Interpenetrating polymer networks (IPNs) based on polybutadiene (HTPB) and polymeric ionic liquid (PIL) networks were prepared using an in situ strategy. PIL network is obtained from a free radical co-polymerization of an ionic liquid monomer (ILM) and poly(ethylene glycol) dimethacrylate initiated with azobisisobutyronitrile. HTPB network is formed by isocyanate-alcohol addition between the hydroxyl end groups of telechelic HTPB and a pluri-isocyanate cross-linker, catalyzed by dibutyltindilaurate. The IPN containing 72 wt% PIL network exhibits one mechanical relaxation temperature as determined by Dynamic Mechanical Thermal Analysis (DMTA).