The relationship between glass transition (T-g) and ionic conductivity (sigma) of an amorphous crosslinked polymer electrolyte membrane (PEM) was examined based on ion -dipole complexation between dissociated lithium cations and ether oxygen of poly(ethylene glycol diacrylate) and plasticization by succinonitrile (SCN). In a binary PEM consisting of a lithium salt/polymer network, T-g increased due to a strong ion-dipole interaction, whereas sigma declined due to lower ion mobility coupled to reduced chain mobility. Above the threshold salt concentration of 7 mol %, dual loss tangent peaks were observed in dynamic mechanical studies, which may be ascribed to segmental relaxations of ion -dipole complexed networks and that of polymer chains surrounding the undissociated lithium salt acting like "fillers". Upon SCN plasticization, these two peaks merged into one that was further suppressed below T-g of the pure network, whereas a improved to the superionic conductor level. The role of plasticization on the ionic conductivity enhancement is discussed.