M.Sc Thesis

M.Sc StudentStern Aya
SubjectElastomeric, Emulsion-templaed Polymers for the Controlled
Release of Water
DepartmentDepartment of Materials Science and Engineering
Supervisors PROF. Michael Silverstein
DR. Eric Assouline
Full Thesis textFull thesis text - English Version


High internal phase emulsions, HIPEs, are highly viscous emulsions consisting of two immiscible phases, one hydrophobic, one hydrophilic, with more than 74 % of the volume comprised of the dispersed, internal phase. While the lower limit of the dispersed, internal phase, defined by the maximum packing fraction of monodisperse hard spheres, is 74 %, HIPEs with 90 % internal phase are not unusual. HIPEs are water-in-oil (w/o) emulsions or oil-in-water (o/w) emulsions, prepared by adding the internal, major phase to the external, minor phase while constantly stirring. The HIPE’s viscosity and characteristics are affected by the type of mixing, the fraction of the internal and external phases, and the type of stabilization used. PolyHIPEs are highly crosslinked, porous polymers formed when the external phase is polymerized. PolyHIPEs have large surface areas, low weights, and low densities, lending to their use in absorbing and insulating purposes.

The main objectives of this research were to develop elastomeric polymeric materials with water storage capacities that enable water retention over time and to characterize the controlled release of the water for use in plant irrigation. The sixteen different polyHIPEs synthesized can be divided into two groups: (1) polyHIPEs based on elastomeric monomers 2-ethylhexyl acrylate (EHA) or lauryl acrylate (A12) with different initiators and loci of initiation, and (2) polyHIPEs based on the same monomer but with an oligomer, 1,2-polybutadiene (PB), added to increase crosslinking. The different initiators can be divided into organic-soluble initiators, such as benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN), and water-soluble initiators such as potassium persulfate (KPS) for interfacial initiation. Two polymerization mechanisms were tested, free radical polymerization (FRP) and atom transfer radical polymerization (ATRP).

The structures of the polyHIPEs were characterized using cryogenic scanning electron microscopy. The water content and the thermal properties were evaluated via differential scanning calorimetry (DSC) and the thermal properties were also characterized using dynamic mechanical thermal analysis (DMTA). The mechanical properties of the samples were evaluated through uniaxial compression stress-strain tests to 70 % strain and water release was determined through mass loss. Plant growth experiments were performed by adding polyHIPEs to the soil.

The average size of the water droplets was 2 µm, significantly smaller than expected from previous studies. The glass transition temperature of -60 °C from the second heat DSC thermogram is typical of an EHA-based polyHIPE based on EHA. PA12 can undergo crystallization and a melting temperature of 1.7 oC was determined by DSC and DMTA. Generally, the modulus of A12-based polyHIPEs is higher than those of EHA-based polyHIPEs, and copolymerization with PB produces an order of magnitude increase in the modulus. The water release tests indicated that, for both EHA- and A12-based polyHIPEs, interfacial initiation reduces the release rate and that copolymerization with PB increases the release rate. The plants whose pots contained open-cell polyHIPEs (organic-phase initiation) exhibited survival superior to plants whose pots contained closed-cell polyHIPEs (interfacial initiation).