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Nanoindentation tests to assess polymerization of resin-based luting cement Mitsuha Sato a , Akihiro Fujishima b , Yo Shibata b,∗ , Takashi Miyazaki b , Mitsuko Inoue a a

Department of Pediatric Dentistry, Showa University School of Dentistry, 2-1-1 Kitasenzoku, Ohta-ku, 145-8515 Tokyo, Japan b Department of Conservative Dentistry, Division of Biomaterials & Engineering, Showa University School of Dentistry, 1-5-8 Hatanodai, Shinagawa-ku, 142-8555 Tokyo, Japan

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. The optimal polymerization of resin-based luting cements plays a critical role in

Received 6 November 2013

the long-term clinical success of dental prostheses and indirect restorations. This study

Received in revised form

investigated a mutual action between the conformational changes and mechanical proper-

16 April 2014

ties of a dimethacrylate resin-based luting cement with and without pre-application of the

Accepted 29 May 2014

acidic functional monomer 10-methacryloxydecyl dihydrogen phosphate. Methods. Degree of conversion in the luting cement was measured using conventional infrared spectrophotometry. Mechanical properties of the luting cements were also eval-

Keywords:

uated by quasi-static and dynamic nanoindentation tests.

Dimethacrylate

Results. The results of infrared spectrophotometry and nanoindentation testing were propor-

10-Methacryloxydecyl dihydrogen

tional in samples without functional monomer pretreatment. When considerable residual

phosphate

monomer remains within the final products, the mechanical properties of the resin-based

FTIR

luting cements could possibly be impaired. Although the apparent degree of conversion

Nanoindentation

increased with a mixture of functional monomer, a reduction in the cross-linking poly-

Polymerization

mer network may have resulted in the highest viscoelastic creep behavior of the luting cement. The time-dependent behaviors found in the nanoindentation tests likely resulted from linear polymerization chains of the functional monomer. Significance. The application of an acidic functional monomer may affect the viscosity of resin-based luting cements. Quasi-static or dynamic nanoindentation is a useful tool for assessing the polymerization qualities of resin composites. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The integrity of luting cements is an important determinant of the long-term success of dental prostheses and indirect



restorations [1,2]. With increasing demand for esthetic properties and adequate marginal seal, the use of resin-based luting cement has become dominant. Despite their superior esthetic and mechanical properties, conventional resin-based cements require pre-application of

Corresponding author. Tel.: +81 3 3784 8178; fax: +81 3 3784 8179. E-mail address: [email protected] (Y. Shibata). http://dx.doi.org/10.1016/j.dental.2014.05.034 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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Table 1 – Sample preparation protocol. Sample group Control LC MDP-pretreated MDP-mixed

Treatment Mixed resin-based luting cement (Rely-XTM ARC, 3M ESPE). Mixed resin-based luting cement was photo-irradiated for 30 s with a light cure unit. Mixed resin-based luting cement was placed on a CaF2 disc pretreated with MDP monomer (Epricode, Kuraray). MDP monomer and resin-based luting cement were mixed and then immediately placed on a CaF2 disc.

total etch adhesives or a self-etching primer containing an acid-functionalized monomer such as 10-methacryloxydecyl dihydrogen phosphate (MDP), because the resin substrate lacks chemical adhesive properties [3,4]. Acidic monomers may compromise the curing mechanism of resin-based cements [5,6] and hence optimal polymerization of the cements with MDP application is uncertain. Inferior polymerization of resin-based materials may negatively affect their mechanical properties, causing significant deterioration of clinical performance [7,8]. Infrared (IR) spectrophotometry has been used to evaluate the polymerization of dental composites [9,10]. In this technique, the relative intensities of the C C double bond peak at 1637 cm−1 and of the phenyl group peak at 1608 cm−1 indicate the degree of conversion. However, IR spectrophotometry also detects MDP vibration modes, which theoretically overlap with the cement spectra, so that isolation of polymerization is unlikely [11]. Thus, an evaluation technique that combines IR spectrophotometry and direct mechanical characterization is needed. IR spectrophotometry using two-beam transmission devices allows visualization of conformational changes over time in thin film preparations of luting cements. Concurrent microscale mechanical testing on the thin film is desirable. Nanoindentation is depth-sensing mechanical testing that continuously measures hardness and elastic modulus by quasi-static load displacement in the thin film preparation [12]. A drawback in mechanical testing of resin-based luting cements is the unavoidable viscoelastic response, which increases with decreased polymerization because of delayed fluid movement of the residual monomer [13,14]. However, nanoindentation systems currently have high placement precision and can capture a material’s responses over a range of imposed frequencies using dynamic force and displacement amplitudes [15,16], enabling measurement of the viscoelastic properties of resin-based materials. This study evaluates the polymerization of dimethacrylate resin-based luting cement with and without MDP pre-application by measuring conformational changes over time with quasi-static and dynamic nanomechanical testing.

2.2.

Table 1 shows sample preparation protocols. Mixed resinbased luting cement was placed directly between CaF2 discs (control) or was photo-irradiated for 30 s with a light cure unit (LC) (DP-075, Morita, Tokyo, Japan). The intensity of the halogen lamp was assessed (470 nm and >500 mW/cm2 ) with a photoelectric sensor before testing and its intensity was maintained throughout the study. In the MDP-pretreated samples, CaF2 discs were pretreated with MDP monomer, and then resin-based luting cement was pressed between a pretreated and an untreated disc (MDP-pretreated). Pretreatment was performed according to manufacturer’s instructions. For the mixed samples, the same volume of MDP monomer and resin-based luting cement were mixed and then immediately placed between CaF2 discs (MDP-mixed). CaF2 discs were used because of their IR translucency and their distinctive mechanical properties compared with the samples.

2.3.

Materials and methods

2.1.

Materials

This study used dual-polymerizing resin-based luting cement (Rely-XTM ARC, 3M ESPE, Tokyo, Japan) and MDP monomer (Epricode, Kuraray, Tokyo, Japan).

IR spectrophotometry

The samples were subjected to a Fourier Transform Infrared (FTIR) analyzer (FT/IR-660, JASCO, Tokyo, Japan). Conformational changes of each composite over time were monitored for 24 h. At a resolution of 4 cm−1 , we performed 200 iterations within the range from 400 to 4000 cm−1 to characterize the various functional groups. The degree of conversion was measured by the intensities of the C C peak at 1638 cm−1 and the C- - -C reference peak at 1608 cm−1 , using a standard baseline technique [9,17]. The peak at 1608 cm−1 originated from aromatic rings, whose intensity remains unchanged during polymerization. Therefore, the ratio of the absorbance intensities of C C/C- - -C reveals the polymerization ratio of the samples using following formula: Degree of conversion (%) =100−

2.4.

2.

Sample preparation

 cured peak intensities of C = C/C − − − C uncured peak intensities of C = C/C − − − C



×100

Phase detection by scanning probe microscope

One CaF2 disc was removed to expose a bare LC sample surface, which was subjected to phase detection. Scanning probe microscopy (SPM) (SPM-9700; Shimadzu, Kyoto, Japan) was performed in the phase mode using rectangular silicon cantilevers with a spring constant of ∼40 Nm−1 and typical resonance frequencies between 250 and 300 kHz. Imaging was accomplished in the attractive tip-sample interaction regime,

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recording height and phase images, which indicate the distribution of mechanical properties of the sample surface [18].

2.5.

Nanoindentation experiments

The bare sample surfaces were subjected to nanomechanical testing using a quantitative nanomechanical test instrument (TS70 TriboScope; Hysitron, Inc., MN, USA) interfaced with a scanning probe microscope (SPM-9700; Shimadzu) with a diamond Berkovich indenter probe (Hysitron). Fused quartz acted as the calibration material to determine the indenter tip area function and the machine compliance [19]. To minimize errors arising from surface roughness, appropriately smooth regions were chosen with a scanning range of 50 ␮m × 50 ␮m and then 1 ␮m × 1 ␮m, so that the surface roughness was less than 10% of the minimum measurement range of the indenter penetration depth [20], assuming a perfect relationship between contact depth and elastic deformation. The distance between indentations was more than 10 ␮m to avoid any influence of residual stresses from adjacent indentations. A constant thermal drift was monitored with 2 ␮N of pre-load for 40 s before indentation. The second 20 s of drift rate was subtracted from the overall contact depth. Indentation tests were performed within the estimated oxygen inhibition region at the center of the sample surface so that superior mechanical properties associated with the degree of conversion could be expected. The tests were performed perpendicular to the selected regions using a loading/partial unloading technique with a load function comprising a total of 33 loading and unloading portions with hold time. Segment time was 1 s. Typical nanoindentation method is based on the assumption of isotropic elastic–plastic materials [20]. However, synthetic polymers often exhibit viscoelastic or time-dependent behavior. The most readily observed effect of viscoelasticity on indentation is creep under constant load. When unloading follows loading without a hold time at peak load, displacement increases slightly in the initial portion of the unloading phase, because the creep rate of the materials is initially higher than the imposed unloading rate [20]. This phenomenon results in a negative, changing slope in the initial unloading phase, making accurate assessment of the elastic modulus impossible. To eliminate this negative influence, each loading portion was followed by a hold time, so that the unloading portion was assumed to be purely elastic rather than viscoelastic. Moreover, an incremental loading rate was applied to a maximum loading force of 200 ␮N during a single test. The elastic moduli of polymers may vary with different loading rates because viscoelasticity causes a varied unloading slope. A larger number of loading/unloading portions enables multiple loading rates resulting in the final loading force. Therefore, this technique also evaluates the load-dependent behavior of resin-based luting cements by performing multiple loading rates followed by partial unloading during a single test [21]. The hardness and elastic moduli at indenter contact depths between 10 and 100 nm were noted, based on knowledge of the effective range of the Berkovich tip area function. An applied closed-loop (load) control algorithm was used throughout the tests, unless the dynamic indentations were superimposed in a load function. For the closed-loop control, the indenter tip

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was first withdrawn from the surface to a set distance (lift height) and then re-captured the sample surface, because the 2 ␮N pre-load during thermal drift monitoring may influence the ultra-low loading portion of this partial unloading test. The elastic moduli were calculated from force–displacement curves using the standard unloading analysis within the TS70 proprietary software (Hysitron). The ideal curves fit to each displacement curve were corrected manually. The effective measurement range and constant elastic moduli were confirmed prior to each nanoindentation experiment.

2.6.

Dynamic nanoindentation test

Force–displacement curves with a loading/partial unloading portion were recorded using 50 ␮N/s to a maximum load of 200 ␮N. The initial loading portion was followed by a 10 s holding time and partial unloading to 100 ␮N. The second loading portion regained the maximum load of 200 ␮N to enable nearly pure elastic deformation. The second loading portion was followed by a 20 s holding time. Oscillations and five sinusoidal indentations were superimposed after the 20 s holding time on the second force–displacement curve of the loading/partial unloading tests (Fig. 1). The applied amplitudes were set at 20 ␮N with frequencies at 0 (creep), 1, 2, and 4 Hz. The initial oscillations mimicked the subsequent sinusoidal indentation profiles with appropriate amplitudes and instantaneous holdings, so that material amplitudes by dynamic indentations became constant prior to the sinusoidal frequency. The above load functions were determined in consideration of effective area functions and measurement range, according to suggestions of depth-dependent loading/partial unloading tests. During superimposed dynamic sinusoidal indentations, a phase lag between the applied stress and the measured strain signal reveals viscoelastic behavior. For a pure elastic solid, stress and strain should be in phase, while a pure viscous fluid has a 90◦ phase lag of strain relative to applied stress [22]. The tangent of phase lag (tan ı) is the ratio of the storage modulus (E ) to the loss modulus (E ), according to the trigonometric function tan ı = E /E .

2.7.

Statistical analysis

Five samples were evaluated by FTIR. Five quasi-static and dynamic indentation tests were performed on each sample surface, and the data reproducibility was confirmed by at least five samples in each sample group. Results are expressed as the mean ± standard deviation. The normal distribution of each dataset was confirmed using the Kolmogorov–Smirnov test. The appropriateness of the hypothesis of homogeneous variances was investigated by Bartlett’s test. Data were analyzed by ANOVA followed by a post hoc Tukey test. A p-value less than 0.05 was considered significant.

3.

Results

3.1.

IR spectra

Fig. 2a shows the conformational changes of the control samples’ IR spectra between 1 min and 24 h compared to

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Fig. 1 – A load function for dynamic nanoindentation tests. The initial loading portion was followed by a 10 s holding time and partial unloading to 100 ␮N. The second loading portion regained the maximum load of 200 ␮N to enable nearly pure elastic deformation. The second loading portion was followed by a 20 s holding time. Oscillations and five sinusoidal indentations were superimposed after the 20 s holding time on the second force–displacement curve of the loading/partial unloading tests.

a

0.12

0.1

control 1 min

0.08

Abs

control 5 min control 10 min 0.06

control 60 min control 24 hrs

0.04

LC 24 hrs

0.02

0

1650

1630

1610

LC’s spectrum for 24 h. The IR spectra of all resin-based luting cements indicated the reduction of a peak at 1637 cm−1 derived from C C against a peak at 1608 cm−1 derived from C- -C due course of time. The LC samples showed just over 80% polymerization throughout the analysis (Fig. 2b). The degree of conversion of the control samples increased rapidly to approximately 75% by 30 min. The ratio then dropped to just under 70% by 40 min before increasing to just over 70% by 60 min. The degree of conversion of MDP-mixed samples rose sharply by 20 min and then increased steadily to just over 80% by 60 min and the values were approached to LC samples. Though the MDP-pretreated samples showed increasing polymerization by 40 min, it was the lowest of the sample types, at approximately 60%.

1590

Wavenumber(cm-1)

3.2.

Phase detection by SPM

Degree of conversion(%)

b 100 90

control

80

LC

70

MDP-mixed

60 MDPpretreated

50 40 30

Phase detection on the LC sample revealed a variety of delayed responses within the elastic deformation and thus the image revealed variations in relative mechanical properties as different colors (Fig. 3a). The indentation image obtained by SPM at the maximum loading force indicated that the nanoindentation tests used sufficient measurement ranges to evaluate the mechanical properties of the sample surface (Fig. 3b).

20 10

3.3.

Nanoindentation test

0 0

10

20

30

40

50

60

70

Time(min)

Fig. 2 – Conformational changes of each sample over time. (a) Representative spectra of control samples over time between 1 min and 24 h and a spectrum of LC sample for 24 h. (b) Polymerization ratio of the samples for 60 min. The degree of conversion was measured by the intensities of the C C peak at 1638 cm−1 and the C- - -C reference peak at 1608 cm−1 , using a standard baseline technique.

The depth-dependent nanoindentation tests showed greater hardness and elastic modulus at shallow contact depth, with decreasing values as indenter contact depth increased (Fig. 3c). The values at the shallow contact depth were therefore varied and were merged as a function of contact depth on the basis of the estimated size-dependent mechanical properties of composite materials that consisted of plastic fillers and viscous matrix. The LC sample showed the greatest hardness (0.43 ± 0.07 GPa) and elastic

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Fig. 3 – Depth dependent loading/partial unloading tests. (a) Phase detection on the LC sample revealed a variety of delayed responses within the elastic deformation. (b) The indentation image obtained by SPM at the maximum loading force. (c) Hardness or elastic modulus vs. contact depth on the sample surfaces. The average values of five indentations were merged.

modulus (8.52 ± 1.27 GPa) at maximum contact depth (p < 0.05) (Fig. 4a). Although the average hardness of the MDP-mixed sample was the lowest with increasing contact depth, the hardness and elastic moduli of all non-photo-irradiated samples were not statistically different (p > 0.05) and all values were significantly lower (p < 0.05) than those of the LC sample. The delayed displacement at 0 Hz revealed the timedependent creep behavior of the samples. The LC sample had the lowest average creep over five indentations, while the MDP-mixed sample had the highest (p < 0.05) (Fig. 4b). The average tan ı of the samples decreased with increasing frequencies. The MDP-mixed sample had the lowest value at 0.09 (p < 0.05), while those of the other samples were approximately 0.17 (Fig. 4c).

4.

Discussion

Dental composites consist primarily of inelastic fillers within a flexible organic matrix [23]. The phase responses obtained by SPM reveal a homogeneous distribution of mechanical properties because of the time-dependent behavior of the viscoelastic resin matrix. The present depth-dependent nanoindentation tests therefore measured the samples’ sizedependent mechanical properties. Namely, samples with greater hardness and elastic moduli with shallow contact depth had stiffer inorganic fillers. As indenter depth and contact area between the indenter tip and sample surface increased, the values illustrated a combination of fillers and deformable elastic matrix.

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Fig. 4 – Quasi-static and dynamic nanoindentation tests. (a) Hardness and elastic modulus at the maximum contact depth obtained from the depth dependent loading/partial unloading tests. (b) The delayed displacement at 0 Hz revealed the time-dependent creep behavior of the samples. The average values of five indentations were merged and displayed. (c) The tangent of phase lag (tan ı) is the ratio of the storage modulus (E ) to the loss modulus (E ), according to the trigonometric function tan ı = E /E . Results are expressed as the mean ± standard deviation. A p-value less than 0.05 (*) was considered significant.

Moisture conditions are clinically relevant because MDP requires the presence of water to be acidic [24,25]. The acidity of the MDP mixture may affect polymerization, as photochemical polymerization is highly dependent on activation of the amine co-initiator that generates radicals by an acidic redox system [26]. Although moist conditions would simulate clinical conditions more realistically, the presence of water often spoils the FTIR data. For this reason, we maintained dry conditions throughout this study. As mentioned (in Section 2), the nanoindentation experiments were performed on the estimated oxygen inhibition region. Thus, the present study implies an internal layer between the prosthesis and the prepared tooth with no contact with water. The results of FTIR spectrophotometry and nanoindentation testing were presumably proportional to the degree of conversion and the mechanical properties of the LC samples compared with the controls, at least for samples without MDP treatments. A camphorquinone-tertiary amine photo initiator system generates primary radicals with appropriate irradiation that decomposes the C C double bond of resin monomers. Such photo-reactions increase the number of

cross-linking polymers in the resin matrix so that LC samples develop harder and stiffer mechanical properties than those of controls based on their higher polymerization rate [26]. If considerable residual monomer remains within the final products, the superior mechanical properties of resin-based luting cements might be impaired. The self-etch dental adhesive used in this study consists essentially of a phosphoric acidic functional monomer [25]. The molecular structure of this monomer comprises C C aliphatic groups that are involved in the polymerization of resin-based luting cements. Therefore, the coated monomer on a CaF2 disc could affect the apparent reduction of conversion in the MDP-pretreated samples, assuming that the ratio of the absorbance intensities of C C/C- - -C reveals the degree of conversion of the samples (see Section 2). Alternatively, C C aliphatic groups of the MDP mixture could conjugate efficiently during the polymerization of the resin-based luting cement in MDP-mixed samples, and hence the reduction of C C groups enabled an apparent enhanced degree of conversion, while the MDP-mixed samples exhibited the highest viscoelastic properties as discussed below.

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As a viscoelastic material, resin-based luting cement generated time-dependent behaviors such as large creep rate during constant load (0 Hz) or tan ı with dynamic indentation (1–4 Hz). According to a simple viscoelastic Maxwell model represented by a viscous dashpot and elastic spring connected in series [27], high viscoelasticity often demonstrate frequency-dependent stiffening [28] because the dashpot cannot instantaneously respond to high frequencies (1–4 Hz), resulting in less deformation and leading to apparently lower tan ı, while large creep deformation may occur at low frequencies such as constant loading (0 Hz), resulting in a large creep rate. Although the apparent degree of conversion ratio increased, a reduction in the cross-linking polymer network may have resulted in the highest viscoelastic creep behavior of MDP-mixed samples [24]. Thus, the extremely high frequency-dependent behavior of the MDP-mixed samples is likely related to the linearity of the polymer chains due to the conjugated MDP monomer, while cross-linked polymers are capable of effective creep deformation resistance [24]. In addition to the dynamic nanoindentation tests, the depth-dependent partial loading/unloading test enabled quasi-dynamic oscillations with relatively higher strain rates (see Section 2). Frequency-dependent stiffening might also result in overestimation of hardness and elastic modulus of the MDP-mixed samples compared with those of the control or MDP-pretreated samples, despite their presumed inferior cross-linking. Recently, self-adhesive resin-based luting cements, which contain acidic monomer such as MDP, have been intensively introduced [29]. In these cements, a “linearity of polymerization” is unavoidable event at the interface between the prosthesis and the resin substrate, represented in this study by the MDP-mixed sample. Adhesion systems should be developed with awareness of in situ molecular structures as well as mechanical properties, based on nanomechanical testing technologies such as the quasi-static or dynamic nanoindentation tests described here. The nanoindentation testing in the present study indicated that the degree of conversion observed by conventional FTIR did not entirely correlate with the nanomechanical properties of resin-based luting cements with adhesive monomers. A previous study reported that micro-Raman spectroscopy provides an in situ map analyzing the interface between the tooth and the adhesive [30], while an in situ FTIR often suffers from difficulties related to sample preparation and moisture. Thus, future investigations involving laser microRaman spectroscopy for polymerization quality assessment are promising.

5.

Conclusion

While the conventional FTIR technique reveals the apparent polymerization of resin-based luting cements, the addition of functional monomers such as 10-methacryloxydecyl dihydrogen phosphate might reduce the effectiveness of precision analysis of polymerization qualities. The quasi-static or dynamic nanoindentation tool is useful in the assessment of the effective polymerization qualities of the resin composites. A good combination of in situ nanoindentation tests and

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molecular structure analysis could be expected in the future dental composite studies.

Acknowledgements This work was supported by two Grants-in-Aid for Scientific Research (B: 24390445 and C: 24592966) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. None of the authors have any conflicts of interest to declare.

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Nanoindentation tests to assess polymerization of resin-based luting cement.

The optimal polymerization of resin-based luting cements plays a critical role in the long-term clinical success of dental prostheses and indirect res...
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