Graft copolymerization of D,L-lactic acid (LA) onto chitosan (CS) was attempted without using a catalyst, pH-sensitive hydrogels were obtained which are based on two different components: a natural polymer and a synthetic polymer. These polyester substituents provide the basis for hydrophobic interactions that contribute to the formation of hydrogels. The swelling mechanisms in enzyme-free simulated gastric fluid (SGF, pH 2.2) or simulated intestinal fluid (SIF, pH 7.4) at 37 degrees C were investigated. Meanwhile, structural changes of the graft copolymers in the different pH buffers were studied by FTIR, and these are discussed together with the swelling mechanisms. The effect of pH on the water uptake of hydrogel was investigated by using McIlvaine buffer with the same ionic strength. The morphological change of hydrogels in different aqueous solutions is investigated by scanning electron microscopy (SEM).