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We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate in detail the miscibility behavior, interaction mechanism, and ionic conductivity of polymer electrolytes composed of lithium perchlorate (LiClO4) and monomethoxypoly(ethylene glycol)-block-poly(ε-caprolactone) (MPEG−PCL) diblock copolymer. The existence of the PCL phase in the MPEG−PCL block copolymer tends to retard the crystallinity of MPEG and results in a higher ionic conductivity for the LiClO4/MPEG−PCL based polymer electrolyte. DSC analyses indicate that the phase separation occurs for some compositions of the LiClO4/MPEG−PCL blends. In addition, FTIR spectroscopy studies reveal the complicated interactions that occur within the LiClO4/MPEG−PCL blend system upon varying the LiClO4 content and increasing the temperature. For the LiClO4/MPEG−PCL blend system, the relative intensity of the “complexed” carbonyl stretching band of the PCL block tends to increase when either the LiClO4 concentration or the length of the PCL block increases. For some compositions of LiClO4/MPEG−PCL, when the temperature is increased from 120 to 180 °C, the relative intensity of the “complexed” CO group increases; this phenomenon is different from that observed for the LiClO4/PCL homopolymer blend.