Poly-ethylene-vinyl alcohol microgels prepared through salting out: Rationalizing the aggregation process and tuning the microstructural properties

The ion-specific effect on the solubility of macromolecules in water, rationalized through the Hofmeister series, can be used as an effective method to obtain polymer particles with desired microstructural properties thanks to the so-called salting out effect. By choosing the proper salt and optimizing the polymer and the salt concentration, it is possible to tailor the particle structural features like the radius and the degree of compactness and to speed up the aggregation process until complete conversion from free chains to aggregates. Here we rationalize the segregation of poly-ethylene-vinyl alcohol (EVOH) chains in the presence of sodium chloride (NaCl). The effect of polymer and salt concentrations on the kinetics and the thermodynamics of the aggregation process is considered. In particular, we show how such parameters influence the microstructural properties of the particles through a combination of light and neutron scattering techniques as well as microscopy. We found that by increasing sodium chloride concentration it is possible to speed up the aggregation process, obtaining a complete conversion from free chains to aggregates at shorter times, whereas increasing poly-ethylene-vinyl alcohol concentration from 0.70% w/w to 1.30% w/w determines an increase of the microgel size of a two-times factor. Moreover, NaCl concentration directly affects both the packing degree of the polymer chains within the aggregate and the morphology of the polymer particle, leading to a more compact structure at higher NaCl concentrations. Understanding how it is possible to tune both aggregation kinetics and EVOH particle microstructure is fundamental in order to give a general picture which could be broadened to the class of vinyl alcohol-based polymers.