Colloidal and polymeric contributions to the yielding of dense microgel suspensions

Abstract

Hypothesis: Soft microgel colloids can be densely packed since particle networks can compress and interpenetrate. This evolution of the particle's internal structure associated with packing is expected to determine the linear viscoelastic properties and the yielding behavior of dense suspensions of microgel colloids. Experiments: We investigated the volume fraction-dependent linear and non-linear rheological response of suspensions of soft core-shell particles formed by a poly(Nisopropylacrylamide) (PNIPAM) microgel core and a thin poly(ethylene glycol) (PEG) shell. Findings: The linear viscoelasticity of suspensions reveals a transition from a fluid to a jammed glass state. Increasing volume fraction within the jammed state, the linear storage modulus and the yield stress show distinct regimes associated with the evolution of particle contacts, which involve progressive compression and interpenetration of the shell and core. The yielding of jammed suspensions occurs in two-steps: At small strains jammed cages are rearranged, while full disentanglement of interpenetrating networks only occurs at large deformations and results in fluidization. Yield strains and stresses increase with increasing shear rate or frequency, suggesting a progressive dominance of the timescale associated with shear over that associated with the internal dynamics of the system.

Link between Morphology, Structure, and Interactions of Composite Microgels

“…Note that this reduction would be significantly larger in pure PNIPAM particles synthesized with the same method and cross-linking density . The experimental data in ref correspond to measurements of the hydrodynamic radius using dynamic light scattering. The value of R g obtained through the modeling of the SANS experimental data presented here shows a smaller relative reduction, suggesting that the R g value estimates the size of the denser core of the particles.…”

“…The presence of a depleted region prevents PNIPAM monomers from concentrating in the center of the microgel, thus diverting the collapsed regime volume, as we can observe at α = 1.5. The experimental values of R H / R H max versus T , with R H max being the hydrodynamic radius measured at T = 20 °C, are reported for comparison in the inset of Figure b ( R H data taken from ref ). The extent of the R H reduction is closely comparable to that found for the inside case in the simulations.…”

“…Deuterated water was chosen to increase the contrast in neutron scattering measurements. Particle characterization obtained by dynamic light scattering in a previous work showed that the hydrodynamic radius R H ≈ 166 nm at small T and that the volume phase transition (VPT) occurs at T c ≈ 36 °C, leading to R H ≈ 90 nm at high T. 30 This value of T c is sensibly larger than that usually found for standard PNIPAM microgels (T c ≈ 32 °C).…”

“…The vertical red line is placed at α ∼ 0.65, indicating the occurrence of the VPT for a pure PNIPAM microgel. Inset: Experimental swelling curve reporting R H / R H max versus T , where R H data are taken from ref .…”

Linear and nonlinear viscoelasticity of concentrated thermoresponsive microgel suspensions

“…Others have also studied their transition from a repulsive glass-like to attractive gel-like state upon heating a highly packed pNIPAM microgel suspension beyond the LCST [15], [20], [21], [55][56].…”