Deswelling behaviour of ionic microgel particles from low to ultra-high densities

Abstract

The swelling of ionic microgel particles is investigated at a wide range of concentrations using a combination of light, X-ray and neutron scattering techniques. We employ a zero-average contrast approach for small-angle neutron scattering experiments, which enables a direct determination of the form factor at high concentrations. The observed particle size initially decreases strongly with the particle concentration in the dilute regime but approaches a constant value at intermediate concentrations. This is followed by a further deswelling at high concentrations above particle overlap. Theory and experiments point at a pivotal contribution of dangling polymer ends to the strong variation in size of ionic microgels, which presents itself mainly through the hydrodynamics properties of the system.

Modeling deswelling, thermodynamics, structure, and dynamics in ionic microgel suspensions

“…The electrostatic repulsion between the microgels is quantified by the reduced electrostatic coupling strength Γ el ≡ Z net λ B /a, which in the present study is in the range of 1-9, comparable to values for typical ionic microgel systems [4,5,51]. The electrostatic repulsion between the microgels is here strong enough that configurations of microgels that are in contact or overlapping are very unlikely, such that g(r ≤ 2a) ≈ 0.…”

“…For u eff and ε V , we use the analytic linear-response expressions of Denton et al [18,19], and for f p (a t ) the Flory-Rehner expression given in Eq. (5).…”

“…For weak crosslinking, the soft particle swells to an equilibrium size that can be substantially larger than that in the dry state. It has been shown experimentally [4][5][6] and theoretically [5][6][7][8][9][10][11] that the particles deswell with increasing concentration, which is most pronounced for low background ionic strength and smaller concentrations well below the threshold where the microgels start to overlap. This behavior distinguishes ionic microgels from nonionic ones, since for the latter deswelling (or interpenetration and facetting) is observed only at very high concentrations [12,13].…”

Soft particles in an electric field – a zero average contrast study

“…Hydrogenated, Hm, and deuterated, Dm, ionic poly(N-isopropyl-(acrylamide)), pNIPAM, particles were synthesised via precipitation polymerisation in the presence of the co-monomer acrylic acid, AA, and a surfactant 18 to control the internal charge and the final size of the particles, respectively, as described elsewhere. 11 During the synthesis N-isopropyl(acrylamide), NIPAM, was used as the main monomer and N,N-methylenebisacrylamide, BIS, was used as a crosslinker. The exact amount of the used chemicals can be seen in Table 1.…”

“…9 Due to the interplay between internal and external osmotic pressures and the sensitivity of the long dangling chains to the bulk ionic strength, ionic microgels have shown a complex response to their environment and variations in particle concentration, pH, ionic strength and temperature. 10,11 Previous work using ionic microgel particles have demonstrated their capability to form field-induced structures when subjected to alternating external fields. [12][13][14][15] These particles possess a strongly enhanced polarizability, caused by the combination of a large fraction of internal and external mobile counterions and the polarizability of the polymer network.…”