Preparation of Polystyrene Nanoparticles by Seed Polymerization Using Amphiphilic Random Copolymer Micelles as Seeds

Abstract

In recent years, with the progress of techniques to prepare monodisperse fine polymer particles, various colloidal polymer particles are being used for practical applications, such as surface coatings, cosmetics, surface modifiers for photo films, and carriers for virus diagnosis. The sizes of the polymer colloidal particles prepared so far lie in the range from 10 nm– 10 mm depending on the method employed, which includes emulsion polymerization, microemulsion polymerization, miniemulsion polymerization, soap-free polymerization, dispersion polymerization, seed polymerization, and suspension polymerization. Among these methods, microemulsion polymerization is the most commonly-used technique to prepare polymer colloids with diameters <100 nm and narrow size distributions. Such small polymer particles are desirable for surface coatings because it takes only a short time for film formation and the particles can penetrate easily into porous substrates producing a film with good optical and mechanical properties. For pharmaceutical applications, colloidal polymer particles are expected to be used as a long-circulating drug delivery vehicle because particles smaller than 100 nm can be prevented from being detected by the human body’s reticuloendothelial system. However, a disadvantage of the microemulsion polymerization is that one has to use a large amount of a surfactant and a cosurfactant, i.e., nearly comparable amounts of a surfactant and a monomer may be necessary to be used. This often creates a problem of the purity of final products. In order to overcome this disadvantage, reactive surfactants are often used. These surfactants have a reactive group being able to react with a growing polymer chain during radical polymerization, resulting in a covalent incorporation of surface active molecules into the polymer chain. Another possibility to surmount this disadvantage is to use polymeric amphiphiles in place of conventional surfactant molecules. Although no covalent bonding is formed between the polymer amphiphile and resulting polymer particle, the polymer amphiphiles are strongly anchored to the surface of resulting particles through hydrophobic interactions. A number of studies using various amphiphilic block copolymers have demonstrated that they act as a powerful stabilizer in emulsion polymerization. In aqueous emulsion polymerization, hydrophobic block sequences can anchor onto the particle surface while the hydrophilic blocks extend into the water-phase and create a hydrophilic shell. Recently, the use of block copolymer micelles as a seed for emulsion polymerization has been proposed. With the seed polymerization, it may be possible to control the final number of the particles, and hence the size of the particles, whereas in the case of emulsion polymerization with use of conventional surfactants, the particle size depends on many parameters and thus the size can not easily be predicted. Comparing to amphiphilic block copolymers, amphiphilic random copolymers are much easier to synthesize, and one can choose a much wider range of monomers for their synthesis. Therefore, it should be worthy testing amphiphilic random copolymers for seed polymerization. There is a general trend for amphiphilic random copolymers to form micelle-like aggregates on the nanometer scale in aqueous solution due to mainly intramolecular hydrophobic associations, the association properties depending strongly