pH-Sensitive organic-inorganic copolymers of hydrogels were developed as drug delivery systems (DDS) to improve the swelling behavior of polyacrylic acid (PAA). They were represented through FTIR, TGA and XRD characterization which revealed that the functional groups of methacryl-phenyl polyhedral oligomeric silsesquioxane (POSS) were successfully added to the acrylic acid (AA) molecular chains through radical solution polymerization. The DSC test results indicate that the addition of POSS could improve the thermal properties of the copolymers. The swelling properties at the pH range of 1.25-8.01 exhibited the pH sensitivity of POSS/AA copolymers (POSS-co-AA) and the lower swelling ratio in acidic conditions indicated that the DDS had low amount of release in SGF; this phenomenon suggested that the copolymer was available as DDS of theophylline. And it was proved by drug release curve and scanning electron microscopy. Since the addition of POSS reduced the release rate of theophylline and prolonged the release time of the drug, the concentration range of theophylline could remain low for an extended duration. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013