Submicro-capsule Preparation Via Interfacially Confined Living Free Radical Miniemulsion Polymerization

A novel strategy for nanoencapsulation via interfacially confined controlled/living radical miniemulsion polymerization was proposed. The principle of the strategy is based on the self-assembly of amphiphilic molecules and reversible addition fragmentation transfer (RAFT) radical polymerization chemistry. The RAFT agent was designed to be amphiphilic and was used as a stabilizer of the miniemulsion. The resulted miniemulsion is composed of mini-droplets of monomer/core material oil solution with an average size of about 100 nm. The RAFT molecules are confined at the interface of water/oil. When a water-soluble initiator like KPS is added, water-soluble primary radicals are born in the water. After several additions of monomer, the radicals become surface active and enter the mini-droplets. As the RAFT agent is has very high chain transfer coefficient, the radical would transfer among the RAFT agents located at the interface of the oil/water. By this way, the radical remains to be anchored in the interface, so the polymerization is confined in the interface. The polymer chains then grow inwards gradually, leading to the formation of a polymer shell. Such a unique approach of nano-encapsulation is very efficient for obtaining well-defined core-shell morphology. In this thesis, styrene was used as the shell monomer and nonadecane as the core material. It was demonstrated that the interfacially confined RAFT miniemulsion was very efficient, and the factors influencing and controlling the encapsulation efficiency was studied.Based on the strong tendency toward alternative copolymerization of styrene (St) and maleic anhydride (MAn), three types of Poly(St-alt-MAn-b-St) RAFT agents (SMA-RAFT) with different chain lengths and block compositions were synthesized by the bulk copolymerization of St and MAn. The copolymerization was carried out at 60℃ with 1-phenylethyl phenyldithioacetate as a RAFT agent. SMA-RAFT 1 has only an alternative copolymer segment of St and MAn (hydrophilic segment) with the RAFT end group. SMA-RAFT2 has a hydrophobic segment of about 3 units St added to the alternative segment. SMA-RAFT3 has 6 PS units in the hydrophobic segment. The polydispersity index of the three SMA-RAFT agents is all below 1.13.By tuning the degree of ammonolysis and hydrolysis of SMA-RAFT 1, the effect of the SMA-RAFT1 hydrophilicity on the encapsulation efficiency was studied. It wasfound that when one in three of the MAn units were ammonolized and the rest units were partially hydrolyzed in the hot water, the resulted latex had a well-defined core-shell morphology. But either insufficient ammonolysis or excessive ammonolysis would cause second nucleation, leading to the formation of a large number of pure polystyrene particles with sizes around 20-50 nm. The low initiator concentration could destroy the encapsulation efficient, which needs futher investigations.As SMA-RAFT2 and SMA-RAFT3 are more hydrophobic than SMA-RAFT1, so they would not solve in the aqueous phase even they were totally ammonolized. When SMA-RAFT2 and SMA-RAFT3 were used, the 20~50nm polystyrene particles disappeared but some larger polystyrene particles were still observed.The addition of a small amount of anion surfactant (SDS) combined with the SMA-RAFT agents turned out to be a good method to improve the encapsulation efficiency. When 0.5wt% of SDS (based on monomer) and 0.3 mol% SMA-RAFT1 was used, the encapsulation is very successful. The nanoencapsules have high symmetry and intergrity. The ratio of core/shell is close to that calculated form the recipe and particle size distribution is narrow. However, when the amount of SDS increases, the encapsulation was deteriorated.The evolution of the shell thickness of the nanocapsules during the polymerization course was also investigated. The shell thickness was found indeed to increase gradually with monomer conversion. The success in achieving crosslinked shell is also demonstrated.We, for the first time, proposed and demonstrated the utility of an interfacial RAFT radical miniemulsion polymerization in nano-encapsulation. The principles and methodology behind this technique are readily scalable up and highly efficient. The "living" radical polymerization nature of the system offers great opportunities to tune the properties of the polymer shell like thickness, surface functionality, molecular weight, inner-wall functionality, and crosslinking simply by using semi-continuous polymerization technique.