Application Of Efficient And Smart Emulsifiers Prepared By RAFT Polymerization

Emulsions are widely used in cosmetics, foods, pharmaceuticals and petroleum products. However, emulsions are inherently unstable thermodynamically and will eventually coalesce into separate phases. To slow this process, a tensioactive stabilizer, such as surfactant and particle, is usually added. However, surfactants are potential environmental pollutants, especially in the case of high internal phase emulsions(HIPEs), which contain more than 74.05% v/v dispersed phase, and require high surfactant concentrations(5~50 wt%) for stability. Most colloidal particles, especially inorganic particles, require chemical treatment by surface-absorbing polymers to modify the wettability.Core cross-linked star(CCS) polymers have interesting properties combining the nanosize of particles and configurational flexibility of soluble polymers. We hypothesized that CCS polymers with stimuli-responsive arm polymers can make excellent stimuli-responsive stabilizers for emulsions, because the distal ends of the arm chains still have a high degree of flexibility and would rapidly respond to external stimuli and therefore the emulsions could be triggered to demulsify very efficiently. We have synthesized three types of stimuli-responsive CCS polymers and investigated their role as responsive emulsifiers.Firstly, pH responsive poly(2-(dimethylamino)ethyl methacrylate)(PDMAEMA) CCS polymers were synthesized by RAFT dispersion polymerization using 1, 6-hexanediol diacrylate as the crosslinker. When n-dodecane was used as the oil, stable and gelled O/W HIPEs over a wide pH range(2-12) were obtained. Maximum oil volume fraction as high as 83% was achieved at a polymer concentration as low as 0.3 wt%. pH-trigerred complete demulsification of HIPEs was easily realized by addition of base. By adjusting the pH back to its original value and applying a shearing force again, stable emulsions were reformed. This emulsification-demulsification process was repeated several times. The use of CCS-stabilized HIPEs as templates allowed the preparation of hydrophilic polyHIPEs. In addition to dodecane, PDMAEMA CCS polymers also stabilized toluene. With increasing pH, emulsions from O/W HIPEs inverted to common W/O emulsions via intermediate multiple emulsions.Secondly, a series of five CCS polymers which exhibited responsiveness to both temperature and salt were presented. These CCS polymers consisted of arms of poly(MEAx-co-PEGAy)(MEA is 2-methoxyethyl acrylate and PEGA is poly(ethylene glycol) acrylate) of different compositions and hydrophobic cores. They have different cloud points(CPs) and hydrodynamic diameters because of the different polymerization ratio of crosslinkers to arm polymers. These CCS polymers stabilized HIPEs with oil fractions up to 92 vol% at relatively low concentrations of CCS polymers. The HIPEs were thermally triggered to completely demulsify at mild temperatures within a short time. While the addition of kosmotropes enhanced the demulsification efficiency, the addition of chaotropes reduced the demulsification efficiency or enhanced the thermal stability of the HIPEs.Thirdly, polyelectrolyte-based CCS polymers were synthesized and their emulsification properties were studied. Styrenic imidazole-based CCS polymers(S-PVBn Im) were prepared by RAFT-mediated dispersion polymerization, followed by quaternization of S-PVBnIm and anion exchange reaction. Different from the above CCS polymers, polyelectrolyte-based CCS polymers stabilized oils of wide-ranging polarity in the form of HIPEs. Moreover, S-PVBnEtImBF4 showed upper critical solution temperature(UCST) properties. Above its UCST, S-PVBEtImBF4 displayed effective emulsion properties evidenced by formation of HIPEs. Below UCST, low internal phase emulsions were solidified because of enhanced interactions among the droplets.Though CCS polymers showed excellent emulsification properties, the preparation, especially the purification, always required significant synthetic efforts. The use of simple homopolymers as effective emulsifiers are highly desirable. For this, poly(4-vinylphenylboronic acid)(PVPBA) homopolymers were investigated as potential homopolymer emulsifiers, because different ionization degrees result in the formation of amphiphilic polymers. PVPBA polymers were synthesized by RAFT polymerization of VPBA. PVPBA homopolymers stabilized various oils with a wide range of polarities, such as n-dodecane, hexane, polydimethylsiloxane(PDMS) and toluene, to form oil-in-water(O/W) HIPEs with polymer concentrations as low as 0.5 wt%. The intermolecular hydrogen bonds of boronic acid in the three-dimensional network result in formation of solid-like gel emulsions with a wide range of oil volumes from 50% to 88%. The HIPEs are pH-sensitive and base(i.e., NaOH) can break HIPEs completely and instantly. However, the pH-sensitivity of HIPEs can be modified by addition of glucose, thereby increasing the applicability in basic media. In addition, hydrophobic porous materials and microcapsules were prepared by emulsion-template methods.