Positron annihilation lifetime spectroscopy and differential scanning calorimetry (DSC) measurements were performed for blends of polyacrylamide (PAM) and poly(ethylene glycol) (PEG) and blends of poly(dimethylacrylamide) (PDMAM) and PEG. The samples were prepared by codissolution in a concentration range of 0-100 wt % PEG. The thermal behavior, characterized by DSC measurements, showed similar variations of the glass-transition temperatures (T-g's) with the PEG concentration for the two systems. Pure PAM and PDMAM presented T-g's of 188 and 111degreesC, respectively. A relatively small and nearly linearly decreasing T-g was observed for the two systems in the range of 20-80 wt % PEG. PEG crystals were present in all blend compositions, and no melting point depression was observed. The thermal results pointed to the partial miscibility of the blends. The degree of crystallinity of PEG increased with increasing PEG concentration for the PDMAM/PEG systems. The ortho-positronium lifetime (tau(3)) increased with increasing PEG concentration for both blends. However, the parameter of the ortho-positronium formation probability (I-3) decreased with the PEG concentration. The product tau(3)(3)I(3), which was proportional to the total free volume fraction, was approximately constant with the PEG concentration for PDMAM blends and increased with the PEG concentration for PAM systems. This result may be interpreted as a consequence of a more heterogeneous structure in PAM blends. Scanning electron microscopy micrographs of blends with 40 and 80 wt % PEG provided evidence of the regions associated with PEG crystallites. (C) 2003 Wiley Periodicals, Inc.