Double network (DN) hydrogels composing of poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network (FN) and poly(acrylamide) (PAM) as the second network (SeN)

which were prepared by ultraviolet irradiation polymerization

were modified by poly(ethylene glycol) (PEG).By adding PEG into single network (SN) hydrogels

many tiny meshes have been made in the network structures; meanwhile the PAM particles of SeN are in different sizes.DN hydrogels modified by PEG become tougher and softer than those without PEG.The swelling behavior was measured gravimetrically at 28℃.The absorbed amount of acrylamide (AM) in SN hydrogels was measured by brominate titration.The structures of different SN hydrogels were successfully characterized by scanning electron microscopy (SEM).The compressive and tensile properties were examined by using Zwick/Roell tester.The results showed that:this PEG modified DN hydrogels exhibited higher swelling ratio than that of unmodified DN hydrogels

the former was twice times higher than the latter.Interestingly

the volume of DN hydrogels became larger with the gradual increasing amount of PEG in the hydrogels

likewise

the transmittance of DN hydrogels with various PEG amounts became lower.Modified SN hydrogels prefer to absorb AM

because the small meshes

which are in the network of hydrogels

enhance the absorbability of water and AM.Pictures of SEM demonstrate apparently that many small meshes of about 0.05～10 μm are in the network structures and the thickness of mesh walls ranges from 0.02 μm to 0.2 μm.However

in the unmodified SN hydrogels no obvious meshes were found and the thickness of mesh walls is about 3 μm.Some compressive and tensile properties were enhanced

for example when the PEG = 1.0 g

the compressive strain of modified DN hydrogels was 90%

while that of the unmodified ones was 80%.Twice times higher tensile strain than that of unmodified DN hydrogels indicated that modified DN hydrogels had become tougher and softer.