The present investigation revealed that the first and second hypothesis must be accepted, once the physicochemical properties tested were not negatively affected by the addition of CTMA and there was a reduction in the viscosity, water sorption and solubility in comparison with BisGMA-based resin by the addition of the novel trimethacrylate monomer. Also, CTMA groups attained mechanical properties similar to traditional systems based on Bis-GMA. Consequently, third hypothesis should be accepted.
An effective polymerization plays a very important role on the physicochemical and mechanical properties of resin-based dental materials. Double bond conversion of multi-methacrylate polymers is rarely complete because of the flexibility of monomers during propagation, and due to the limited mobility of partially cured macromolecules as the reaction progresses [12]. The long flexible carbon chain of CTMA probably induced a delayed gelation, increasing the mobility of the active species after the formation of microgels during polymerization and achieving a similar degree of conversion as the traditional Bis-GMA/TEGDMA control resin.
The long aliphatic carbon chain of CTMA, devoid of polar hydroxyls, also contributes to its high hydrophobicity [17], thus explaining why CTMA groups showed lower water sorption. This may have led to smaller amounts of leachable monomers from CTMA composites, and overall soluble products, along with the acceptable degree of conversion. In contrast to CTMA, the BisGMA-rich composites may be partially degraded into BPA when they are in contact with human saliva [5] and might contaminate the body.
Apart from reduced water sorption and solubility, a relatively low viscosity is desired for monomers employed in resin-based dental materials, in order to facilitate handling and incorporation of filler particles. The Bis-GMA content reduces side-chain mobility, as it increases the formation of strong hydrogen bonds through its hydroxyls [22]. The observed decrease in viscosity was attributed to the substitution of the viscous Bis-GMA by CTMA, since the latter has a high molecular weight but a long flexible carbon chain free of hydroxyl groups, thereby not forming hydrogen bonds which increase viscosity. This assertion finds validation in CTMA-50 group, which eliminated all content of Bis-GMA and the consequent formation of hydrogen bonds, resulting in a substantial reduction in the viscosity of the composites.
In terms of thermal degradation stability, the incorporation of CTMA in composites up to 20% was able to improve it, which could be attributed to the bulky aromatic structure and long side alkyl chains of CTMA. These characteristics may have prevented the packing of the polymer chains leading to an increase in the voids in the system. Also, all groups were stable up to 200 ºC (Fig. 6), indicating a similar and acceptable thermal stability of resin composites for safe use in the oral cavity. The survey of Jaillet et al. (2014) investigated the thermal degradation of novel vinylester prepolymers from cardanol in comparison to diglycidyl ether of bisphenol A (DGEBA), which showed similar thermal stability [23]. A maximum decomposition rate around 430°C for cardanol-based resins was found, quite similar to the values obtained for CTMA-based resin composites (Table 3; Tmáx: 422 to 427°C).
Concerning mechanical properties, dynamic mechanical analysis provides information about the properties of polymer networks, such as storage modulus and glass transition temperature (Tg), by evaluating the structure and stiffness of the materials [21]. Control composite obtained the highest value of storage modulus at the rubbery zone (180 ºC), indicating greater entanglement of polymer networks. Crosslinking density is an important variable in the viscoelastic behavior of the polymers, and, typically, resin-based materials with multimethacrylates are highly crosslinked polymer networks, once a higher number of functionalities is beneficial for the storage modulus in the rubbery zone [12]. However, our results showed that the trimethacrylate addition reduced the storage modulus, and, therefore, the crosslinking density of the composites. Furthermore, the incorporation of CTMA revealed a plasticizing effect on Bis-GMA composites, preventing close packing between the polymer backbones, as seen in the lowering of Tg in comparison to the control (Table 4). However, the Tg obtained by CTMA composites are still above body temperature and food/beverage consumed (> 115 ºC); therefore, their physical and mechanical properties are preserved, ensuring optimal intraoral performance of these materials.
The height of the maximum tan δ peak on DMA reflects the extent of mobility of the polymer chain segments as a function of temperature. When CTMA was used as a comonomer, all composites showed higher tan δ peaks than the control. This result reveals a high mobility of the CTMA polymer networks (especially for CTMA-40 and CTMA-50) due to the flexible long carbon chain, causing an increase in the viscous behavior (less energy is stored in the material) at the expense of the elastic component. Also, regarding the width of tan δ peaks, the samples revealed similar wide peaks, which means that the glass transition occurs over a wide temperature range. This wide glass transition is apparently related to the chain-growth polymerization in heterogeneous networks and usually occurs with increasing crosslink density of the polymer network [21].
One of the few properties correlated with the clinical performance of resin composite restorations is the flexural strength, as this in vitro test is correlated to the clinical wear and tensions undergone in restorations and plays an important role in the acceptance of restorative materials [24]. The statistical analysis demonstrated similarity of mechanical properties of CTMA-based composites when compared to control. The main reason that could explain this behavior is the CTMA chemical structure, which has one aromatic ring that confer high mechanical stability to the material similar to the rigid backbone bisphenol-A structure of the Bis-GMA monomer. The similar degree of conversion outcomes may also correlate to the results obtained for flexural strength and elastic modulus, since physical and mechanical properties of resin-based composites are influenced by the degree of polymerization [25]. As Bis-GMA based resin composites have decades of clinical success and currently still are standard composites employed around the world. The proposed newly synthesized monomer CTMA then achieved similar or superior physicochemical properties to the control Bis-GMA, proving the clinical suitability, with the advantages of lacking bisphenol-A as well as CTMA-synthesis thresholds from a plant-derived compound, thereby turning this a monomer derived from a renewable source.