Sun Y, Fong H. Electrospun nano-scaled glass fiber reinforcement of Bis-GMA/ TEGDMA dental composites. J Appl Polym Sci. 2008;110: 2063–70. 8. Pawlowska E, Poplawski T, Ksiazek D, Szczepanska J, Blasiak J. Genotoxicity and cytotoxicity of 2-hydroxyethyl methacrylate. Mutation Res. 2010;696:122–9. 9. Andrzejewska E, Marcinkowska A. New aspects of viscosity effects on the photopolymerization kinetics of the 2,2-bis [4-(2hydroxymethacryloxypropoxy)phenyl] propane/triethylene glycol dimethacrylate monomer system. J Appl Polym Sci. 2008;110: 2780–6. 10. Lambrechts P, Goovaerts K, Bharadwaj D, De Munck J, Bergmans L, Peumans M, Van Meerbeek B. Degradation of tooth structure and restorative materials: a review. Wear. 2006;261: 980–6. 11. Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: a systematic review. Dent Mater. 2005;21:962–70. 12. Khatri CA, Stansbury JW, Schultheisz CR, Antonucci JM. Synthesis, characterization and evaluation of urethane derivatives of Bis-GMA. Dent Mater. 2003;19:584–8. 13. Pereira SG, Nunes TG, Kalachandra S. Low viscosity dimethacrylate comonomer compositions [Bis-GMA and CH3Bis-GMA] for novel dental composites; analysis of the network by stray-field MRI, solid-state NMR and DSC & FTIR. Biomaterials. 2002; 23:3799–806. 14. Satsangi N, Rawls HR, Norling BK. Synthesis of low-shrinkage polymerizable methacrylate liquid-crystal monomers. J Biomed Mater Res Part B. 2005;74B:706–11. 15. Boland EJ, Carnes DL, MacDougall M, Satsangi N, Rawls R, Norling B. In vitro cytotoxicity of a low-shrinkage polymerizable liquid crystal resin monomer. J Biomed Mater Res Part B. 2006;79B:1–6. 16. Matinlinna JP, Lassila LVJ, Kangasniemi I, Yli-Urpo A, Vallittu PK. Shear bond strength of Bis-GMA resin and methacrylated dendrimer resins on silanized titanium substrate. Dent Mater. 2005;21:287–96. 17. Viljanen EK, Skrifvars M, Vallittu PK. Dendritic copolymers and particulate filler composites for dental applications: degree of conversion and thermal properties. Dent Mater. 2007;23:1420–7. 18. Pavlinec J, Moszner N. Monomers for adhesive polymers, 8a Crosslinking polymerization of selected N-substituted bis(acrylamide)s for dental filling materials. J Appl Polym Sci. 2009; 113:3137–45. 19. Moszner N, Gianasmidis A, Klapdohr S, Fischer UK, Rheinberger V. Sol-gel materials 2. Light-curing dental composites based on ormocers of cross-linking alkoxysilane methacrylates and further nano-components. Dent Mater. 2008;24:851–6. 20. Klapdohr S, Moszner N. New inorganic components for dental filling composites. Monatsh Chem. 2005;135:21–45. 21. Ernst CP, Galler P, Willershausen B, Haller B. Marginal integrity of class V restorations: SEM versus dye penetration. Dent Mater. 2008;24:319–27. 22. Weinmann W, Thalacker C, Guggenberger R. Siloranes in dental composites. Dent Mater. 2005;21:68–74. 23. Moszner N, Fischer UK, Angermann J, Rheinberger V. A partially aromatic urethane dimethacrylate as a new substitute for Bis-GMA in restorative composites. Dent Mater. 2008;24:694–9.

24. Kerby RE, Knobloch LA, Schricker S, Gregg B. Synthesis and evaluation of modified urethane dimethacrylate resins with reduced water sorption and solubility. Dent Mater. 2009;25:302–13. 25. Atai M, Ahmadi M, Babanzadeh S, Watts DC. Synthesis, characterization, shrinkage and curing kinetics of a new low-shrinkage urethane dimethacrylate monomer for dental applications. Dent Mater. 2007;23:1030–41. 26. Antonucci JM, Icenogle TB, Regnault WF, Liu DW, O’Donnell JNR, Skrtic D. Polymerization shrinkage and stress development in bioactive urethane acrylic resin composites. Polym Prepr. 2006;47:498–9. 27. Ferracane JL. Resin composite—state of the art. Dent Mater. 2011;27:29–38. 28. Buruiana T, Melinte V, Costin G, Buruiana EC. Synthesis and properties of liquid crystalline urethane methacrylates for dental composite applications. J Polym Sci Part A. 2011;49:2615–26. 29. Buruiana T, Melinte V, Jitaru F, Aldea H, Buruiana EC. Photopolymerization experiments and properties of some urethane/urea methacrylates tested in dental composites. J Compos Mater. 2012;46:371–82. 30. Buruiana EC, Buruiana T, Melinte V, Zamfir M, Colceriu A, Moldovan M. Synthesis of oligomeric urethane dimethacrylates with carboxylic groups and their testing in dental composites. J Polym Sci Part A. 2007;45:1956–67. 31. Buruiana T, Melinte V, Stroea L, Buruiana EC. Urethane dimethacrylates with carboxylic groups as potential dental monomers. Synth Prop Polym J. 2009;41:978–87. 32. Buruiana T, Buruiana EC, Melinte V, Colceriu A, Moldovan M. Urethane dimethacrylate oligomers for dental composite matrix: synthesis and properties. Polym Eng Sci. 2009;49:1127–35. 33. Antonucci JM, Regnault WF, Skrtic D. Polymerization shrinkage and stress development in amorphous calcium phosphate/urethane dimethacrylate polymeric composites. J Compos Mater. 2010;44:355–67. 34. Cantwell JB, Dart EC, Jaworzyn JF, Nemcek J, Traynor JR. Photocurable dental filling compositions. 1978;US4110184. 35. Sideridou I, Tserki V, Papanastasiou G. Effect of chemical structure on degree of conversion in light-cured dimethacrylatebased dental resins. Biomaterials. 2002;23:1819–29. 36. Imazato S, Torii M, Tsuchitani Y, McCabe JF, Russell RRB. Incorporation of bacterial inhibitor into resin composite. J Dent Res. 1994;73:1437–43. 37. Xiao YH, Chen JH, Fang M, Xing XD, Wang H, Wang YJ, Li F. Antibacterial effects of three experimental quaternary ammonium salt (QAS) monomers on bacteria associated with oral infections. J Oral Sci. 2008;50:323–7. 38. He J, Soderling E, Osterblad M, Vallittu PK, Lassila LVJ. Synthesis of methacrylate monomers with antibacterial effects against S Mutans. Molecules. 2011;16:9755–63. 39. Zhou C, Weir MD, Zhang K, Deng D, Cheng L, Xu HHK. Synthesis of new antibacterial quaternary ammonium monomer for incorporation into CaP nanocomposite. Dent Mater. 2013;29:859–70. 40. Xu X, Wang Y, Liao S, Wen ZT, Fan Y. Synthesis and characterization of antibacterial dental monomers and composites. J Biomed Mater Res Part B. 2012;100B:1151–62. 41. Antonucci JM, Zeiger DN, Tang K, Lin-Gibson S, Fowler BO, Lin NJ. Synthesis and characterization of dimethacrylates containing quaternary ammonium functionalities for dental applications. Dent Mater. 2012;28:219–28. 42. Liang X, Huang Q, Liu F, He J, Lin Z. Synthesis of novel antibacterial monomers (UDMQA) and their potential application in dental resin. J Appl Polym Sci. 2013;129:3373–81. 43. Waschinski CJ, Zimmermann J, Salz U, Hutzler R, Sadowski G, Tiller JC. Design of contact-active antimicrobial acrylate-based materials using biocidal macromers. Adv Mater. 2008;20:104–8.

123

J Mater Sci: Mater Med 44. Weng Y, Guo X, Chong VJ, Howard L, Gregory RL, Xie D. Synthesis and evaluation of a novel antibacterial dental resin composite with quaternary ammonium salts. J Biomed Sci Eng. 2011;4:147–57. 45. Buruiana EC, Stoleriu A, Ottenbrite RM, Negulescu I. Quaternary ammonium intermediates and monomers with liquid crystal properties. Des Monom Polym. 1999;2:173–84. 46. Ono HK, Jones FN, Pappas SP. Relative reactivity of isocyanate groups of isophorone diisocyanate. Unexpected high reactivity of the secondary isocyanate group. J Polym Sci B Polym Lett Ed. 1985;23:509–15. 47. He J, So¨derling E, Vallittu PK, Lassila LVJ. Investigation of double bond conversion, mechanical properties, and antibacterial activity of dental resins with different alkyl chain length quaternary ammonium methacrylate monomers (QAM). J Biomater Sci—Polym Ed. 2013;24:565–73. 48. Dickens SH, Stansbury JW, Choi KM, Floyd CJE. Photopolymerization kinetics of methacrylate dental resins. Macromolecules. 2003;36:6043–53. 49. Cramer NB, Couch CL, Schreck KM, Boulden JE, Wydra R, Stansbury JW, Bowman CN. Properties of methacrylate–thiol– ene formulations as dental restorative materials. Dent Mater. 2010;26:799–806.

123

50. Rueggeberg FA, Ergle JW, Lockwood PE. Effect of photoinitiator level on properties of a light-cured and post-cure heated model resin system. Dent Mater. 1997;13:360–4. 51. Ge J, Trujillo M, Stansbury J. Synthesis and photopolymerization of low shrinkage methacrylate monomers containing bulky substituent groups. Dent Mater. 2005;21:1163–9. 52. Jeon MY, Yoo SH, Kim JH, Kim CK, Cho BH. Dental restorative composites fabricated from a novel organic matrix without an additional diluent. Biomacromolecules. 2007;8:2571–5. 53. Malacarne J, Carvalho RM, de Goes MF, Svizero N, Pashley DH, Tay FR, Yiu CK, Carrilho MRD. Water sorption/solubility of dental adhesive resins. Dent Mater. 2006;22:973–80. 54. Soh MS, Yap AUJ. Influence of curing modes on crosslink density in polymer structures. J Dent. 2004;32:321–6. 55. EN ISO 4049. Dentistry-Polymer-based filling, restorative and luting materials. International Organization for Standardization, Geneva, Switzerland. 56. Elliot JE, Lovell LG, Bowman CN. Primary cyclization in the polymerization of bis-GMA and TEGDMA: a modeling approach to understanding the cure of dental resins. Dent Mater. 2001; 17:221–9.