Colloidal templating : a route towards controlled synthesis of functional polymeric nanoparticles

Abstract

Template-directed synthesis of polymeric nanoparticles offers better control over particle morphology, shape, structure, composition and properties compare to the conventional emulsion polymerization routes. For the production of anisotropic polymer-clay composite latex particles and polymeric nanocapsules, exploring template-directed routes are of immense technological interest owing to the vast and rapidly expanding application fields of these morphologies and the associated disadvantages of the presently used approaches for their synthesis. Presently, templating approaches such as layer-by-layer deposition enable the controlled synthesis of a huge variety of composite latex particles and nanocapsules. However despite simplicity and versatility, the current approaches suffer from lengthy procedures rendering them often difficult to scale-up. The main objective of the research described in this thesis is to explore synthetic routes towards useful latex particle morphologies by combining emulsion polymerization with colloidal templating. Two different morphologies, anisotropic polymer-clay composite latex particles and polymeric nanocapsules were selected for the templated synthesis. As template material for the synthesis of anisotropic composite latex particles, gibbsite and Na-montmorillonite platelets were chosen, while unilamellar vesicles of dimethyl dioctadecylammonium bromide (DODAB) were selected for the templated synthesis of nanocapsules. A RAFT copolymer approach based on the adsorption and subsequent chain extension of amphipatic copolymer chains, synthesized by the RAFT process, on oppositely charged substrate particles (platelets and vesicles) was explored. Vesicles were also explored for morphosynthetic templating where the polymerization of various hydrophobic monomers inside the vesicle bilayer was performed. Use of surfactant vesicles in templating approaches requires an efficient and reproducible protocol capable to produce stable vesicle dispersions with relatively narrow size distribution. Two simple and readily available approaches, sonication and membrane extrusion, were explored for the preparation of DODAB vesicles. Sonication resulted in dispersions containing a mixture of lens-shaped vesicles and bilayer fragments with broad particle size distribution. Besides, sonication methods were only capable of handling small preparation volumes and were not reproducible. Membrane extrusion was found to be an efficient method capable of producing large unilamellar vesicles of desirable size ranges and with relatively low polydispersity and hence was selected as a model method to synthesize DODAB vesicles for templating studies. In the next step, DODAB vesicles were explored as template for morphosynthetic templating reactions. A series of different hydrophobic monomers were solubilized in the vesicle bilayers. Monomer solubilization was found to reduce the gel-to-liquid-crystalline phase transition temperature (Tm) of the vesicles, resulting in smoother, spherical vesicles, compared to the virgin vesicles where angularities and sharp edges in the bilayers were observed. Polymerization in DODAB vesicles resulted in phase separated vesicle-polymer hybrid morphologies (parachute and necklace). For the templated synthesis of anisotropic (flat) nanocomposite latex particles, a RAFT copolymer based templating/encapsulation approach was explored using gibbsite platelets as template. Using dibenzyltrithiocarbonate (DBTTC) as a RAFT agent, a series of amphipatic living random RAFT copolymers with different combinations of acrylic acid and butyl acrylate units were synthesized. These copolymers were adsorbed on to the oppositely charged gibbsite platelets and subsequently, taking advantage of their living RAFT moieties, were chain extended to form a polymeric shell by starved feed emulsion polymerization. Characterizations of the resulting composite latexes using techniques such as cryo-TEM revealed the successful formation of anisotropic (flat) latex particles. The most important factors found to have influence on the overall efficiency of the encapsulation include hydrophilic-lipophilic balance (HLB), chain length and monomer feed composition. More hydrophobic and long chain RAFT copolymers were found to give rise to more secondary particle formation. Monomer feeds comprising more hydrophobic monomer resulted in loss of control over platelet orientation which gave rise to the formation of armored morphology. The RAFT copolymer approach was successfully extended to encapsulate Na-montmorillonite platelets using cationic RAFT copolymers of dimethyl aminoethyl acrylate and butyl acrylate. For the templated synthesis of polymeric nanocapsules, the RAFT copolymer approach was explored on DODAB vesicles. For this purpose, anionic RAFT copolymers of acrylic acid and butyl acrylate were adsorbed on vesicles where they were subsequently chain extended by feeding desirable monomers under starved feed conditions. Characterization of the resulting latexes using cryo-TEM demonstrates that the approach was successful to form a polymer layer on the surface of DODAB vesicles leading to the formation of nanometer-sized hollow capsules. The hydrophilic-lipophilic balance (HLB) balance and monomer feed composition were found to have significant effects on the morphology of the vesicle-templated nanocapsules. Finally, responsive nanocapsules were synthesized using a monomer feed comprising methyl methacrylate (MMA), tertiary butyl acrylate (t-BA) and the crosslinker ethylene glycol dimethacrylate (EGDMA). Subsequent hydrolysis of the tertiary butyl ester units resulted in the formation of pH-responsive nanocapsules. Higher concentration of the crosslinker in the monomer feed gave rise to polymer segregation. The synthesized nanocapsules undergo reversible swelling upon changing the pH which makes them potentially useful system for controlled release applications.