## 2 Materials and Methods
### Measurements
#### 2.4.3 Physicochemical Properties  
Polymerization shrinkage kinetics were conducted according to the \"bonded-disk\" method which was initially published and further refined by [PERSON] and co-workers [PERSON] and co-workers [PERSON] (1974); [PERSON] (1974); [PERSON] (1975). Briefly, a disk-shaped unset specimen with dimensions of 1.0 mm \(\times\) 8.0 mm (height \(\times\) diameter) was formed and centrally positioned upon a 3-mm-thick rigid glass plate. A flexible cover-slip diaphragm, supported by an outer peripheral brass ring with internal diameter circa 15 mm, was rested on the upper surface of the specimen disk so as to be adherent. A uniaxial LVDT (linear variable displacement transducer) measuring system was positioned centrally onto the cover slip. The signal from the LVDT was transmitted to a computer by a transducer indicator (E 309, RDP Electronics Ltd., Wolverhampton, UK) and a high-resolution analog-to-digital converter (ADAM-4016 acquisition module). The data acquisition was supported by the datalogger software AdvantechAdam/Apax.NET Utility, version 2.05.11. Measurements records were taken by continuous irradiation of specimens with the above LED polymerization unit for 5 min directly from beneath the glass plate at room temperature. Five repetitions (\(n\) = 5) were performed at each specimen. Strain was calculated as:  
\[\epsilon(\%)=100\times\frac{\Delta\mathrm{L}}{\mathrm{L}_{0}}\]  
where \(\epsilon\) (%) represents the strain (%) and \(\Delta\mathrm{L}\) and \(\mathrm{L}_{0}\) are the shrinkage displacement and the initial specimen thickness, respectively. Polymerization kinetics were performed by placing a small amount of each composite between two translucent Mylar strips, which were pressed to produce a very thin film. The films of unpolymerized composites were exposed to visible light as previously described, and immediately scanned by a Spectrum One Perkin-Elmer FTIR spectrometer (PerkinElmer Inc., Waltham, MA, USA) at different curing time intervals (0, 5, 10, 15, 20, 25, 30, 40, 60, 80, 120, 180 s). A spot of samples was placed between two NaCl crystals and spectra were obtained over the 4000-600 cm\({}^{-1}\) region and acquired with a resolution of 4 cm\({}^{-1}\) and a total of 32 scans per spectrum. A commercial software Spectrum v5.0.1 (Perkin-Elmer LLC 1500F2429) was used to process and calculate all the data from the spectra. The area of aliphatic C=C peak absorption at 1637 cm\({}^{-1}\) and the aromatic C=C peak absorption at 1580 cm\({}^{-1}\) were determined utilizing a base line technique which proved the best fit to the Beer-Lambert law [PERSON] (1974). The aromatic C=C vibration was used as an internal standard. The percent degree of monomer conversion (DC%) of the cured specimen, which expresses the percent amount of double carbon bond reacted at each time period, was determined according to the equation:  
\[\mathrm{DC}(\%)=\left[1-\frac{\left(\frac{\Delta\mathrm{A}_{1637}}{\Delta \mathrm{L}_{1580}}\right)_{\mathrm{polymer}}}{\left(\frac{\Delta\mathrm{A}_{1 637}}{\Delta\mathrm{L}_{1580}}\right)_{\mathrm{monomer}}}\right]\times 100\]  
Sorption of water and solubility characteristics were determined according to the method described in ISO 4049 (1988). For this purpose, four specimen discs (15 mm diameter \(\times\) 1 mm thickness) were prepared for each composite material by filling a Teflon mold with the unpolymerized material as described above. The samples were irradiated for 40 s on each side by dividing them into nine overlapping irradiation zones and using the abovementioned LED polymerization unit. All the specimens were placed in a desiccator and transferred to a pre-conditioning oven at 37 \(\pm\) 1 \({}^{\circ}\)C. After 24 h they were removed, stored in a second desiccator (23 \(\pm\) 1 \({}^{\circ}\)C) for 2 h, and weighed to an accuracy of \(\pm\)0.1 mg using a Sartorius TE 124S balance (Sartorius AG, Goetingen, Germany). This cycle was repeated until a constant mass (\(m_{1}\)) was obtained.