A series of cellulose graft poly(ethylene glycol) (cellulose-graft-PEG) copolymers as phase change materials (PCMs) were synthesized in ionic liquid by using 4, 4-diphenylmethane diisocyanate as coupling reagent. FTIR and (1)H NMR characterized the structure of the copolymers. Differential scanning calorimetry (DSC) indicated that the copolymers showed phase transitions with large thermal storage density, and the phase change temperature is in the range of 40-60 degrees C. The transition temperature and enthalpy can be tailored by changing the molecular weight and the content of poly(ethylene glycol) (PEG) side chains. The copolymers are solid-solid PCMs with good thermal stability. Expanded graphite (EG) with 2, 5 and 10 wt% was added to the PCMs to improve the thermal conductivity. It was found that the thermal conductivity enhanced significantly with increasing EG content. The cellulose-graft-PEG/EG composite PCMs have potential applications in thermal energy storage and temperature control. (C) 2009 Published by Elsevier B.V.