Synthesis of Polymeric Janus Nanoparticles through Seeded Emulsion Polymerization

Abstract

Methods to prepare anisotropic or Janus particles are well documented. However, the preparation methods available to prepare polymeric Janus nanoparticles still suffer from significant challenges. In this Thesis, three methods are explored to synthesize polymeric Janus nanoparticles with amphiphilic properties. In the Janus nanoparticles prepared, one side is hydrophilic, being coated by a “hairy” layer of poly(acrylic acid) (AA) and one side is hydrophobic, being made of styrene (Sty) or 2,2,2-triflioroethyl methacrylate (TFEMA). The first method described the synthesis of anisotropic or Janus nanoparticles from seed particles prepared through the self-assembly of amphiphilic diblock copolymers using solvent exchange method. The ability of the amphiphilic diblock copolymers to self-assemble into seed particles depended on the ratio between the hydrophobic and hydrophilic blocks. Using a fixed ratio between the hydrophobic and hydrophilic block of 4.9 and 4.8 respectively, the amphiphilic diblock copolymers, PABTC-(Sty)49-b-(AA)10 and PABTC-(Sty)87-b-(AA)18 self-assembled, forming seed particles with a diameter of 14 and 19 nm respectively. These particles were then swollen with different amounts of cross-linker, divinylbenzene (DVB), producing cross-linked seed particles. The cross-linked seed particles were then swollen with DVB/styrene or tetraethylene glycol diacrylate (TEGDA)/TFEMA mixtures, which were added by batch addition at room temperature before polymerizing at high temperature. For the smallest seed particles of 14 nm, higher amounts of cross-linker imparted lower surface active property on the anisotropic particles once swollen and polymerized with TEGDA/TFEMA. The contrary was true for larger seed particles of 19 nm. The anisotropic particles with a TEGDA/TFEMA bulge were very surface active, therefore markedly amphiphilic. Anisotropic particles made from the 19 nm seed particles were more surface active than those made from 14 nm seed particles, due to their ability to form bigger hydrophobic bulge. Through the method described here, amphiphilic Janus nanoparticles as small as 17 nm measured through the longest dimension were synthesized. The second method studied produced seed particles by the continuous addition of styrene into a micelles solution of PABTC-(nBA)5-b-(AA)5 under basic conditions. Using this approach, the seed particles prepared had an average size of 21 nm. The seed particles were then cross-linked using DVB, before styrene was added continuously to the cross-linked seed latex to yield anisotropic particles. The size of the bulge was conveniently adjusted by the amount of monomer being fed and was not restricted by the size of the cross-linked seed particles. Once the bulge of the anisotropic particles prepared was cross-linked, tri-lobed anisotropic nanoparticles were synthesized. The continuous monomer addition method produced shape anisotropic particles at high solid content and was more convenient and simple than the batch method. The flexibility of the continuous addition method made it attractive for application in industry. The drawback of this method was that large micrometric polymer beads were formed as well during the seed particles synthesis, which needed to be removed by centrifugation. And finally, miniemulsion polymerization was used to synthesize cross-linked seed particles. This approach proved to be advantageous over the continuous addition method, with no micrometric polymer beads observed during the seed latex synthesis. As DVB was added during the homogenization of the monomer and amphiphilic diblock copolymer, the cross-linked seed was conveniently polymerized in a single step. Styrene was then added by batch or in a continuous manner, while TFEMA was added only by batch. The size of the bulge formed increased with the continuous addition of styrene to the cross-linked seed latex. The anisotropic/Janus nanoparticles prepared with a TFEMA bulge imparted the largest reduction on the surface tension of water. The anisotropic or Janus nanoparticles prepared by the continuous addition of styrene were less surface active than those prepared using batch addition of styrene. The decomposition products resulting from the water-soluble initiator used rendered the bulge produced more hydrophilic, resulting in particles with lower surface active properties than expected. Miniemulsion polymerization was a simpler and quicker method to form the cross-linked seed particles compared to the other two methods tested, eliminating the need to purify the seed latex from large micrometric polymer beads as required in the second method. Through the methods described in this Thesis, it was possible to synthesis for the first time small amphiphilic Janus nanoparticles of less than 20 nm measured through the longest dimension. Shape anisotropic particles and amphiphilic Janus particles of more than 40 nm could be directly used in industrial applications thanks to the fast and simple synthesis method and in the case of continuous addition, to the high solid content of particles synthesized.