Semi interpenetrating polymer networks (s-IPN) hydrogels composed of poly(vinyl acohol) (PVA), poly(4-acetyl acryloyl ethyl acetate-co-acrylic acid) [P(AAEA-co-AA)] were synthesized via solution polymerization using acrylic acid (AA), 4-acetyl acryloyl ethyl acetate (AAEA) and PVA. Fourier-transform infrared (FT-IR) was used to confirm the chemical structure of the s-IPN hydrogels. The results showed that the AAEA mainly existed as enol form, making the peak of C—O stretching in the IR spectra of PVA-P(AAEA-co-AA) flow to low wave numbers. This is attributed to the formation of strong hydrogen bonds between PVA, PAA and PAAEA. SEM macrographs revealed that PVA impenetrated into the P(AAEA-co-AA) ploymer networks homogeneously, and barricaded the phase separation of the hydrogels. XRD analyses showed that the hydrogels formed perfect s-IPN structure when the dosage of PVA was low. However, when the dosage of PVA was high, PVA could not impenetrate in P(AAEA-co-AA) ploymer networks homogeneously, making the crystallization of PVA increase significantly. The volume phase transition temperature (VPTT) of the s-IPN hydrogels was determined by their respective DSC thermograms of swollen hydrogel specimens. The results showed that the VPTT of the hydrogels ranged from 54.0 to 57.8 ℃, and increased with the increasing dosage of PVA. The stress strength of s-IPN hydrogels was studied, the results showed that the s-IPN structure formed between PVA and P(AAEA-co-AA) improved the strength of the hydrogels significantly, and the maximum stress strength of the hydrogel was about 8.4 MPa. The temperature sensitivity of the s-IPN hydrogels was also investigated, the results showed that the hydrogels exhibited excellent temperature sensitivity. When the temperature was lower than VPTT, the hydrogels kept at a swelling state, but when the temperature was higher than VPTT, the hydrogels deswelled evidently.

Key words: hydrogel, temperature sensitivity, mechanical strength, volume phase transition