Theoretical Aspects of Particle Swelling in Living Free Radical Miniemulsion Polymerization

Abstract

Living free radical reactions originally were carried out in bulk or solution. Recently, this chemistry has been carried out in water-based systems (emulsion and miniemulsion living free radical polymerization). Significant colloidal instability early in the polymerization has been found in several of these systems. With the hypothesis that something unique to living free radical polymerization was causing colloidal instability beyond that found in conventional free radical emulsion polymerization, the swelling of polymer particles during the early stages of living free radical miniemulsion polymerization was investigated. A new superswelling state caused by the presence of a large number of oligomers was found. This superswelling could be used to explain the instability issues reported. The effect of various factors on the superswelling state was studied. It was found that superswelling state is rather sensitive to recipe variations. Simply increasing the costabilizer level and/or using a nonionic polymeric surfactant would probably eliminate superswelling and hence the instability.

Nitroxide‐Mediated Controlled Free‐Radical Emulsion Polymerization of Styrene and n‐Butyl Acrylate with a Water‐Soluble Alkoxyamine as Initiator

“…As shown in Table 1, both experiments are also reproducible from the particle-size viewpoint, although the diameters are rather large and the particle-size distribution broad, in contrast to what is expected in a classical emulsion polymerization. Nucleation might lead to a change in the thermodynamic properties of the system, in close analogy with the theory of "superswelling" proposed for miniemulsions by Luo et al [17] The presence of a large concentration of oligomers in the growing particles during the early stages of the reaction would lead to a lower chemical potential than in the non-nucleated micelles or droplets, which would enhance swelling of the former by the monomer. This would favor the formation of large particles with a broad particle-size distribution.…”

Block copolymer synthesis by controlled/living radical polymerisation in heterogeneous systems

“…through the aqueous phase. However, for CLRP in miniemulsion, one of the main issues initially encountered was superswelling 57 , which leads to colloidal instability and phase separation, particularly in the case of RAFT-controlled polymerisations. This issue can be overcome by careful choice of polymerisation conditions.…”

Recent advances in controlled/living radical polymerization in emulsion and dispersion

“…159 Superswelling hypothesis has been theoretically and experimentally explored to explain the observed loss of colloidal stability during RAFT miniemulsion polymerization. 160 This hypothesis is based on slow nucleation in miniemulsion CRP including RAFT polymerization, resulting in a large concentration of oligomer radicals in monomer droplets in the early stage of polymerization. Because the oligomeric species are very effective swelling agents with monomers, the presence of the oligomers results in the formation of micronsized oligomer particles.…”