|M.Sc Student||Weinshtok Mordechay Liora|
|Subject||Encapsulation of "Phase Change Materials" within|
Polymeric Microcapsules and Monoliths
|Department||Department of Materials Science and Engineering||Supervisor||PROF. Michael Silverstein|
|Full Thesis text|
Phase change materials (PCMs) are capable of releasing and absorbing thermal energy as latent heat. The uniqueness of PCMs is their ability to release and absorb relatively large amounts of heat at a specific temperature and this makes PCMs potential candidates for integration inside energy systems with no external source of energy. PCMs can be divided into 3 main groups: organic materials (paraffins and non-paraffins), inorganic materials (salt hydrates and metallic), and eutectic materials (organic-organic, inorganic-inorganic). The objectives of this research were: to synthesize microcapsules (MCs) that encapsulate organic PCMs using interfacial step-growth polymerization (ISGP) and free radical polymerization (FRP); to synthesize porous monoliths that encapsulate organic PCMs using ISGP within high internal phase emulsions (HIPEs); to evaluate the macromolecular structures, porous structures, and properties of the resulting materials; and to evaluate the potential of these materials for energy storage and release applications.
Four PCMs were chosen for the encapsulation: two paraffins (octadecane (OD) and tetracosane (TC)) and two non-paraffins (methyl palmitate (MP) and myristic acid (MA)). Polyurea (PUA) MCs successfully encapsulated OD and TC through ISGP within a surfactant-stabilized emulsion and within a Pickering emulsion stabilized using nanoparticles (NPs). The ISGP reaction was between a diisocyanate (one of hexamethylene diisocyanate (HDI), toluene-2,4-diisocyanate (TDI), or hydrogenated methylene diphenyl diisocyanate (HMDI)) and water containing tris(2-aminoethyl)amine (TDAEA). Polystyrene (PS) MCs, crosslinked using divinylbenzene (DVB) as a comonomer, successfully encapsulated all four PCMs through FRP within both surfactant-stabilized emulsions and NP-stabilized Pickering emulsions. Poly(urethane urea) (PUU) polyHIPEs were synthesized to encapsulate OD through ISGP reactions between a diisocyanate (HDI, TDI) or a triisocyanate and water containing tannic acid and sodium alginate (which also stabilized the HIPE). The structures of the MCs and the structures of the polyHIPEs were characterized using scanning electron microscopy (SEM). The thermal properties were characterized using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The mechanical properties of the polyHIPEs were characterized using uniaxial compression tests. The crystalline structures of the encapsulated OD were characterized using X-ray diffraction (XRD). The potential of the encapsulated PCMs for energy storage and release applications was evaluated by monitoring infrared heating usig a thermal camera and thermocouples.
The MCs, 2 to 5 µm aggregates of 20 to 200 nm primary particles, contained a maximum of 73 wt % OD, which is suitable for application in textiles and foams. The latent heat of cooling for the HDMI-based MCs was 158 J/g (216 J per gram OD). The HDI-based polyHIPE had a capsule-like structure, 10 to 50 µm in diameter, and contained 90 wt % OD. The latent heat of cooling for the polyHIPE was 211 J/g (234 J per gram OD), superior to that of the MCs. The successful encapsulation of PCMs in both nanometer-scale MCs and in polyHIPEs (which is unprecedented) can help broaden and enhance their use in energy storage and release applications. This work demonstrates that a judicious choice of synthesis parameters can be used to optimize the properties of the MCs and polyHIPEs for the desired application.