Vibrational Spectroscopy

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1 Vibrational Spectroscopy 51 (2009) Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: Combinatorial investigation of structure properties relationships and microcosmic curing mechanism of dental adhesives functional monomers Bing-Jie Sun a, Wei-Hui Shang b, Qiu Jin a, Qian-Wei Xu b, Pei-Yi Wu a, * a The Key Laboratory of Molecular Engineering of Polymers(Ministry of Education) and Department of Macromolecular Science and Advanced Materials Laboratory, Fudan University, 220 Handan Road, Shanghai , PR China b School of Materials Science and Engineering, Tongji University, Shanghai , PR China ARTICLE INFO ABSTRACT Article history: Received 24 June 2008 Received in revised form 17 October 2008 Accepted 28 October 2008 Available online 13 November 2008 Keywords: Dental adhesive functional monomer Mid-infrared Near-infrared Two-dimensional correlation spectroscopy Pyromellitic dimethacrylate (PMDM), a dental adhesive functional monomer has been investigated by FTIR and two-dimensional infrared spectroscopy (2D-IR) methods. With the comparison of the curing processes among three samples (Base-Resin, PMDM and monomethacrylate monomer 4-methacryloxyethyl trimellitic anhydride (4-META)), better properties and faster curing of newly used PMDM has been found. Assignments of Base-Resin, PMDM and 4-META in the mid-infrared (MIR) and near-infrared (NIR) regions are ascertained detailedly. According to the high resolution of 2D-NIR maps, assignments of the dental adhesive functional monomers are further validated and the mechanism of the curing process has been accurately concluded as well; similar 2D-NIR spectra of all three samples show the same transformation process from the double-bonds to the single-bonds during visible-light curing. ß 2008 Elsevier B.V. All rights reserved. 1. Introduction Adhesive monomers are desirable compounds in direct restorative dentistry [1 7]; in general, they have twofold adhesion functional groups, one is a hydrophilic adhesive group and the other one is a hydrophobic polymerizable group. The adhesive group mainly includes a carboxylic acid group and the polymerizable group usually refers to C C double-bonds [8 11]. Besides, the matrix resin and reactive diluents which also contain these functional groups are always mixed with the adhesive functional monomers to achieve proper viscosity [8,11 16]. During curing with UV or visible light irradiation, the adhesive groups adhere to enamel or dentin of the patient s teeth, while the polymerizable groups would be polymerized with other monomers, the methacrylate resins or reactive diluents [17,18]. Although monomethacrylate monomer 4-META (4-methacryloxyethyl trimellitic anhydride) is one of the best-known adhesive functional monomers, it still has a few disadvantages, like low polymerizability and high cost, etc. [19 21]. Therefore, new adhesive monomers with more adhesive and better polymerizable groups should be synthesized to enhance their properties and curing efficiency in restorative dentistry applications. * Corresponding author. Fax: address: peiyiwu@fudan.edu.cn (P.-Y. Wu). Near-infrared (NIR) spectroscopy covers the wavenumber range between the mid-infrared (MIR) and the visible region. This technique has been widely used to analyze the overtone and combination bands of compounds since the 20th century [22 24]. NIR has lots of advantages: easy preparation, rapid and nondestructive analysis of bulk materials, especially, NIR spectra can separate peaks which are overlapped in the MIR region and thereby improve the resolving power of the infrared spectra [25]. Unfortunately, the spectra-structure correlation in NIR region received less attention compared to MIR spectroscopy [22]. Generalized two-dimensional (2D) correlation spectroscopy was proposed by Noda [26 28]. This technique looks into the spectral intensity fluctuation under variable perturbations, such as temperature [25,29], time [30,31], concentration [32] or ph [33], etc. Therefore, specific event order and the dynamical mechanism of the system in the perturbing process can be concluded [25,29,34]. Through spreading the peaks over the second dimension, 2D spectra can reveal subtle information not obvious in 1D spectra, help to ascertain the unknown assignments and efficiently improve the spectral resolution. Many NIR studies have been done to investigate the curing process of adhesive monomers, matrix resin and reactive diluents, nevertheless, the frequencies of vibrations in the NIR region are still faintly assigned. The detailed curing mechanism on the molecular level is also uncertain. In the present study, the above gaps are supplied. We synthesized a kind of dental adhesive /$ see front matter ß 2008 Elsevier B.V. All rights reserved. doi: /j.vibspec

2 94 B.-J. Sun et al. / Vibrational Spectroscopy 51 (2009) functional monomer (PMDM) rather than 4-META, and detailedly assigned vibrations of adhesive monomers, matrix resin and reactive diluents in NIR region for the first time; additionally, 2D-IR correlation spectroscopy serves as an innovation studying the curing mechanism on the molecular level. 2. Experimental 2.1. Materials Camphorquinone (CQ, 98%), N,N-dimethyl aminoethyl methacrylate (DMAEMA, 99%) were purchased from Fluka (Germany). Triethyleneglycol dimethacrylate (TEGDMA, 98%), 2-hydroxyethylethyl methacrylate (HEMA, 98.5%), were obtained from Sigma Aldrich. 4-META was supplied by Sun Medical Ltd. (Japan). Other chemicals were kindly supplied by Sinopharm Reagent Co., Ltd. Chemicals. Based on previous description [35] we changed the catalyst and its concentration to improve the efficiency of the reaction, and Pyromellitic dimethacrylate (PMDM) (Fig. 1) was synthesized by the following procedure: 0.22 mol of HEMA and 0.1 mol of pyromellitic dianhydride (PMDA) were dissolved in 100 ml THF (refluxed with metallic sodium and distilled previously) then refluxed in nitrogen at 60 8C for 0.5 h in the presence of 0.01 g hydroquinone monomethyl ether (HQME, inhibitor) until solid PMDA completely dissolved in the solution. Then 0.2 mol of triethylamine was added to the reactor as catalyst. The mixture of the reagents was refluxed in nitrogen at 80 8C for 2 h, and then dissolved in diethyl ether after being cooled, therefore any unreacted PMDA and tetra-acid byproducts could be removed by vacuum filtration. Afterward, the organic solution was washed with distilled water and hydrochloric acid (1 mol/l 150 ml) for three times, and finally the solvent was removed under vacuum condition. The products were examined by NMR spectroscopy with the aid of a Bruker model AVANCE DMX-500 spectrometer working at 500 MHz. The samples were prepared with dimethylsulfoxide (DMSO) as the solvent and tetramethylsilane (TMS) as internal standard. Based on the NMR result, we observe that there are two isomers of PMDM within the products, para-pmdm (d = 8.0 ppm) and meta-pmdm (d = 7.9 and 8.1 ppm) with a ratio of 1:1.05, these two isomers are only different in the positions of HEMA groups, and both of the isomers could be used as the adhesive functional monomers. In order to obtain the pure para-isomer, the recrystallization method was utilized. The crude solid was first dissolved completely into methanol and heated to 65 8C, and then water was added dropwise until the solution became turbid, finally the mixture was cooled at 4 8C for 1 week. Afterwards, the crude Fig. 1. Chemical structure of the dental adhesive functional monomers: 2,2-bis 4-(2-hydroxy-3-methacryloxyprop-1-oxy) phenyl propane (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), 4-methacryloxyethyl trimellitic anhydride (4-META), and pyromellitic dimethacrylate (PMDM).

3 B.-J. Sun et al. / Vibrational Spectroscopy 51 (2009) crystals were filtered and re-crystallized in the same way for two more times. Till the third time of the re-crystallization ended, the para-pmdm isomer was gained as a shiny and white crystalline material with a melting-point range of C, and 1 H NMR result shows the final para-pmdm achieves a purity of 99.3%. Three systems of samples were prepared for the infrared experiments: (i) methacrylate resin 2,2-bis 4-(2-hydroxy-3- methacryloxyprop-1-oxy) phenyl propane (Bis-GMA) and reactive diluents TEGDMA (Fig. 1) was mixed in a 70:30 weight ratio (named Base-Resin below for short); (ii) PMDM:Bis-GMA:- TEGDMA = 10:60:30 (named PMDM below); (iii) 4-META (Fig. 1): Bis-GMA:TEGDMA = 10:60:30 (named 4-META below). CQ/DMAEMA photoinitiating system (0.2 and 0.2 wt%, respectively, in respect to the total resin) was dissolved in all resin samples Methods Fourier transform infrared spectroscopy The NIR and MIR spectra, based on a 4 cm 1 spectral resolution and signal-averaging 32 scans, were recorded by a Nicolet Nexus 470 FT-NIR/MIR spectrometer with a liquid-nitrogen-cooled MCT detector and DTGS detector, respectively. Microscope slides/caf 2 windows, which have no absorbance in the NIR/MIR regions, were used as transmission cell windows. In the NIR experiment, two pieces of microcope slides and one Teflon spacer (2 mm thickness) were used to make the sample cell; while in the MIR experiment, two pieces of CaF 2 windows and one Teflon spacer (20 mm thickness) were used to make the sample cell. And then the sample cells were filled with each sample and taken to do the NIR or MIR experiment, respectively. The variable spectra during curing of three samples in visible light irradiation at the same distance were collected every 20 s, and the intensity of light was measured as 550 mw/cm 2 with a radiometer (Model 100, Demetron, Danbury, CT, USA). The original MIR spectra were baseline-corrected by OMNIC 7.3 software, while the NIR spectra were analyzed without any data pretreatment. Fig. 2. Near-infrared spectrum of Base-Resin, PMDM and 4-META without curing. MIR spectrum; while assignments of PMDM and 4-META should be the same with those of Base-Resin. In Fig. 3 the fundamental vibrations of Base-Resin before and after curing are shown. A summary of the MIR frequency positions Two-dimensional correlation analysis The spectra at an interval of 20 s in certain wavenumber ranges were selected, and the generalized 2D correlation analysis were made by the 2D Shige software composed by Shigeaki Morita (Kwansei-Gakuin University, Japan). The 2D correlation spectrum is shown in the center of the map, while the averaged 1D reference spectrum is at the side and the top. In the 2D correlation maps, the red-colored regions are defined as the positive correlation intensities, whereas the blue-colored ones are regarded as the negative correlation intensities. 3. Results and discussion 3.1. Preliminary considerations In Fig. 2 the NIR spectra of three dental adhesives functional monomers (Base-Resin, PMDM and 4-META) before curing are shown. As most bands of the samples are not clearly assigned, their assignments should be ascertained before detailed analysis, and the bands in MIR region were first analyzed to speculate on the assignments of those overtone and combination bands in the NIR region. Comparing the spectra in Fig. 2, all bands of the three samples are almost the same, which reflect the limitation of infrared spectra in distinguishing similar samples with only small structural differences (same functional groups, but of different numbers). As their infrared spectra are nearly the same, we took the simplest Base- Resin sample for example to affirm the assignments of bands in the Fig. 3. Mid-infrared spectrum of Base-Resin before and after curing: (A) in the range from 3130 to 2776 cm 1 ; (B) in the range from 1660 to 1425 cm 1. The black lines represent the spectrum of the sample before curing, the red lines represent the spectrum of the sample after the longest curing, and the blue line in (A) refers to the second derivative spectrum of the black line in (A). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

4 96 B.-J. Sun et al. / Vibrational Spectroscopy 51 (2009) Table 1 Observed MIR bands of the dental adhesives functional monomer. See also text (s, symmetric; as, asymmetric; Ar, aromatic) [22,25,36]. Wavenumber (cm 1 ) Assignment a 3100 n( C H) b 3040 n(ar H) c 2979 n as (CH 3 -I) d 2964 n as (CH 3 -II) e 2929 n as (CH 2 ) f 2890 n s (CH 3 -I) g 2872 n s (CH 3 -II) h 2848 n s (CH 2 ) i 1640 n(c C) j 1510 d(ar H) k 1460 d(ch 2 -a) Table 2 Frequency position and assignment of calculated and observed NIR absorption bands of dental adhesives functional monomer [8,12,22,25]. Wavenumber (cm 1 ) Assignment Fig. 4. Near-infrared spectrum of Base-Resin before and after curing in the region of cm 1. Observed Calculated n( C H) n(ar H) n as (CH 3 -I) n as (CH 2 ) n s (CH 3 -I) n s (CH 2 ) n( C H) + n(c C) n as (CH 3 -I) + n(c C) nar(c C) + d(ar H) n( C H) + d(ch 2 -a) and their assignments are listed in Table 1 [22,25,36], according to it the calculated NIR bands (without taking anharmonicity into account) are listed in Table 2 [8,12,22,25]. There are two types of methyl groups in Bis-GMA, CH 3 -I and CH 3 -II (denoted in Fig. 1), and the positions of their vibration bands are different in the MIR region (Table 1). The band j in Fig. 3(B) is assigned to the d(ch 2 ) vibration, however its quite difficult to make sure whether the CH 2 group is on the terminal group (like a in Fig. 1) or on the chemical chain. The 2D-IR spectra could help to solve this problem and we will discuss this later. Fig. 4 shows the NIR spectrum of Base-Resin before and after curing in the region of cm 1, according to the calculated values in Table 2, peaks found in the spectrum are tentatively assigned [8,12,22,25] and also listed in Table 2. By comparison with the theoretical values and the experimental results, we can find good agreement. Furthermore, as combination vibration is generally formed between adjacent groups, and the 4681 cm 1 band is the combination between 2979 and 1640 cm 1 band, the assignment of 2979 cm 1 should be referred to the vibration of n as (CH 3 -I) in MIR region, thats because CH 3 -I group is next to the C C double-bond, whose stretching vibration band is at 1640 cm 1. Base on assignments of Base-Resin in the MIR and NIR regions, we can also clearly conclude assignments of PMDM and 4- META in those regions FT-near-infrared analysis It has been known that during curing of the dental adhesive monomers by visible light activated, the adhesive groups adhere to the patient s teeth, and the polymerizable groups like double bonds would be polymerized with other monomers, Bis-GMA or TEGDMA, which means the double-bonds of monomers will transfer into single-bonds during curing. In order to compare the Fig. 5. The extent of cure of 6165 cm 1 in Base-Resin, PMDM, and 4-META during curing in visible light. properties of the newly used monomer PMDM and traditional monomer 4-META with simplest Base-Resin, we take the doublebond related 6165 cm 1 band for example to investigate the curing process. Fig. 5 shows the change in the amount of double-bonds (6165 cm 1 ), we can observe that the maximum cure extent of PMDM (52.28%) is almost the same as for Base-Resin (51.61%), but it is larger than 4-META (46.74%), which indicates the newly used monomer of PMDM has higher curing efficiency than traditional 4- META. Moreover, in the curing process, we can see that PMDM starts curing even earlier than Base-Resin, while 4-META needs more time to finish curing, which also reveals the faster curing of PMDM. We also investigated other bands in the NIR region of these three samples, and list the extent of their changes in Table 3. According to the assignments in Table 2, 4623 cm 1 is assigned to the vibration of the aromatic group, as a result it changes only little (less than 5%) during curing. In visible light irradiation, doublebonds will transfer into single-bonds and finish the curing process, hence the intensities of double-bond related peaks like 6165, 4743 and 4564 cm 1 decrease; on the other hand, the intensity of 4681 cm 1 band increases at the same time, which indicates the 4681 cm 1 band is related to a single-bond group. However, also in

5 B.-J. Sun et al. / Vibrational Spectroscopy 51 (2009) Table 3 The maximum extent of change of 6165, 4743, 4681, 4623 and 4564 cm 1 in Base- Resin, PMDM, and 4-META during curing with visible light ( + means increase, means decrease in the curing process). Samples Extent of change (%) 6165( ) 4743( ) 4681(+) cm 1 ( ) Base-Resin PMDM META Table 3, the increase extent of three double-bonds related peaks is much larger than the decrease degree of 4681 cm 1, which helps to conclude that the band of 4681 cm 1 is the combination of a single-bond and a double-bond vibration, the former consequentially should be of a larger contribution, as the 4681 cm 1 peak exhibits an increase during curing Two-dimensional correlation analysis The red-colored and blue-colored areas in the 2D-IR correlation contour maps represent positive and negative cross-peaks, respectively. The 2D-IR correlation spectra are characterized by two independent wavenumber axes (n 1, n 2 ) and a correlation intensity axis. Two types of spectra, 2D synchronous and asynchronous spectrum are obtained in general. The correlation intensity in the 2D synchronous and asynchronous maps reflects the relative degree of in-phase or out-of-phase response, respectively. The 2D synchronous spectra are symmetric with respect to the diagonal line in the correlation map. Some peaks appearing along the diagonal are called the autopeaks and the symbols of them are always positive, autopeaks represent the degree of autocorrelation of perturbation-induced molecular vibrations, where there appears the autopeak, the peak at this wavenumber would change greatly under the environmental perturbation. Off-diagonal peaks, named cross-peaks (F(n 1, n 2 )), may be positive or negative, which represent the simultaneous or coincidental changes of spectra intensity variations measured at n 1 and n 2. Positive cross-peaks (the symbol of F(n 1, n 2 ) is positive) demonstrate that the intensity variations of the two peaks at n 1 and n 2 are taking place in the same direction (both increase or both decrease) under the environmental perturbation; while the negative cross-peaks (the symbol of F(n 1, n 2 ) is negative) help to infer that the intensities of the two peaks at n 1 and n 2 change in the opposite direction (one increases, while the other one decreases) under perturbation. The 2D asynchronous spectra are asymmetric with respect to the diagonal line in the correlation map. Unlike synchronous spectra, only off-diagonal cross-peaks would appear in asynchronous spectra, and these cross-peaks can also be either positive or negative. The intensity of the asynchronous spectrum (C(n 1, n 2 )) represents sequential or successive changes of spectral intensities observed at n 1 and n 2. With the cross-peaks both in synchronous and asynchronous maps, we can get the specific order of the spectral intensity changes taking place while the sample is subjected to an environmental perturbation. According to Noda s rule [26 28], when F(n 1, n 2 ) > 0, if C(n 1, n 2 ) is positive (redcolored area), band n 1 will vary prior to band n 2 ;ifc(n 1, n 2 )is negative (blue-colored area), band n 1 will vary after n 2. However, this rule is reversed when F(n 1, n 2 ) < 0. In brief, if the symbols of the cross-peaks in the synchronous and asynchronous maps are the same (both positive or both negative), band n 1 will vary prior to band n 2 ; while if the symbols of the cross-peak are different in the synchronous and asynchronous spectra (one positive, and the other one is negative), band n 1 will vary after n 2 under the environmental perturbation PMDM Fig. 6 shows the synchronous and asynchronous maps of PMDM in the NIR spectral region of cm 1. The synchronous map develops two strong autopeaks, one at 4743 cm 1 is assigned to the combination of two double-bond related peaks, and this autopeak indicates the double-bonds change prominently in the curing process; the other autopeak at 4564 cm 1 is also a combination band. Additionally, these two peaks develop a positive cross-peak at (4743, 4564) cm 1 in the synchronous map, according to Noda s rule [26 28] the positive cross-peak in the synchronous map reveals that these two peaks at 4743 and 4564 cm 1 change in the same direction during curing; this is quite consistent with the 1D-NIR result that in Fig. 4 we can see both bands of 4743 and 4564 cm 1 decrease during curing. Moreover, in the corresponding asynchronous spectrum of Fig. 6(B), there s also a cross-peak at (4743, 4564) cm 1, but is negative. From different symbols in the synchronous and asynchronous maps, we can infer that the peak at 4564 cm 1 changes prior to the one at 4743 cm 1. During curing, the main change of dental adhesives functional monomers is the transformation from the double-bonds to the single-bonds, and the double-bonds should first decrease and the Fig. 6. Synchronous (A) and asynchronous (B) 2D NIR correlation spectra of PMDM in the cm 1 region.

6 98 B.-J. Sun et al. / Vibrational Spectroscopy 51 (2009) single-bonds could appear afterwards, therefore the double-bond related peaks would change prior to the single-bond peaks. The 4743 cm 1 band is made up of two double-bond related vibrations: n( C H) + n(c C), the 4564 cm 1 band is the combination of n( C H) + d(ch 2 ), as 4564 cm 1 changes prior to 4743 cm 1 during curing, we can ascertain that the band at 4564 cm 1 should be definitely made of all double-bonds components. Therefore, the CH 2 group in d(ch 2 ) at 4564 cm 1 (Tables 1 and 2), is of type a on the terminal groups (Fig. 1). As 4564 > 4743 cm 1 ( > means change prior to ), we can also derive that the hydrogen atoms within the CH 2 terminal groups are more vivacious and change faster than the C C framework of compounds. In Fig. 6(A), two negative cross-peaks at (4743, 4693) and (4743, 4665) cm 1 can also be observed. According to Noda s rule [26 28], we can conclude the peak at 4743 changes in opposite directions to the peaks at 4693 and 4665 cm 1. As 4743 cm 1 band is double-bond related peak which decreases in the curing process, both bands of 4693 and 4665 cm 1 must contain single-bond related components, which would increase during curing. In Fig. 6(B) both cross-peaks at (4743, 4693) and (4743, 4665) cm 1 appear to be negative as well, same symbols in the synchronous and asynchronous maps help to get the sequence of 4743 > 4693, 4665 cm 1, which clearly manifests the fact that double-bonds would turn out to be single-bonds components after curing. To further investigate these two single-bonds components contained bands of 4693 and 4665 cm 1, we should combine the FTIR and 2D-IR spectra; as we observe single 4681 cm 1 band in 1D-NIR spectra, but doublet peaks at 4693 and 4665 cm 1 in 2D-NIR maps, we can tentatively conclude that single 4681 cm 1 peak actually splits into doublet bands at 4693 and 4665 cm 1, and the high resolving power of 2D correlation spectra is able to observe such phenomenon. From Table 2 we know that the peak of 4681 cm 1 is the combination of the n as (CH 3 -I) (2979 cm 1 ) and n(c C) (1640 cm 1 ) vibrations, therefore the peak of 4665 cm 1 at lower wavenumber should contain more contribution from n(c C), while the one of 4693 cm 1 at higher wavenumber contains more contribution from n as (CH 3 -I). Further to draw the slice spectrum (not shown in this paper), we can detect that the cross-peak at (4693, 4665) cm 1 is positive in the synchronous map but negative in the asynchronous map, which reveals: 4665 (more doublebonds component) >4693 cm 1 (more single-bonds component). Fig. 7. Synchronous (A) and asynchronous (B) 2D NIR correlation spectra of Base- Resin in the cm 1 region. Fig. 8. Synchronous (A) and asynchronous (B) 2D NIR correlation spectra of 4-META in the cm 1 region.

7 B.-J. Sun et al. / Vibrational Spectroscopy 51 (2009) Their assignments also help to note the fact that double-bonds transform into single-bonds during curing. With the sequence gained before, we can get the whole order of change for PMDM: 4564 > 4744 > 4665 > 4693 cm 1, which shows the holistic transformation from double-bonds to singlebonds in the visible light curing process Base-Resin The synchronous (A) and asynchronous (B) of Base-Resin in Fig. 7 are quite similar to Fig. 6, with the cross-peaks we can also conclude that the order of change of Base-Resin is: 4560 > 4744 > 4666 > 4693 cm 1, from which we can derive that the positions of several peaks only show small differences compared to PMDM. However, the trend in transformation from double-bonds to single-bonds is exactly the same META Fig. 8 shows the synchronous (A) and asynchronous (B) of 4- META, which is also quite similar to PMDM and Base-Resin. The whole changing sequence of 4-META in the curing process can be gained with cross-peaks in Fig. 8 as: 4560 > 4744 > 4676 > 4693 cm 1. Besides some different peak positions, the double-bond to single-bond transformation can be clearly observed as well. 4. Conclusion The kind of newly used dental adhesive functional monomer PMDM has been detailedly investigated. With the help of midinfrared spectra, we have clearly ascertained the assignments of the dental adhesives monomers PMDM, 4-META and also Base- Resin in the near-infrared region. Comparing different extents of curing among three samples (Base-Resin, PMDM and 4-META), we have observed that PMDM has higher extent of cure and faster curing speed than the other two. In the 2D-NIR analysis, assignments of the monomers are further confirmed, and change sequences of various vibrations are accurately concluded. 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