Abstract
Non-aqueous microgel particles are commonly synthesised in water, dried, and then redispersed in non-aqueous solvents. An important factor to consider when synthesising such particles is the initiator, which can determine how well the particles disperse in solvents. Polystyrene microgel particles were made with three different initiators. When a neutral, oil soluble initiator (azobisisobutyronitrile) was used the particles dispersed in toluene as well as cyclohexane and decalin. In contrast, anionic, water-soluble initiators (potassium persulfate or azobis(4-cyanovaleric acid)) created particles that only redispersed in toluene and not the other two solvents. Of the three considered, toluene is the best solvent for polystyrene and also has the highest polarizability, making it most effective at redispersing particles with polar or ionisable functional groups. Zeta potential and conductivity measurements, however, did not detect a direct relationship between particle charging and redispersibility. Oil soluble initiators result in “inside out” polymerisation where the initiator groups are buried inside the growing particle, whereas water-soluble initiators result in “outside in” polymerisation, with the polar initiator groups residing on the particle surface. By tailoring the ratio between water and oil soluble initiators, it may be possible to synthesise microgel particles with uniform or designed charge profiles from the core to the surface.
Highlights
Non-aqueous polymer microgels are often made in poor solvents, notably water, where they are un-swollen, and transferred into good solvents where the particles swell [1]
M1, M5 and M6 microgel particles were made via a microemulsion polymerisation [10], M2 microgel particles were made via an emulsion polymerisation [11] and M4 microgels were made via a surfactant free emulsion polymerisation [1, 12]
Toluene is a better solvent for polystyrene than cyclohexane or decalin and it is expected that the particles will be largest in toluene
Summary
Non-aqueous polymer microgels are often made in poor solvents, notably water, where they are un-swollen, and transferred into good solvents where the particles swell [1]. The polymer–solvent interaction parameter, χ is a key factor that controls whether or not particles will redisperse in solvent. The choice of both the monomer and solvent governs the χ parameter, as does the temperature. To what extent particles do swell in good solvents is determined by the molar mass between cross-links, as shown by the FloryRehner theory [2]. There are two main ways by which the initiator can influence the properties of a microgel particle. Nair [5] made microgel particles using two different initiators, 4,4 -azobis(4-cyanovaleric acid) (ACVA) and potassium persulfate (KPS) and found that particles made
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