Influence of gelation step for preparing PEG–SiO2 shape-stabilized phase change materials by sol–gel method

Abstract

An in situ shape-stabilized phase change material (ssPCM) from polyethylene glycol (PEG) has been produced by sol–gel method. The inorganic matrix was in situ formed from tetraethyl orthosilicate (TEOS), controlling the condensation rate in a second alkaline step using NaOH. ssPCMs having a latent heat up to 113.8 J/g were synthetized using a sol with a molar ratio H2O:EtOH:H2SO4:PEG1000:TEOS of 2:0.34:0.021:0.50:1 and an equivalent ratio NaOH/H2SO4 of 1.15 for promoting the gel step. The presence of high-density hydrogen bonds between silanol groups and the ether oxygen atoms of PEG and the existence of latent heat allowed to confirm that the PEG worked in two ways. It either forms the PEG–SiO2 matrix or adsorbs onto the surface of the previous polymeric matrix, losing or conserving its latent heat, respectively. The addition of NaOH allowed to change the functionality of the silicon matrix which strongly affected the water content, the thermal stability, and the amount of active PEG in the ssPCMs, leading to an optimal neutralization condition when an equivalent ratio NaOH/H2SO4 of 1.15 was used. The obtained ssPCM has an appropriate range of operative temperatures, a high latent heat in the range of common thermoregulating materials, and a proper thermal reliability.