2.3. Synthesis of Poly(urethane-acrylate) (PUA)

All glassware was oven dried at 100 °C for more than two hours prior to the synthesis of poly(urethane-acrylate).

In a 25 mL two-necked round bottom flask, 2.5 mL of PEG 400 was heated at 70 °C with 3.6 mg of dibutylbis(lauroyloxy)tin and 3 mL of IPDI for 20 min under nitrogen atmosphere. At the end of the first step, the oligomer was cooled down to 50 °C and then end-capped with HEMA (3 mL), by adding it dropwise to the reaction mixture. The synthesis of UAO was performed at 50 °C for 4 h. Then the reaction was cooled down to room temperature and a specific amount of filler was added and stirred for 30 min at room temperature. Afterwards, benzophenone (120 mg) and methyl diethanolamine (120 mg) were added and the reaction mixture was cast in a Teflon mold and was exposed to the UV-lamp for 10 min in order to obtain PUA.