The role of initiator on the dispersibility of polystyrene microgels in non-aqueous solvents

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. Electronic supplementary material The online version of this article (doi:10.1007/s00396-017-4023-y) contains supplementary material, which is available to authorized users.


Supplementary Information: Conductivity and Mobility
The conductivity of a solution, κ in S m −1 , depends on how many ions are present in the solution: where Λ m is the molar conductivity of the solution and c is the concentration of the electrolyte 1 .
Λ m varies with concentration. One reason for this is that the ions do not always fully ionize in solution. For strong electrolytes, where substances are fully ionized, the conductivity depends only slightly on the molar concentration whereas for weak electrolytes, where substances are not fully ionized, the conductivity is highly dependent on the molar concentration 1 . To gauge the extent of charging in the microgel samples, in the calculations that follow, it is assumed that in non-aqueous solvents, all the microgel particles ionise with either a +1 or -1 charge, see Fig. 1.
Another reason that Λ m depends on concentration is due to interactions between the particles. For strong electrolytes at low concentrations, the molar conductivity varies linearly with the square root of concentration, as described by Kohlrausch's law: where K is the correlation factor. Λ • m is the limiting molar conductivity, defined as the molar conductivity in the limit of zero concentration and can be expressed as the sum of contributions from individual ions: where Λ + and Λ − are the limiting molar conductivities of the cations and the anions respectively and v + and v − are the numbers of cations and anions per formula unit of electrolyte. The limiting molar conductivities of ions can be determined from the ionic mobilities (µ i ) using: where v is the valency of the ions. Faraday's constant is defined as F = N av e where N av is Avagadro's number and e is the elemental charge. The mobility of an ion is calculated using: where η is the solvent viscosity and r H is the particle hydrodynamic radius. Using Eq. 4 and Eq. 5 it is possible to link measurable and theoretical quantities and hence for a symmetric v + : v − electrolyte Eq. 3 becomes: The square root concentration dependence of the molar conductivity, seen in Eq. 2, can be explained in terms of the ionic atmosphere around each ion. When the central ion moves, the ionic atmosphere around it also moves, however it does not move immediately and thus the atmosphere becomes incomplete just in front of and behind the central ion. The centre of charge consequently becomes displaced and retards the moving ion, this is known as the relaxation effect. Additionally, the central ion experiences a viscous drag, which is enhanced in the presence of the ionic atmosphere because the ionic atmosphere moves in the opposite direction to the central ion. The enhanced viscous drag is know as the electrophoretic effect, which reduces the mobility and hence the conductivity of the ions 1 .
The Debye Hückel Onsager theory attempts to quantify these complicated concentration effects and leads to the Kohlrausch-like expressions: 1 with where is the electric permittivity of the solvent, R is the gas constant, T is the temperature. b=0.586 for a 1:1 electrolyte. The agreement of the Debye Hückel Onsager theory with experimental conductivities is good at very low molar concentrations 1 .
The Debye Hückel Onsager theory can be used to predict the conductivities of a suspension of charged microgel particles in solution using r H and η and assuming a 50:50 split of v = +1 and v = −1.