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4. Answer the following questions on double-network (DN) hydrogel consisting of poly(sodium 2- acrylamido-2-methylpropanesulfonate) (PNaAMPS) as the short, pre-stretched network and poly(acrylamide) (PAAm) as the long, coiled stranded network. Each network is covalently cross- linked by methylenebis(acrylamide) (MBAA). NaAMPS AAm MBAA TADETA (a) The following are stress-strain curves obtained by cycled tensile tests for the DN hydrogel. Stress-strain curve (tensile test) 1.2 5th 2nd 3rd 1st 0 1 2 3 4 5 6 7 8 Strain & (-) (i) What is the fracture stress of the gel? (3 marks) (ii) Explain the differences of the profiles in the 1st-5th runs. (3 marks) (b) Explain the result of the following experiment: The DN gel was supplied with Fe²+ and then mechanically stretched. It was revealed that Fenton reaction (Fe²+ + H₂O₂ → Fe³+ + HO + OH-) occurred in the gel, whereas the Fenton reaction did not occur in the unstretched gel. لد سل سلال SO,Na Stress (MPa) NH₂ سلم حمله
(c) Explain the result of the following experiment: The DN gel (note: the gel contains no Fe²+) was immersed in an aqueous solution of NaAMPS and N,N,N"-triacryloyl diethylenetriamine (TADETA) and subjected to cycled tensile tests (1 h intervals between each cycle) in the same solution. The gel became stiffer with an increasing number of cycles as shown below. (4 marks) Force-length curve Tensile-test in a solution of NaAMPS and TADETA 1st cycle 2nd DN Gel 3rd F 4th L TADETA 15 30 F F(N) 32 N - 0 45 60 L (mm) 75 90