Influence of surfactants and organic polymers on monolithic shape-stabilized phase change materials synthesized via sol-gel route

Shape-stabilized phase change materials (ss-PCMs) can store up to five times more thermal energy than commercially available composite PCM boards based on core-shell PCMs, but have low mechanical stabilities. Therefore, we have recently synthesized ss-PCMs with high mechanical stability via a novel porogen-assisted solgel process. Here, we track the ss-PCM formation in situ and analyze the influence of the porogens sodium dodecyl sulfate (SDS) and poly(vinyl alcohol) (PVA) on ss-PCM properties. During gelation the silica scaffold is built around PCM droplets covered with SDS in an O/W emulsion. All ss-PCMs exhibit a high shape-stability (similar to 100%), a high long-term performance (> 2000 thermal cycles) and a high chemical stability, regardless of the amount of porogen used in the synthesis. Higher PVA amounts separate the emulsion in large hydrophilic and small hydrophobic regions and increase the silica macropore width in ss-PCMs from 1007 nm to 8801 nm due to silica fragmentation during drying. The silica fragmentation lowers the compressive strength (1.2 MPa) and thermal conductivity (0.37 W/mK) of the ss-PCMs by 50% and by 17% (10 degrees C), respectively. In contrast, low SDS amounts can not stabilize all PCM droplets in the emulsion from phase-separation, resulting in a partially fragmented silica phase. Too high SDS concentrations decrease the viscosity and hence, stability of the emulsion. Consequentially, the silica macropores collapse during drying, which lower the compressive strength (1.02 MPa) and thermal conductivity (0.37 W/mK) of ss-PCMs by 63% and 25% (10 degrees C), respectively.