Influence of SiO₂ pore structure on phase change enthalpy of shape-stabilized polyethylene glycol/silica composites

Polyethylene glycol (PEG2000)/silica (SiO₂) composites with various weight percentages of PEG were prepared as solid–liquid shape-stabilized phase change materials using sol–gel method. In the composite, PEG and SiO₂ were chosen as the phase change substance and the supporting material, respectively. The composites were characterized by differential scanning calorimetry and scanning electron microscope. The pore structure of the SiO₂ matrix with removal of PEG was studied using N₂ adsorption analysis. The phase change enthalpy of PEG in the composite was determined. It was lower than the theoretical value, and decreased with the increase of PEG content. PEG in the composite was strongly confined during the phase transition, and the confinement effect was related with the pore structure of the silica matrix. By correlating the phase change enthalpy with the average pore diameter of the SiO₂ matrix by employing a confined phase change model with a constraint layer, the effect of the pore structure on phase transition of PEG was quantitatively evaluated. The phase change enthalpy of PEG in the composite depended on the average pore diameter of the SiO₂ matrix, the pore geometrical shape, and the thickness of the PEG constraint layer.