*2.3. Characterization of Dental Composites*

### 2.3.1. Fourier Transform Infrared Spectroscopy (FTIR)

Infrared spectra were obtained with a Thermo Scientific Nicolet 8700 spectrometer (Madison, WI, USA) by the attenuated total reflectance (ATR) technique using a germanium crystal. Samples were analyzed in the 4000 to 650 cm−<sup>1</sup> wavenumber range with a resolution of 4 cm−<sup>1</sup> and averaging 100 scans.

### 2.3.2. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed in a Perkin Elmer TGA-7 thermogravimetric analyzer (Norwalk, CT, USA), from 45 to 750 ◦C at a heating rate of 10 ◦C/min, under nitrogen atmosphere. From the first derivative curve, the decomposition temperature of composites was determined.

#### 2.3.3. Dynamic Mechanical Analysis (DMA)

The glass transition temperature (Tg) of composites was determined by dynamic mechanical analysis using a Perkin Elmer DMA-7 (Norwalk, CT, USA) in bending mode. Bars of 30 × 10 × 0.5 mm were heated from 35 to 200 ◦C at 3 ◦C/min, under nitrogen atmosphere, using a frequency of 1 Hz.

### 2.3.4. Mechanical Properties

An overlapping irradiation regime was applied to photo-polymerize the specimens for mechanical properties dependent on size and shape; five overlapped irradiations were employed on both sizes (40 s each irradiation; 400 s in total) to cure bending specimens, while one irradiation was used on both sizes (40 s each irradiation; 80 s in total) in compression specimens. All photo-cured specimens were immersed in distilled water at 37 ± 1 ◦C until mechanical testing (7 days).

In order to establish the influence of both nanoclay-type and filler content on the mechanical properties of dental resins, flexural tests were carried out according to the ISO 4049 standard [17] while compressive tests were performed according to the ASTM D695 standard [18]. For 3-point bending tests, five rectangular samples (25 mm length, 2 mm width and 2 mm thickness) were tested at 1 mm/min in a Shimadzu Autograph AGS-X (Kyoto, Japan) Universal Testing Machine, using a load cell of 1 kN and a distance between supports of 20 mm. For compression tests, five cylindrical specimens (8mm height and 4mm diameter) were deformed at 1 mm/min in a Shimadzu Autograph AG-I (Kyoto, Japan) Universal Testing Machine employing a load-cell of 5 kN.

Flexural strength (σ, MPa) and modulus (E, MPa) were calculated according to Equations (1) and (2), respectively.

$$
\sigma = \frac{3Fl}{2bh^2} \tag{1}
$$

$$\mathbf{E} = \frac{F l^3}{4bh^3 d} \tag{2}$$

where *F* is the maximum load recorded (N), *l* is the span between the supports (mm), *b* is the width of the specimen (mm), *h* is the height of the specimen (mm) and *d* is the deflection at load F (mm).

Compressive strength (σ, MPa) was determined according to Equation (3) and the elastic modulus (E, MPa) was calculated as the slope of the elastic part of the stress–strain curve.

$$
\sigma = \frac{4F}{\pi D^2} \tag{3}
$$

where *F* is the maximum load recorded (N) and *D* is the diameter of the specimen (mm).

Fracture surfaces from flexural test samples were analyzed by Scanning Electron Microscopy (SEM) using a JEOL JSM-6360 LV (Akishima, Tokyo, Japan) operated at 20 kV. Samples were adhered on aluminum cylinders using a double-sided tape of copper and coated with a thin layer of gold in a Denton Vacuum Desk-II (Moorestown, NJ, USA) sputter coater system for 1 min prior to examination.

#### 2.3.5. Depth of Cure

Depth of cure (Dc, mm) tests were performed on cylindrical specimens (6 mm height; 4 mm diameter) according to Section 7.10 of the ISO 4049 standard [17]. Three samples of each composite were irradiated during 40 s on one side, and the Dc was calculated according to Equation (4)

$$\mathbf{D}\mathbf{c} = \frac{l}{2} \tag{4}$$

where *l* is the height of the specimen (mm) after removing the uncured material.

#### 2.3.6. Sorption and Solubility

For water sorption and solubility tests, disc-shaped specimens (1 mm thickness; 15 mm diameter) were used according to Section 7.12 of the ISO 4049 standard [17]. Nine overlapped irradiations of 40 s were applied to specimens on one side (360 s in total). Water sorption (Wsp, µg/mm<sup>3</sup> ) and solubility (Wsl, µg/mm<sup>3</sup> ) were calculated according to Equations (5) and (6), respectively.

$$\text{Wsp} = \frac{m\_2 - m\_1}{V} \tag{5}$$

$$\text{Wsl} = \frac{m\_1 - m\_3}{V} \tag{6}$$

where *m*<sup>1</sup> is the mass (µg) of the conditioned specimen, *m*<sup>2</sup> is the mass of the specimen (µg) after immersion in distilled water at 37 ± 1 ◦C for 7 days, *m*<sup>3</sup> is the mass of the reconditioned specimen (µg) and *V* is the volume of the specimen (mm3).

#### *2.4. Statistical Analysis*

One-way analysis of variance (ANOVA) and Tukey's test (P < 0.05) were used to determine significant differences between properties of dental resin composites prepared with either MMT or PLG.

#### **3. Results and Discussion**

#### *3.1. Fourier Transform Infrared Spectroscopy (FTIR)*

FTIR spectra of neat resin and its nanocomposites prepared with either MMT or PLG are shown in Figure 1a,b, respectively. As noted, spectra of nanocomposites were very similar to those obtained for pure resin, probably due to the low filler content. Thus, a broad band was observed at 3344 cm−<sup>1</sup> , attributed to O-H stretching vibration of hydroxyls in Bis-GMA structure; bands at 2951 and 2877 cm−<sup>1</sup> related to asymmetric and symmetric stretching vibrations of the methylene group, and an intense

band at 1723 cm−<sup>1</sup> owing to C=O stretching of ester groups from dimethacrylates (Bis-GMA and TTEGDMA). Spectra also showed bands at 1640 and 1608 cm−<sup>1</sup> which correspond to the stretching vibration of the aliphatic (from vinyl group of monomers) and aromatic (from benzene ring) C=C bonds, respectively [19]; in fact, the height ratio of these bands is generally used to determine the degree of conversion of dental resins [20]. Finally, an intense band around 1130 cm−<sup>1</sup> was also detected and associated with symmetric vibration of C-O-C linkage from TTEGDMA structure.

**Figure 1.** FTIR spectra of nanocomposites prepared with either (**a**) montmorillonite (MMT) or (**b**) palygorskite (PLG).

#### *3.2. Thermogravimetric Analysis (TGA)*

*3.3. Dynamic Mechanical Analysis (DMA)* 

*3.4. Mechanical Properties* 

immobilized polymeric chains at the polymer/filler interphase [23].

Figure 2 shows the DTGA (First derivative of the TGA curve) curves for pure resin, obtained from Bis-GMA and TTEGDMA, and its composites prepared with either MMT (Figure 2a) or PLG (Figure 2b). As can be seen, pure resin presented two well-defined degradation stages (Td) at 380 and 435 ◦C, although a broad transition at lower temperatures (320 ◦C) was also observed. Teshima et al. studied the thermal degradation of resins prepared from Bis-GMA and TEGDMA and detected three degradation steps during thermal decomposition of this material. They found that methacrylic acid and 2-hydroxyethyl methacrylate are released during the first and second stages, whereas propionic acid and phenol are produced in the final stage [21]. *Polymers* **2020**, *12*, 601 6 of 13 Finally, it should be also mentioned that the broad transition observed at 320 °C was shifted to lower temperatures (290 °C) in nanocomposites, being more evident when nanoclay content was increased. This event could be related to the emission of volatiles such as water and fragments of organic modifier from PLG and MMT nanoclays respectively.

**Figure 2**. DTGA curves for nanocomposites prepared with either (**a**) MMT or (**b**) PLG. depending on the type and concentration of the inorganic aggregate. **Figure 2.** DTGA curves for nanocomposites prepared with either (**a**) MMT or (**b**) PLG.

a one broad peak at around 110 °C, which suggests a homogeneous polymeric network; this value is in agreement with that reported by Terrin et al. for dental resins composed mainly of Bis-GMA and TEGDMA [9]. Composites containing PLG showed slightly higher values than those obtained for the corresponding MMT composites. It is also noted that dental resins prepared with PLG exhibited an increase in this parameter as nanoclay content was increased. In contrast, when MMT was added to dental resin formulations, values decreased slightly from 110 to 108 °C returning to higher Tg values (112 °C) at nanoclay contents of 8 and 10 wt.%; a similar trend was reported by Terrin et al., who studied resin-based composites with organically modified MMT [9]. The shift of the relaxation temperature in composites containing nanoclays towards higher temperatures is attributed to

**Table 2.** Glass transition temperatures (Tg) of dental composites.

0 110 2 108 111 4 110 112 6 110 116 8 112 116 10 112 116

Figures 3 and 4 show the flexural and compressive properties of the nanocomposites prepared in this study, respectively. As can be seen, the mechanical properties of dental resin composites varied

**MMT PLG** 

**Nanoclay Content (%) Tg (°C)** 

On the other hand, it has been suggested that addition of nanoclays into polymeric matrices could improve the thermal stability of the resulting material [22]. This fact was not observed in nanocomposites prepared in this study; i.e., temperatures observed for dental resin composites were practically similar to those obtained for neat resin. The latter is in agreement with Munhoz et al., who pointed out that it was not possible to identify an impact of the presence of the modified clay on the thermal stability of the composites [23]. However, a close inspection of thermograms allowed detection of small changes in thermal degradation behavior in some composites. For instance, the degradation stage at 380 ◦C in some MMT nanocomposites was shifted to higher temperatures overlapping with that of 435 ◦C in composites containing 10 wt.%. This could be associated with interaction between surfactant and crosslinking polymer structures. Mahnoodian et al. studied the thermal degradation of Bis-GMA/TEGDMA/Cloisite 30B nanocomposites and found that the pure resin exhibited two decomposition stages at 292 and 392 ◦C. The first temperature did not change when MMT was added to resins, although temperature corresponding to the second decomposition step was reduced with increasing clay content [7].

Finally, it should be also mentioned that the broad transition observed at 320 ◦C was shifted to lower temperatures (290 ◦C) in nanocomposites, being more evident when nanoclay content was increased. This event could be related to the emission of volatiles such as water and fragments of organic modifier from PLG and MMT nanoclays respectively.
