The functionality of porous isotactic (it) poly(methyl methacrylate) (PMMA) thin films, which were previously developed by the selective extraction of syndiotactic (st) poly(methacrylic acid) (PMAA) from the it-PMMA/st-PMAA stereocomplex thin film on a substrate using the layer-by-layer assembly method was investigated after thermal treatment (70, 80, and 90 degrees C) in water for 4 h. Quartz crystal microbalance analysis and infrared spectra measurements revealed that the st-PMAA incorporation ability of the porous it-PMMA thin film decreased in order at 80 and 90 degrees C, while there was no decrease observed at 70 degrees C. X-ray diffraction analysis also supported the thermal stability of the porosity at 70 degrees C, whereas two it-PMMA crystalline peaks (20 = 9 degrees and 14 degrees) were generated during heating at 90 degrees C. The loss of the functionality of the it-PMMA thin film was thus shown to be due to crystallization, which was caused by the increase in polymer-chain mobility during the heating process. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3265-3270, 2010