טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentInna Gurevitch
SubjectPorous Polymers Synthesized within Nanoparticle-Based
Pickering Emulsions
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professor Silverstein Michael
Full Thesis textFull thesis text - English Version


Abstract

High internal phase emulsions (HIPEs) are concentrated emulsions with the dispersed phase occupying more than 74 % of the emulsion volume. PolyHIPEs are highly crosslinked porous polymer materials obtained by polymerization of monomers in the HIPE external phase. PolyHIPEs exhibit the advantages of large surface areas, low densities, absorbing and insulating abilities. The amount of surfactant needed for HIPE stabilization can often reach up to 30 wt % of the external phase. After HIPE polymerization, the surfactant must be removed. Surfactants are often expensive, difficult to remove, and costly to remove. Replacing the surfactant in HIPEs should prove advantageous, especially for the synthesis of polyHIPEs. Pickering emulsions are stabilized through the spontaneous assembly of solid particles at the oil-water interface rather than through the use of organic surfactants.

The main objectives of this research were to develop Pickering HIPEs using surface-functionalized silica nanoparticles, to polymerize the Pickering HIPEs, and to characterize the resulting materials. Organic alkoxysilanes were used to functionalize the surfaces of the silica nanoparticles, replacing the hydrophilic hydroxyl groups with hydrophobic organic groups. Surprisingly, changing the locus of initiation for the polymerization reaction produced different structures. Interfacial initiation with a water-soluble initiator and organic phase initiation with an organic-soluble initiator were shown to have significant effects upon the porous structures and upon the locations of the nanoparticles within the structures. Pickering HIPEs were polymerized using controlled radical polymerization (CRP). An organic-soluble CRP initiator seems to have similar effects on the polyHIPE structure as the interfacial initiation in conventional free radical polymerization. However, nanoparticle surface initiation produced polyHIPEs with different structures, not seen in commonly synthesized polyHIPEs.

PolyHIPEs are almost always crosslinked to prevent their collapse. The relatively large amounts of crosslinking comonomer used usually yield stiff and brittle polyHIPE. PolyHIPEs with unique properties would result if they could be synthesized without crosslinking comonomers. The crosslinking comonomer was successfully replaced by silane-modified silica nanoparticles, when the silane has polymerizable vinyl group. Elastomeric porous foams were synthesized from Pickering HIPEs using organic phase initiation. Novel water-retaining liquid droplet elastomers (LDEs) were synthesized using interfacial initiation. The ability of LDEs to retain water seemed to be dependent on the void wall thickness.

PolyHIPEs based on long side chain polymers (poly(stearyl acrylate) and poly(stearyl methacrylate)) were crosslinked by the silane-modified nanoparticles used to stabilize the Pickering HIPEs and exhibited shape memory behavior through reversible crystallization and melting of the side chains. Excluding the crosslinking comonomer from the polyHIPE, significantly improved the mobility of the long side chains needed for shape memory behavior. These polyHIPEs combined a high degree of crosslinking with relatively high crystallinity and exhibited rapid, reversible shape memory behavior. Surprisingly, the acrylate-based polyHIPE exhibited a two-stage recovery. Visoelastic models based on Williams-Landel-Ferry (WLF)-like equation were used to describe the recovery of polyHIPEs.