# In-situ sol-gel process for the formulation of Bis-GMA/TEGDMA/silica dental nanocomposite

Rayenne Latoui, Djallel Bouzid, Pierre-Yves Dugas, Edgar Espinosa, Olivier Boyron

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Rayenne Latoui, Djallel Bouzid, Pierre-Yves Dugas, Edgar Espinosa, Olivier Boyron. In-situ sol-gel process for the formulation of Bis-GMA/TEGDMA/silica dental nanocomposite. Polymer, 2024, 313, pp.127731. 10.1016/j.polymer.2024.127731. hal-05448843

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# In-situ sol-gel process for the formulation of Bis-GMA/TEGDMA/silica dental nanocomposite

Rayenne Latoui<sup>a</sup>, Djallel Bouzid<sup>a,**</sup>, Pierre-Yves Dugas<sup>b</sup>, Edgar Espinosa<sup>b</sup>, Olivier Boyron<sup>b,*</sup>

## ARTICLE INFO

**Keywords:**

Dental composite

In-situ sol-gel process

Bis-GMA/TEGDMA resin

Silica nanoparticles

Photo-polymerization

Monomer functionalization

## ABSTRACT

A novel dental composite based on Bis-GMA/TEGDMA resin incorporating silica nanoparticles was developed using an innovative in situ sol-gel process. This method ensures a homogeneous dispersion of fillers, effectively preventing their agglomeration during preparation. It results in an interpenetrating network in which polymer chains and silica particles are intimately mixed, thereby enhancing the structural and mechanical properties and minimizing the volumetric shrinkage of the dental restorative material.

The formation of organic and inorganic network was confirmed by infrared spectroscopy. The in situ synthesis enabled uniform dispersion of silica nanoparticles within the material as evidenced by transmission electron microscopy, scanning electron microscopy and energy dispersive spectroscopy.

The results showed that the prepared composite containing 50 wt% fillers achieved hardness, modulus of elasticity, compressive strength and flexural strength values of 72 HV, 10 GPa, 240 MPa, and 110 MPa, respectively. These properties are comparable to those of the commercial “Nt Premium” composite containing 75 % preformed silica nanoparticles, which has hardness, modulus of elasticity, compressive strength and flexural strength values of 68 HV, 10 GPa, 220 MPa and 95 MPa, respectively. Additionally, the study revealed a significantly reduced volumetric shrinkage of 0.5 % for our composite, compared with over 2.2 % for Nt Premium. This groundbreaking approach holds substantial promise for developing advanced dental materials, offering superior performance and enhanced longevity in clinical applications.

## 1. Introduction

Development of hybrid composite materials using a variety of fillers has grown considerably over the last 30 years [1–3]. Methacrylic resin and silica-based hybrid materials can be employed as biocompatible dental composites with greater mechanical strength, thermal stability and lower volumetric shrinkage. BisGMA is considered as the basic monomer of the resin matrix, coupled with another methacrylic monomers as diluents, in order to facilitate the introduction of mineral fillers and the handling of formulations. Among the most common diluents is TEGDMA [7], it has low viscosity and high reactivity.

In recent years, silica (by various means and methods) has gained a reputation as one of the most suitable inorganic fillers for methacrylic resin and the most common choice in dental composites due to its biocompatibility [1,2,4], its availability and ease of synthesis compared to other fillers such as zirconium, etc., and its low cost, which reduces

the overall manufacturing costs of dental composites [5].

Interfacial interactions between the organic and inorganic compounds have a significant influence on the properties of resin-silica hybrids [6–8]. The better the interphase interaction, the better the compatibility and, consequently, properties of the hybrid material were superior [7]. Interphase bonding between resin and silica fillers can be achieved either by functionalizing the monomers composing the organic resin, or by modifying the silica surface with functional silanes [8].

Surface modification of silica nanoparticles used as fillers for dental composites can be done using, post-modification (grafting) - subsequent grafting of nanosilica using an organosilane as modifier or [9,10], in situ modification (co-condensation method)- simultaneous modification of silica via the preparation of colloidal nanosilica using alkoxysilane as sol-gel precursor, and functional silanes as modifier [11–13]. Several reports on the adding of coupling agent for the functionalization of monomers have been published [14–19], E.Müh et al. prepared various

<footnote><sup>a</sup> Laboratoire de Génie des Procédés pour le Développement Durable et les Produits de Santé, Ecole Nationale Polytechnique de Constantine, Algeria</footnote>

<footnote><sup>b</sup> Laboratoire de Catalyse, Polymérisation, Procédés et Matériaux, Université Claude Bernard Lyon 1, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail address: olivier.boyron@univ-lyon1.fr (O. Boyron).</footnote>
alkoxysilyl units by Michael addition of ethylene glycol acrylate methacrylate (EGAMA) and ethylene glycol bisacrylate (EGBA) to various R, ω-alkoxysilylamines [16]. To these hybrid monomer mixtures, they added Bis-GMA and silica particles in order to obtain composites with 70 wt% filler. In a parallel work, N. Moszner et al. synthesised new methacryloyloxyalkylaminoalkylalk-oxysilanes by Michael addition of the corresponding acryloyloxyalkyl methacrylates with (3-aminopropyl) triethoxysilane (APTES) [17]. They concluded that the main advantage of monomer functionalization resulting in high-molecular-weight hybrid monomers is, in addition to compatibility, the low volumetric shrinkage of the composites.

The commercialized composites are classified according the fillers type [20], that come in a wide variety of sizes, compositions, shapes and assembly combinations. The dispersion of these fillers in the monomers is a crucial step in the composite preparation process [5,21]. The use of standard ex situ methods for the preparation of dental composites [5], can lead to the formation of agglomerates that are difficult to break up during mixing due to the viscosity of the monomers [22,23]. The resulting inhomogeneity in nanocomposites can deteriorate material properties. An innovative approach to obtaining a homogeneous dispersion is the in situ synthesis of inorganic nanoparticles in the polymer matrix, using techniques such as sol-gel chemistry, inverse microemulsion or hydrothermal/solvothermal synthesis [8,9,15, 24–26].

Relevant literature reports the synthesis of polymer-silica hybrid materials by the in situ sol-gel process [7,15,27–31]. T.Nazir et al. prepared hybrid epoxy-silica polymer films by hydrolysis and condensation of various amounts of tetraethoxysilane (TEOS) within epoxy network matrix [7], and Matejka et al. prepared epoxy-silica hybrids using the one-step process, i.e. all reaction components were mixed and reacted to obtain a finely dispersed SiO₂ network [29,30]. In these studies, it has been shown that in situ generation of inorganic oxide nanoparticles can result in improvements in the mechanical, thermal, as well as the optical properties for epoxy based nanocomposites.

The technological relevance of these hybrid materials and the enhanced properties obtained using this technique prompted us to develop an original and innovative route for the preparation of a hybrid dental composite based on Bis-GMA/TEGDMA methacrylic resin and silica, using the in situ sol-gel process, which enables reinforcement nanoparticles to be synthesised and incorporated directly into the resin without the need to add preformed silica particles as fillers, thus ensuring uniform dispersion and accurate control of the material's properties.

Consequently, this research aims to assess the quality of filler dispersion obtained by the in situ sol-gel method, analyze the impact of this method on the mechanical properties of dental composites, such as hardness, modulus of elasticity, and compressive strength and compare the performance of composites made by in situ sol-gel with that of conventional composites.

## 2. Experimental section

### 2.1. Materials

Tetraethyl-orthosilicate (TEOS, 98 %), Ethanol absolute (>99.8 %), (3-aminopropyl) triéthoxy-silane (APTES, 99 %), Bisphenol A glycerolate dimethacrylate (Bis-GMA), Triethylene glycol dimethacrylate (TEGDMA, containing 80–120 ppm MEHQ inhibitor, 95 %), 2-(Dimethylamino) ethyl methacrylate (DMAEMA, containing 700–1000 ppm monomethyl ether hydroquinone inhibitor, 98 %) were purchased from Sigma Aldrich (Germany) and comphorquinone (CQ, 99 %) came from Alfa Aesar (Germany). All these products were used without any further purification.

Water was purified using a Millipore water purification system (Milli-Q IQ 7005).

A reference commercial dental composite 'NT Premium Enamel'

from coltene brasil containing 75 % inorganic charges, monomers (Bis-GMA, Bis-EMA, TEGDMA), BHT inhibitor, photo initiators, fillers, and dyes was selected for comparative study. It was chosen as a reference composite due to its almost similar resin base and silica nanoparticle fillers, enabling direct comparison with experimental composites. The use of a conventional ex-situ incorporation method provides a benchmark for assessing the benefits of the new in-situ sol-gel method.

### 2.2. In-situ formulation of Bis-GMA/TEGDMA/silica composite

In this study, Bis-GMA/TEGDMA-silica composites containing various content of silica were prepared by hydrolysis and condensation of TEOS within the resin matrix composed of two monomer Bis-GMA/TEGDMA followed by the photopolymerization of the resin. APTES was used to end-link the polymer chains of organic matrix with the silica network as shown in Fig. 1.

#### 2.2.1. Functionalization of TEGDMA monomer

The synthesis of the hybrid monomer TEGDMA-APTES was achieved via Michael addition (1, 4) reaction as shown in Fig. 2. TEGDMA and APTES were mixed with molar ratio (r) of APTES and TEGDMA of 1:1. Mixing was performed with a magnetic stirring at room temperature for 48 h. TEGDMA functionalized with APTES was then blended with Bis-GMA and mixed at 50 °C via magnetic stirring for 15 min.

#### 2.2.2. Hydrolysis and condensation

Depending on the final SiO₂ content (30–50 wt %), the measured quantities of TEOS specified in Table 1 were added to the quantities of Bis-GMA, TEGDMA-APTES, and the reaction mixtures were sonicated for 15 min. This step was performed at room temperature in ethanol as solvent. Silica nanoparticles were generated in situ by a sol-gel process, carrying out hydrolysis of TEOS with pure water and using ammonia as a catalyst to accelerate the process. TEOS and water were mixed in 1:10 M ratios (taking into account the amount of water present in the ammonia solution). The mixture was sonicated for 24 h at 50 °C after addition of water and ammonia, until the evaporation of solvent and the formation of a viscous gel.

Interactions between the monomer chains of the organic matrix and the silica network were produced by the condensation of silanol (SiOH) from the hydrolysis of TEGDMA-bound APTES with that from the hydrolysis of TEOS in the resin matrix, as shown in Fig. 3.

#### 2.2.3. Photochemical polymerization

Bis-GMA/TEGDMA-APTES/Silica, were mixed with 1 wt% of comphorquinone (initiator) and 1 wt% of DMAEMA (co-initiator) and were magnetically and manually stirred until homogenization.

The pastes formed were poured into a polyamide mould, 5 mm in diameter and 2 mm thick, then were irradiated top and bottom at room temperature with a LED lamp for 40 s using an “Eighteeth curing pen” LED dental lamp with an intensity of 1500 mW cm⁻² at a distance of 5 mm [32]. The generated free radicals initiate a polymerization reaction in a similar way as described above for Bis-GMA/TEGDMA resin preparation (SI).

During curing process methacrylate groups in Bis-GMA monomer copolymerize with the methacrylate groups in TEGDMA monomer to form highly cross-linked network structures.

Table 1 shows the amounts of reagents to prepare silica, 1g of Bis-GMA/TEGDMA composite resin as well as of different types of Bis-GMA/TEGDMA/silica composites incorporating 30, 40, 50 wt% silica.

Experimental composites were classified into three groups according to their silica mass fractions 30 %, 40 %, and 50 %. These groups are designated as Cp30, Cp40 and Cp50, respectively. Exceeding 50 % silica content is particularly challenging due to the high viscosity and particle agglomeration. One potential solution is to use two populations of silica particles with different sizes, but producing them through the in-situ sol-gel method remains challenging.
<figure><img src="image_3.png" /><figcaption>Fig. 1. Sequential procedure for the in-situ formulation of composite.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Synthesis of functionalized monomer by Michael addition reaction.</figcaption></figure>

<table><caption>**Table 1**
Compositions of silica, composite resin (1g) and composites (1g).</caption><thead><tr><th>Sample</th><th>SiO<sub>2</sub><sup>a)</sup> (Wt. %)</th><th>TEOS (g)</th><th>APTES (g)</th><th>Bis-GMA (g)</th><th>TEGDMA (g)</th><th>CQ (g)</th><th>DMAEMA (g)</th><th>SiO<sub>2</sub><sup>b)</sup> (g)</th></tr></thead><tbody><tr><td>Silica</td><td>100</td><td>2.1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0.88</td></tr><tr><td>Resin</td><td>0</td><td>0</td><td>0</td><td>0.75</td><td>0.25</td><td>0.008</td><td>0.010</td><td>0</td></tr><tr><td>Cp30</td><td>30</td><td>1.2</td><td>0.13</td><td>0.52</td><td>0.17</td><td>0.006</td><td>0.007</td><td>0.28</td></tr><tr><td>Cp40</td><td>40</td><td>1.6</td><td>0.11</td><td>0.45</td><td>0.15</td><td>0.005</td><td>0.006</td><td>0.37</td></tr><tr><td>Cp50</td><td>50</td><td>2.0</td><td>0.10</td><td>0.37</td><td>0.12</td><td>0.004</td><td>0.005</td><td>0.46</td></tr></tbody></table>

## 2.3. Results and discussion

### 2.3.1. Structural analysis

The organic and inorganic network formation in resin-silica composites has been successfully observed by FTIR spectroscopy. As reported in the literature [8,33–36], the absorption peak of asymmetric Si–O–Si stretching varies between 1010 and 1130 cm⁻¹. The FTIR spectrum in Fig. 4-a shows the presence of a distinct absorption peak at 1033 cm⁻¹. This characteristic absorption band of Si–O–Si asymmetric stretching reveals the formation of a silica network by the sol-gel reaction and it becomes stronger with the addition of the coupling agent. Fig. 4-b shows the decrease as a function of photopolymerization time of the characteristic absorption peak of the methacrylate double bond at 1636 cm⁻¹, which also confirms the formation of the polymeric network resulting from the photopolymerization reaction of the Bis-GMA/TEGDMA monomers.

Fig. 4-b depicts the FTIR peak of the composite Cp50, between 1650 and 1590 cm⁻¹ before and after polymerization. The intensity of the vinyl group peak at 1636 cm⁻¹, which represents the residual mole fraction of aliphatic C=C bonds [36,37], decreases progressively with increasing polymerization time [37,38].

Whereas the FTIR results demonstrate the formation of Si–O–Si by a sol-gel reaction, solid-state ²⁹Si NMR provides additional information on the structure of the silica and the degree of the Si–OH condensation reaction. Fig. 5-a and Fig. 5-b compares, respectively, the spectrum of silica with that of the composite containing 50 % of silica (Cp50).

In the silica spectrum (Fig. 5-a), two distinct peaks can be observed at –110 ppm and –101 ppm with a shoulder at –92 ppm, which were attributed to the presence of the species Q₄ [Si (OSi)₄], Q₃ [Si (OSi)₃(OH)] and Q₂ [Si (OSi)₃(OH)₂], respectively originating from TEOS hydrolysis [8,38,39]. The spectrum of the composite (Fig. 5-b) shows two features: (a) a peak at –66 ppm with a shoulder at –59 ppm,

<footnote>a) SiO₂ mass (Wt.%) calculated based on 100 % conversion of TEOS to SiO₂.</footnote>

<footnote>b) SiO₂ mass (g) calculated on the basis of the final silica content (wt.%) determined using TGA analysis (Table 3).</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Steps of in-situ sol-gel possibly reactions occurring in the procedure for the preparation of Bis-GMA/TEGDMA-SiO₂ composites.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. a) FTIR spectra of silica and composite with 50 wt % SiO₂. Inner small image shows the Si–O–Si absorption peaks of composites with different silica contents, b) FTIR spectra of composites after 40 s of polymerization. Inner small image shows the resin absorption peaks of composite at different polymerization time.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. $^{29}$Si MAS NMR spectra of a) silica and b) composite (Cp50).</figcaption></figure>

which were attributed to the T₃ and T₂ silica atoms of APTES [40,41], and (b) two peaks at –110 ppm and –101 ppm with a shoulder at –92 ppm, which were attributed to the Q₄, Q₃ and Q₂ silicon atoms of TEOS, respectively. The presence of silicon atoms Q₃ and T₂ indicates the presence of pendant Si–OH groups, meaning that the silica in these samples is not fully condensed, this observation is widespread in the literature [8,39,42]. The peak linked to species Q₃ decreased in intensity for the composite, indicating an increase in the proportion of condensed species. This result is possibly attributed to the presence of APTES, whose amino group can act as an internal catalyst during in situ synthesis and promotes faster hydrolysis of TEOS and, consequently, greater condensation.

### 2.3.2. Conversion degree

The conversion degree (CD) in the polymer matrix was determined by FTIR spectroscopy.

Fig. 6-a and Fig. 6-b depicts the evolution of the conversion degree depending of polymerization time, and the change in CD with respect to the content of silica in the composites respectively.

The resins have a CD of 67 ± 2.5 %, which is in accordance with literature data [43]. Polymerization of dimethacrylates is never complete. A significant fraction of the double bonds does not react due to

their immobilization, caused by a complex set of features, such as self-acceleration (Tromsdorff gel effect), self-deceleration, steric isolation and vitrification [44–47]. This has been demonstrated in several studies on the photopolymerization of Bis-GMA/TEGDMA systems [42, 48].

The resin without silica had the highest CD. However, the addition of 30, 40 and 50 % silica by weight reduced the CD to 63 ± 1.9 %, 61 ± 2.9 % and 59 ± 3.1 % respectively. The decrease in CD with increasing silica content is probably caused by the increase of system viscosity [49,50].

However, due to the presence of fillers (SiO₂), as well as the hardening phenomenon which restricts the freedom of radicals, the reaction did not reach a high CD. In dental applications, however, a CD of over 55 % is clinically acceptable [49].

### 2.3.3. Morphological characterization

#### 2.3.3.1. TEM analysis.
Transmission electron microscopy (TEM) was used to directly observe the shape, size, and distribution of silica nanoparticles in liquid dispersions, immediately after their formation by the condensation reaction. This technique also enabled the visualization of interactions between the resin and the newly formed nanoparticles.

<figure><img src="image_8.png" /><figcaption>Fig. 6. (a) Evolution of the conversion degree (CD) depending of photopolymerization time, (b) Evolution of the CD depending of silica content.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** TEM image of silica nanoparticles (a), Particle size distribution of silica nanoparticles (b), TEM images of Cp50 sample (c,d). Particle size distribution of composite containing 50 wt% silica (e).</figcaption></figure>

Fig. 7 shows particles size distribution and TEM images of silica nanoparticles synthesised alone and within the composite (Cp50). The results show a narrow particle size distribution. The morphology consists of almost spherical nanosize particle and monodisperse with low aggregation.

The presence of monomers and APTES has an effect on the silica particle size generated. Fig. 7-b shows that the average particle size of

silica is 87 nm. However, the diameter of particles generated in the composite increase to 204 nm (Fig. 7-e).

Fig. 7-a and Fig. 7-d show the silica particles generated without and with the presence of resin (monomers, and coupling agent and photo-initiator system). Silica particles presents a homogeneous and spherical morphology, and they are coated and interconnected by the resin which is correlated to the increase of particles diameter.

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** SEM-EDS analysis of the composite: (a) EDS spectrum; (b) SEM image of Cp50; (c-e) overlay elemental mapping images of Bis-GMA/TEGDMA/SiO₂ composite: C (red), Si (blue) and O (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
### 2.3.3.2. SEM-EDS analysis.
Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS) was used to investigate the composition and structure of solid composites after photopolymerization.

The EDS spectra are shown in Fig. 8-a, and the surface morphology of the corresponding sample is revealed by the SEM image in Fig. 8-b. As expected, C, Si and O were uniformly observed in the composite, as shown in Fig. 8-c, Fig. 8-d and Fig. 8-e, indicating a uniform distribution of the elements throughout the composites, demonstrating that the in situ route can be used to avoid agglomeration and ensure homogeneous dispersion of the silica nanoparticles in the composite.

To assess the homogeneity of silica nanoparticle distribution within the matrix, multiple EDS analyses were conducted at seven points on SEM image (Fig. 8-b). The results revealed a remarkably uniform distribution of silica nanoparticles, with nearly identical silica percentages detected in all analyzed regions (Figure SI). This finding suggests that the manufacturing process dispersed the nanoparticles efficiently, resulting in a highly homogeneous composite material.

### 2.3.3.3. STEM-EDS (Scanning Transmission Electron Microscopy coupled with EDS mapping).
To gain further insights into the nanoscale structure of the composite, STEM-EDS analysis was conducted on a 25 nm thin section. EDS mapping revealed a homogeneous dispersion of silica nanoparticles in the matrix, confirming the efficiency of the manufacturing process. In addition, the analysis revealed the morphology of the nanoparticles, demonstrating well-spherical, nano-sized particles as shown in Fig. 9.

### 2.3.4. Thermal analysis

To investigate the thermal stability of the composite, TGA (SI) analyses were carried out. Thermograms of silica, Bis-GMA/TEGDMA resin and composites (Cp) made with different amounts of silica are shown in Fig. 10. The TGA curves and their derivatives show a maximum degradation after 400 °C and that the samples are thermally stable up to 350 °C.

Differential scanning calorimetry (DSC) analysis was carried out to

ascertain the formation of a highly cross-linked organic network.

Fig. 11 displays the DSC thermograms of the resin and the composites before and after polymerization. The thermal events observed in the DSC curves within the studied temperature range is attributed to the Tg, consistent with findings reported in the literature [51].

The DSC curves of the composites (Fig. 11) show that the Tg increases with polymerization time within the examined range (0–40 s), rising from –22.7 °C prior to polymerization to 195 °C for the Cp50 sample. This increase is correlated with a high cross-link density within the polymer network. Such an enhancement suggests a greater number of cross-links between polymer chains, which reinforces the material’s structure and contributes to the elevated Tg (see Table 2).

The formation of the highly cross-linked organic network is attributed to the monomer functionality of 2, resulting from the presence of two methacrylate functions within the same molecules of Bis-GMA and TEGDMA. Additionally, the in-situ process employed for generating silica nanoparticles within the resin facilitates the creation of an interpenetrating network.

Bernardi et al. published a report on the thermal and structural properties of three different dental composite resins (Filtek TM Supreme XT, Filtek TM Z-250 and TPH3) cured with blue LEDs for 40 s [52]. The results indicated that Filtek TM Z-250 exhibited the most homogeneous morphology, superior thermal stability, and enhanced cross-linking, along with a Tg that was 58 °C higher than that of the other resins [52]. The elevated Tg observed for the composite resin studied here, as well as in those investigated by Bernardi et al., can be attributed to the well-interconnected and more uniform morphology achieved through the in situ preparation method.

### 2.3.5. Mechanical properties

The mechanical tests of the fabricated composites were conducted. Fig. 12 shows the hardness, modulus of elasticity, compressive strength and flexural strength of resin, composites with different content and Nt premium commercial composite for comparison.

As expected, filler content influences the mechanical response. Results showed an enhancement of the hardness H, the modulus of

<figure><img src="image_11.png" /><figcaption>Fig. 9. STEM-EDS analysis of a 25 nm section of the composite Cp50: (a–c) STEM images of the composite Cp50 section at different scales: (a) 1 µm, (b) 250 nm, (c) 100 nm; (e–m) overlay of elemental mapping images of the composite section: C (white), Si (green) and O (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 10. TG (a) and dTG (b) curves of samples in air and N2 atmosphere.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 11. DSC scan curves of resin (black), composites: Cp30 (red), Cp40 (blue), Cp50 (green), before (dash lines) and after polymerization (solid lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

elasticity E, the compressive strength CS and the flexural strength FS when increasing the silica content.

Zha et al. investigated the mechanical properties of dental resin composites using nanoindentation tests and found that incorporation of silica nanoparticles increases hardness and modulus of elasticity of dental resin composites [53].

To assess the effectiveness of the in situ sol gel process on the mechanical properties of the composite, the measured values were compared with those of a conventional composite containing 75 wt% silica. The results indicate that the composite containing only 50 wt% silica, produced in situ, exhibits slightly better mechanical properties than the selected conventional commercial composite with 75 wt% filler. It achieved a hardness of 72 ± 9 HV, a modulus of elasticity of 10 ± 1 GPa, a compressive strength of 236 ± 13 MPa, and a flexural strength of 110 ± 16 MPa. The mechanical properties of many conventional composites have been reported in the literature [54,55]. A study comparing the Vickers hardness of five dental composites containing fillers between 60 and 70 vol% reported by Anupama et al. showed hardness values between 49 and 84 HV [54].

In a similar study, Sonwane et al. examined five dental composites with high silica content (78–83 wt%) [55]. They reported compressive strengths ranging from 54 ± 8 to 176 ± 50 MPa, and flexural strengths from 88 ± 28 to 171 ± 53 MPa. Our results, despite using lower silica contents with the sol-gel in situ method, are comparable to those reported in the literature for conventional processes. This occurs because physical mixtures of organic polymers and preformed inorganic particles tend to separate into distinct phases, especially when silica particles aggregate, leading to uneven stress distribution within the material and limiting its mechanical properties. In contrast, stronger interactions and improved miscibility between the two phases enhance mechanical performance.

### 2.3.6. Volumetric shrinkage

Minimizing contraction in dental restorations is essential to prevent gaps, voids, and stress build-up, which can compromise the longevity and functionality of the material [53]. The results of volumetric shrinkage of composites are summarized in Table 3.

The results showed that increasing the amount of silica in the Bis-

<table><caption>**Table 2**
Thermogravimetry (TG) and calorimetry (DSC) data.</caption><thead><tr><th>Sample</th><th>T<sub>dmax</sub> (°C)</th><th>Residue at 600 °C</th><th>Residue at 850 °C</th><th>T<sub>g</sub> before polymerization</th><th>T<sub>g</sub> after polymerization</th><th>Yield<sup>a)</sup> (%)</th></tr></thead><tbody><tr><td>Silica</td><td>—</td><td>—</td><td>88</td><td>—</td><td>—</td><td>88</td></tr><tr><td>Resin</td><td>312</td><td>—</td><td>—</td><td>-27</td><td>194</td><td>—</td></tr><tr><td>Cp30</td><td>350</td><td>35</td><td>28</td><td>-21</td><td>186</td><td>93</td></tr><tr><td>Cp40</td><td>355</td><td>45</td><td>37</td><td>-21</td><td>194</td><td>92</td></tr><tr><td>Cp50</td><td>360</td><td>53</td><td>46</td><td>-22</td><td>195</td><td>92</td></tr></tbody></table>

<footnote>a) Reaction yield: theoretical silica content calculated based on 100 % conversion of TEOS to SiO₂ divided by final silica content determined by TGA analysis (residue at 850 °C).</footnote>
<figure><img src="image_14.png" /><figcaption>Fig. 12. (a) Hardness, (b) elastic modulus, (c) compressive strength, and (d) flexural strength of resin, fabricated composites and Nt premium commercial composite (CC).</figcaption></figure>

<table><caption>**Table 3**

Volumetric shrinkage (%) of composites.</caption><thead><tr><th>Composite</th><th>Cp30</th><th>Cp40</th><th>Cp50</th><th>CC<sup>a)</sup></th></tr></thead><tbody><tr><td>Shrinkage (%)</td><td>1.2</td><td>0.73</td><td>0.46</td><td>2.23</td></tr></tbody></table>

GMA/TEGDMA matrix reduced polymerization shrinkage. A low volumetric shrinkage was observed for the composites produced in situ with 50 wt% of silica filler, i.e., 0.5 % compared to a shrinkage of 2.3 % for the commercial composites containing 75 wt% fillers.

This result can be attributed to the homogeneous distribution of fillers generated in situ within the resin matrix. When the fillers are uniformly dispersed, the polymer chains are intimately mixed with the silica nanoparticles, they establish a better interconnected network. This enhanced network increases resistance to compression and occupies space within the material, thereby reducing the overall shrinkage of the composite. This effect has been corroborated by STEM analysis, which shows that interparticle and intraparticle voids are effectively filled with resin (Fig. 9).

Furthermore, the reduction in volumetric shrinkage may also result from the formation of a high-molar-mass monomer within the system, arising from the functionalization reaction of TEGDMA with APTES. This reaction increases the density of the material, thereby minimizing the available free volume for polymerization-induced shrinkage. Previous studies have demonstrated that dental composites incorporating such high-molecular-weight monomers exhibit significantly lower shrinkage (0.6 %) compared to those containing lower molecular weight monomers [56].

### 3. Conclusion

A promising biocompatible dental composite based on Bis-GMA/TEGDMA resin and silica filler was successfully prepared in situ using the sol-gel process. Comprehensive characterization using various analytical techniques demonstrated that the resultant materials exhibit a favorable degree of conversion, excellent mechanical properties, and low volumetric shrinkage. These properties are comparable to, or even exceed, those of commercial composites with higher silica content.

The in situ generation of silica nanoparticles within the resin matrix significantly enhanced the performance of the hybrid composite. This approach facilitated homogeneous and uniform dispersion of silica throughout the material. The improved mixing and robust interactions between the resin and silica phases resulted in superior mechanical properties and reduced volumetric shrinkage, attributable to the formation of a highly interconnected network that effectively resists compression and minimizes void formation.

This study conclusively demonstrates that the sol-gel in situ technique is a viable method for the preparation of dental composites. It provides superior dispersion of silica nanoparticles within the Bis-GMA/TEGDMA system, leading to enhanced material properties. This advancement holds significant potential for the development of high-performance dental composites with improved clinical outcomes.

### CRediT authorship contribution statement

**Rayenne Latoui:** Writing – original draft, Methodology, Investigation, Formal analysis. **Djallel Bouzid:** Writing – review & editing, Supervision, Conceptualization. **Pierre-Yves Dugas:** Formal analysis. **Edgar Espinosa:** Formal analysis, Data curation. **Olivier Boyron:** Writing – review & editing, Supervision, Formal analysis.

<footnote>a) Nt premium commercial composite.</footnote>
## Disclosure

The authors declare that no generative AI and AI-assisted technologies were used in the writing process, and we take full responsibility for the content of publication.

## Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors report no conflicts of interest and no financial support for this study.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Djallel Bouzid reports financial support, equipment, drugs, or supplies, and travel were provided by Direction générale de la recherche scientifique et du développement technologique (DGRSDT) Algeria. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

We thank Pr. Timothy McKenna for his cooperation and the technology platform of the Ecole Nationale Polytechnique de Constantine for its valuable assistance. This work was supported by the Direction générale de la recherche scientifique et du développement technologique (DGRSDT) in Algeria.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.polymer.2024.127731.

## Data availability

Data will be made available on request.

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