*3.3. Dynamic Mechanical Analysis (DMA)*

Glass transition temperatures of dental composites were obtained from the maximum of the tan d versus temperature curves and their maxima are summarized in Table 2. Thermograms exhibited 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 immobilized polymeric chains at the polymer/filler interphase [23].


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

#### *3.4. Mechanical Properties*

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 depending on the type and concentration of the inorganic aggregate.

Regarding flexural properties (Figure 3), it was observed that strength decreased monotonically with the MMT loading; on the other hand, samples containing PLG exhibited a drastic decrease of strength when the additive was added (i.e., 2 wt.%) and remained almost constant at higher

filler concentration. It is worth mentioning that the minimum flexural strength set by the ISO 4049 standard [17] (50 MPa) was fulfilled by all composites containing MMT and that prepared with PLG at 2 wt.%. Typical stress–strain curves for nanocomposites prepared with either MMT or PLG are presented in Figure 3c,d, respectively. concentration. It is worth mentioning that the minimum flexural strength set by the ISO 4049 standard [17] (50 MPa) was fulfilled by all composites containing MMT and that prepared with PLG at 2 wt.%. Typical stress–strain curves for nanocomposites prepared with either MMT or PLG are presented in Figure 3c,d, respectively.

*Polymers* **2020**, *12*, 601 7 of 13

strength when the additive was added (i.e., 2 wt.%) and remained almost constant at higher filler

Regarding flexural properties (Figure 3), it was observed that strength decreased monotonically

The flexural modulus tended to increase with filler concentration regardless of type of nanoclay employed; MMT composites seemed to exhibit slight increments, but the differences were not statistically significant). PGL composites showed higher modulus with PLG up to 4 wt.% clay content, and no statistical differences were found for composites with higher clay content. Several authors [4,24,25] have also reported that flexural strength of nanocomposites decreased and modulus increased [8,10] as filler content increased. The flexural modulus tended to increase with filler concentration regardless of type of nanoclay employed; MMT composites seemed to exhibit slight increments, but the differences were not statistically significant). PGL composites showed higher modulus with PLG up to 4 wt.% clay content, and no statistical differences were found for composites with higher clay content. Several authors [4,24,25] have also reported that flexural strength of nanocomposites decreased and modulus increased [8,10] as filler content increased.

It is well known that the dispersion of nanoparticles and compatibility between phases (filler and matrix) play a key role in the enhancement of mechanical properties of nanocomposites. Thus, it is probable that MMT presented better results than PLG as it was organically modified to improve its interaction with organic polymers and increase the interlayer spacing favoring its dispersion, as observed in the SEM analysis (see Figure 5). It is well known that the dispersion of nanoparticles and compatibility between phases (filler and matrix) play a key role in the enhancement of mechanical properties of nanocomposites. Thus, it is probable that MMT presented better results than PLG as it was organically modified to improve its interaction with organic polymers and increase the interlayer spacing favoring its dispersion, as observed in the SEM analysis (see Figure 5).

**Figure 3.** Flexural properties of dental resin composites: (**a**) Strength; (**b**) modulus; (**c**) stress–strain curves for nanocomposites prepared with MMT and (**d**) stress–strain curves for nanocomposites prepared with PLG.

Compressive properties (Figure 4) were not greatly affected with addition of nanoclay content, except compressive strength of materials containing PLG; these properties decreased with nanoclay concentration as reported by Mucci et al. [8]. Interestingly, the strength of composites containing MMT also showed higher values than corresponding PLG materials. Typical stress–strain curves for nanocomposites prepared with MMT or PLG are presented in Figure 4c,d, respectively. Compressive properties (Figure 4) were not greatly affected with addition of nanoclay content, except compressive strength of materials containing PLG; these properties decreased with nanoclay concentration as reported by Mucci et al. [8]. Interestingly, the strength of composites containing MMT also showed higher values than corresponding PLG materials. Typical stress–strain curves for nanocomposites prepared with MMT or PLG are presented in Figure 4c,d, respectively.

*Polymers* **2020**, *12*, 601 8 of 13

**Figure 3.** Flexural properties of dental resin composites: (**a**) Strength; (**b**) modulus; (**c**) stress–strain curves for nanocomposites prepared with MMT and (**d**) stress–strain curves for nanocomposites

**Figure 4.** Compressive properties of dental resin composites: (**a**) Strength; (**b**) modulus; (**c**) stress– strain curves for nanocomposites prepared with MMT and (**d**) stress–strain curves for nanocomposites prepared with PLG. **Figure 4.** Compressive properties of dental resin composites: (**a**) Strength; (**b**) modulus; (**c**) stress–strain curves for nanocomposites prepared with MMT and (**d**) stress–strain curves for nanocomposites prepared with PLG.

Figure 5 shows the nanoclay dispersion within the dental composites. Unloaded dental resin exhibited a clear brittle fracture producing a smooth surface; the presence of nanoclays produced a different fracture mechanism generating a rough surface upon breaking. SEM observation at higher magnification revealed that nanoclays were dispersed differently within dental resin; MMT seems to be better distributed in the resin due to its surface modification than PLG, which was distributed as agglomerates. Figure 5 shows the nanoclay dispersion within the dental composites. Unloaded dental resin exhibited a clear brittle fracture producing a smooth surface; the presence of nanoclays produced a different fracture mechanism generating a rough surface upon breaking. SEM observation at higher magnification revealed that nanoclays were dispersed differently within dental resin; MMT seems to be better distributed in the resin due to its surface modification than PLG, which was distributed as agglomerates.

Unfilled MMT 10 wt% PLG 10 wt%
