Liora Weinstock, MSc candidate
Department of Materials Science and Engineering
Technion – Israel Institute of Technology, Haifa 32000, Israel
Phase change materials (PCMs) release and absorb a relatively large amount of thermal energy as latent heat while undergoing a phase change. PCMs are of interest for energy storage applications, releasing and absorbing 14 times more heat than conventional materials (water, soil, rocks) at a specific temperature without the use of an external electrical source. PCMs are divided into 3 main groups: organics (paraffins and non-paraffins), inorganics (salt hydrates, metals), and eutectics (organic-organic, inorganic-inorganic, organic-inorganic). The objectives of this research were to encapsulate organic PCMs within polymeric systems, to characterize the resulting systems (structures, properties), and to evaluate the potential of these materials for energy storage applications. Two very different systems were investigated: (1) polymeric microcapsules (MCs) synthesized using both free radical polymerization (FRP) and interfacial step growth polymerization (ISGP); (2) polymeric monoliths (polyHIPEs) synthesized within high internal phase emulsions (HIPEs) through ISGP at the oil-water interface.
Both paraffin PCMs (octadecane (OD), tetracosane (TC)) and non-paraffin PCMs (methyl palmitate (MP), myristic acid (MA)) were investigated. Polyurea (PUA) MCs were synthesized using ISGP within oil-inwater emulsions stabilized with either a surfactant or nanoparticles (NPs). The monomers were a diisocyanate (aliphatic, cyclic, or aromatic), water, and a triamine. Crosslinked polystyrene (PS) MCs were synthesized using FRP. Poly(urethane urea) (PUU) polyHIPEs were synthesized to encapsulate OD using ISGP. The monomers were a diisocyanate, tannic acid, and water. The structures were characterized using scanning electron microscopy, the thermal properties using differential scanning calorimetry and thermogravimetric analysis, and the mechanical properties using uniaxial compression tests. The potential for energy storage applications was evaluated using a thermal camera set-up.
OD and TC were successfully encapsulated within PUA MCs, 2 to 5 µm aggregates of 20 to 100 nm primary particles, using cyclic and aromatic diisocyanates. MP and MA could not be encapsulated within the PUA MCs and the PUU polyHIPEs since they reacted with the diisocyanates. OD, TC, MP and MA were successfully encapsulated within the PS MCs, 5 to 20 µm aggregates of 100 to 200 nm primary particles. The highest latent heats achieved for the MCs were for OD, 123 J/g within NP-stabilized PS MCs and 158 J/g within surfactant-stabilized PUA MCs. A maximum of 73 wt % OD could be encapsulated in the MCs, which is suitable for textile and foam applications. The unique truly closed-cell capsule-like polyHIPE structure (voids of 10 to 50 µm) was quite different from the typical polyHIPE open-cell structure. A maximum of 90 wt % OD could be encapsulated, much higher than in the MCs, with the aliphatic diisocyanate producing the most successful encapsulation, 210 J/g. The OD-containing monolith with a modulus of 6.7 MPa could be subjected to 70 % strain without failing. MCs containing OD and TC exhibited a significant retardation of
temperature rise when evaluated for thermal energy applications. The successful synthesis of two very different types of PCM encapsulation systems can be used to adapt PCMs for a variety of energy storage applications.
Supervisor: Prof. Michael S. Silverstein