A General Route To Synthesize The Polymeric Nanocapsules-RAFT Interfacial Miniemulsion Polymerization

(Hollow) Nanocapsules have shown highly promising applications in a large variety of fields due to the special nanostructures. RAFT (Reversible Addition/Fragmentation Transfer) interfacial miniemulsion polymerization offers a straightforward method to synthesize nanocapsules. Based on the self-assembly of amphiphilic macro-RAFT agents on the interface of droplets/water and the radical polymerization was confined on the interface, a polymeric shell was in-situ formed. This method was not only able to precisely control the structures of nanocapsules, but also offerred such advantages as a simple and environmental process, which is highly efficient, easily scale-up. In principle, this method should offer a generally robust way to synthesize naocapsule, but lots of solid particles were found in many experimental studies, of which the mechanism was not clear yet. In the current thesis, the formation mechanism of the solid particles accompanying the nanocapsules was investigated, aiming at developing RAFT interfacial miniemulsion polymerization as a versatile facile way to synthesize nanocapsules of high selectivity. The following results were achieved:(1).The formation mechanism of the solid particles was proposed that some of the amphiphilic oligomer radicals (Amphi-R'), which were formed from the reaction of macro-RAFT agent anchored on the interface of droplets with the primary radicals, would desorbed into the water phase. Since it took longer time for the radicals (Amphi-R·) to re-enter into the droplets than those water-phase radicals in the conventional (mini)emulsion polymerization, the Amphi-R'would induce the homogenuous nucleation, leading to the formation of the solid particles. Based on the RAFT interfacial miniemulsion, the equations of estimating the feature time of the chain radical for addition/precipitation (τp) in water phase and the feature time of radica entry into the droplets (τe) were developed. The equations predicted that the occurrence of the homogenuous nucleation would depend on the initial droplet size and the molecular structure of the RAFT agent.The interfacial RAFT miniemulsion polymerization respectively mediated by two kinds of macro-RAFT agents, i.e. poly(AAm-b-Stn)RAFT and poly(MAAi6-co-St7)RAFT has been fully investigated in terms of polymerization kinetics, droplet/particle size distibution evolution, molecular weight distribution evolution, and the selectivity of nanocapsules. In the case of poly(AAm-b-Stn)RAFT, only when the diameter of the initial droplets was around 100 nm, the high purity nanocapsules would be synthesized. In the case of poly(MAA16-co-St7)RAFT agent, pH values played a key role to increase the selectivity of the nanocapsules. As pH changed from 8.12 (where, the carboxyl of RAFT agent was fully neutralized) to 6.45, the fraction of the nanocapsules increased from 0.18 to 0.94 respectively due to the suppression of the Amphi-R'desorption. These observations verified the mechanism of the homogeneous nucleation, which was also supported by the polymerization kinetics, droplet/particle size distibution and molecular weight distribution evolution.Also, it was found that replacing water soluble initiator KPS with oil soluble initiator AIBN and introducing the water soluble radical scavenger (NaNO2) could increase the fraction content of nanocapsules, which again suggested that the homogenuous nucleation should be the formation mechanism of the solid particles.(2).Guided by the above insightful understandings, the robust control over the nanocapsule structures was demonstrated. Firstly, the usage of RAFT agent how to influence the size of nanocapsules was investigated in the model system with St as the monomer, ND as the core materials and poly(MAA16-co-St7)RAFT as the RAFT agent at pH=6.45. The size could be tuned from 112 to 480 nm. Secondly, post-addition of SDS to the prepared miniemulsion how to influence the particle size distribution was studied in the model study with the St as the monomer, hexadecane (HD) as the core materials and poly(AA2-b-St2)RAFT as the RAFT agent at pH=8.3. The nanocapsules with uniform particle size could be obtained via centrifugation. The average size of nanocapsule was about 112 nm with the size distribution in the meaning of coefficient variation (CV) as 5.4%, the thickness of polymeric shell was about 20 nm. Thirdly, the core/shell ratio of nanocapsules could be changed from 1/4 to 1/1. When the core/shell ration further increased up to 2/1, the polymeric shell became too thin to support the structures. Fourthly, cross-linking agents how to influence the strenghth of nanocapsule was studied. Nanocapsules with highly cross-linked polymeric shell were synthesized simply by addition of the cross-linking agents. The shell strenghth of these nanocapsules was increased and hollow nanocapsules could be obtained by using solvent to extract out the core materials.(3).RAFT interfacial miniemulsion polymerization was expanded into the inverse miniemulsion polymerization and temperature-sensitive nanocapsules with PNIPAM encapsulation of Na2SO4 were synthesized. The diameter of nanocapsules was about 100-250 nm, and the thickness of polymeric shell was about 30 nm.