A semi-interpenetrating polymer network (semi-IPN) of polyethylene glycol (PEG)/poly(polyethylene glycol diacrylate) (PPGD) composites as novel form-stable PCMs was successfully prepared via in situ polymerization. The mass percentage of PEG reached 70 wt% without any leakage above the melting point of PEG. The prepared form-stable PCMs were investigated by scanning election microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), polarization optical microscopy (POM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TG), respectively. SEM and FTIR results show that the PEG is uniformly dispersed into the PPGD network and there are only physical interactions between PEG and PPGD. POM images and XRD patterns reveal that the crystalline structure of PEG is not affected by PPGD network and the crystal size of PEG in form-stable PCMs decreases due to the restriction of PPGD network. DSC analysis results present that the melting and freezing temperatures and the latent heats of form-stable PCM were measured as 58.62 and 37.45 degrees C and 117.41 and 115.17 J/g, respectively. Thermal cycling test and TG analysis confirm that the prepared form-stable PCMs exhibit good thermal reliability and stability. The prepared form-stable PCMs have great potential application in the area of solar energy storage. (C) 2016 Elsevier B.V. All rights reserved.