Novel SPPESK/PAA semi-interpenetrating polymer network (sIPN) have been synthesized through the in situ polymerization of acrylic acid 14 (AA) in the presence of sulfonated poly (phthalazinone ether sulfone ketone) (SPPESK) with methylene-bis-acrylamide (MBA) as the cross-linker of AA. Their potential applications on proton exchange membranes (PEMs) were investigated. By varying the initial sulfonation degree (DS) of SPPESK and the ratios of MBA/AA and SPPESK/AA, water uptake of the sIPN membranes can be adjusted in a broad range from 0.6 to 2.2 times as that of the corresponding SPPESK membranes while excessive swelling is well restricted even at 90 degrees C in water. Proton conductivity of the SPPESK/PAA sIPN membranes reached a value of 1.882 x 10(-2) S/cm, nearly 3.9 times and 1.2 times as that of the corresponding SPPESK membrane and Nafion, respectively, under the same testing condition. Thermolgravition analysis (TGA) indicates that the SPPESK/PAA sIPN membrane can be stable up to 300 degrees C in N-2. DSC and SEM investigations illustrate that the SPPESK/PAA sIPN membranes exhibit a single Tg and dense morphology in a broad range of MBA/AA and SPPESK/AA ratios. (C) 2007 Elsevier B.V. All rights reserved.