The morphology of low-density polyethylene (LDPE) modified by in situ sorption and thermal polymerization of acrylic acid (AA) in the matrix was examined. The microstructure of the LDPE/poly(acrylic acid) (PAA) materials after Eu3+ and UO22+ ion exchange was investigated. The phase behavior of these materials was analyzed using X-ray diffraction, scanning electron microscopy (SEM), and thermal measurements (DSC). The X-ray diffraction studies showed that PAA is located at amorphous region of the matrix. The LDPE/PAA surface, as investigate by SEM, was apparently homogeneous before and after Eu3+ and UO22+ ion exchange, respectively. Two T-g values were found for the LDPE/PAA material before and after Eu3+ ion exchange. Also, three and four T-g values were found for LDPE/PAA after UO22+ ion exchange depending on the amount of UO22+ in the modified matrix. This indicates microphase domains in the LPDE/PAA-, LPDE/PAA/Eu3+-, and LPDE/PAA/UO22+-modified materials, although a lack of visible phase separation in the micrographs was observed.