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

which were prepared by ultraviolet irradiation polymerization

were modified by poly(vinyl alcohol) (PVA).By adding the solution of PVA (0;82%) into single network (SN) hydrogels

tiny networks have been made in the PAMPS network structures.DN hydrogels modified by PVA become tougher and harder than those without PVA.The swelling behavior was measured gravimetrically at room temperature (25℃).The structures of SN hydrogels modified by different amounts of PVA were successfully characterized by scanning electron microscopy (SEM).The compressive and tensile properties of hydrogels were examined by using Zwick/Roell tester.The results showed that the DN hydrogels modified by PVA exhibited higher swelling ratio than that of unmodified DN hydrogels.Furthermore

with the increase of PVA solutions’ concentration the swelling ratio was increasing.SN hydrogels modified by PVA （0;82%） showed tiny networks in the SN structures.Tiny networks enhanced the mechanical properties of the DN hydrogels.Photos of SEM demonstrated apparently that tiny networks only were found when the amount of PVA is 082% and 1.64%

however

the tiny network structures of SN hydrogels modified by 0.82% PVA were more regular than those of the SN hydrogels modified by 1.64% PVA.Some aspect of the compressive and tensile properties hand been enhanced

for example when the amount of PVA is 082%

the compressive stress of modified DN hydrogels was 12.8 MPa at 90% compressive strain (still keep integrity)

while that of the unmodified ones was 8.7 MPa at 80% compressive strain.Almost 78.6% enhancement of tensile stress compared with unmodified DN hydrogel indicated that the modified DN hydrogels had become tougher and harder.