## 3 Results and Discussion
### Water sorption and Solubility  
The effect of replacing BisGMA by dCl-BisGMA on water uptake (W\({}_{\text{SP}}\)) and solubility (W\({}_{\text{SL}}\)) of resin-based dental composites was also investigated. The results are given as a percentage with respect to disk weight ((wt/wt)%; three cycles). As given in Table 2, the W\({}_{\text{SP}}\) of UTBC100 was significantly different from that of UTBC0 (\(p<0.05\)), indicating great enhancement, even for the three cycles of the sorption-desorption process. However, a significant increase over the three cycles for both the control (UTBC0) and samples (UTBC25 to UTBC100) was observed, apparently due to void generation as a result of soluble materials exclusion. It is clear that increasing dCl-BisGMA decreases W\({}_{\text{SP}}\) but increases W\({}_{\text{SL}}\). The value of W\({}_{\text{SP}}\) is generally associated with DC and the network structure, specifically the hydrophilicity-hydrophobicity characteristics, three-dimensional structure, and free volume [40]. Here, W\({}_{\text{SP}}\) decreased with increasing DC, as expected, owing to the low hydrophilicity of dCl-BisGMA.  
Conversely, although WSL increased with the dCl-amount increase, WSP decreased. However, WSL depended on both the amount of water absorbed and the amount and hydrophilicity of the leachable materials, as well as the microstructure of the polymeric network. A previous study on the effects of dimethacrylate monomer hydrophilicity reported that the hydrophobic monomers are more flexible than hydrophilic ones and contribute to a higher free volume in the polymeric network, despite the value of DC. Moreover, flexible monomers tend to undergo primary and secondary cyclization, which may cause gelation, forming short chains, and leachable species (monomers and oligomers) that contribute to the increased solubility of the final composite [41]. Thus, the increase in WSL with increasing dCl-BisGMA concentrations, despite increasing WSP and DC, may be attributed to the formation of small fragments upon polymerization as a result of the cyclization of more flexible monomers (i.e., dCl-BisGMA and TEGDMA) [40]. This was further supported by the lower WSL for UTBC0, compared with that for UTBC100, despite the lower DC and WSP values. The WSP and WSL behavior, evaluated after three cycles of the sorption-desorption process, as shown in Table 2 and Figure 5, revealed a slight increase in WSP and decrease in WSL from cycle 1 to cycle 3. However, WSL decreased to near zero after cycle 2, indicating that the release of leachable species ceased. The values of WSP and WSL may also be influenced by filler size in the composite. In this regard, previous studies highlighted higher WSP and WSL of nanocomposites compared with hybrid ones [42; 43]. This behavior could be explained on the basis of the high surface area of the nanofillers, making them more prone to ion leaching and hydrolysis of the silane coupling agent.  
Figure 4: Degree of conversion (DC), complex viscosity (CV) at 1.0 rad/s angular frequency, water uptake (WSP), and water solubility (WSL) of the first cycle of sorption—desorption versus the dCl-BisGMA amount in the investigated composites (UTBC0, UTBC25, UTBC50, and UTBC100).