Are Thermoresponsive Microgels Model Systems for Concentrated Colloidal Suspensions? A Rheology and Small-Angle Neutron Scattering Study

Abstract

The structure of concentrated temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel suspensions has been investigated employing rheology and small-angle neutron scattering (SANS). A previously described model expression for the particle form factor P(inho)(q) is extended by a model hard sphere structure factor S(q), and the average radial density profiles phi(r) are calculated from the amplitude of the form factor A(q) and the structure factor S(q). By this procedure, a direct real space description of the spatial ordering in the neighborhood of a single particle is obtained. The overall particle size and the correlation length xi of the concentration fluctuations of the internal polymer network decrease with concentration, revealing the increasing compression of the spheres. Thus, the particle form factor P(inho)(q) of the swollen PNiPAM microgels depends on concentration. The particle-particle interaction potential does not change significantly between 25 and 32 degrees C. Even approximately 1 K below the lower critical solution temperature (LCST), the experimental scattering intensity distributions I(q)/c are described very well by the hard sphere structure factor when an equivalent hard sphere particle size R(HS) and volume fraction eta(HS) are used. Microgels with different degrees of cross-linking and particle size resemble true hard sphere behavior up to effective volume fractions of phi(eff) < 0.35. At higher effective volume fractions phi(eff) > 0.35 strong deviations from true hard spheres are observed. Interpenetration of the outer, less cross-linked regions of the soft spheres as well as particle compression occurred at higher concentrations. In agreement with this, the equilibrium colloidal phase behavior and rheology also has some features of soft sphere systems. At temperatures well above the LCST, the interaction potential becomes strongly attractive and the collapsed microgel spheres form aggregates consisting of flocculated particles without significant long-range order. Hence, an attractive interaction potential in concentrated suspensions of PNiPAM microgels leads to distinctively different structures as compared to attractive hard sphere colloids. When the peculiar structural properties of the PNiPAM microgels are considered, they can be used as model systems in colloidal science.

Interaction of gold nanoparticles with thermoresponsive microgels: influence of the cross-linker density on optical properties

“…Hence, p-NIPAM microgels consist of an inhomogeneous network structure with a gradient in mesh sizes. 33 Increasing the MBA content leads to a larger region of the particle within which the polymer network is highly cross-linked. Therefore, p-NIPAM 0.25 consists of a small highly crosslinked core and a larger, outer region where the network is more open.…”

Effective interactions between soft-repulsive colloids: Experiments, theory, and simulations

“…Various experimental approaches such as small-angle neutron scattering (SANS), 23 confocal laser scanning microscopy (CLSM), 16,17,24 or rheological investigations 10,25 have been used in the attempts to shed light on the effective interaction potential between microgels. Examples are an evaluation of structural data using CLSM at low effective volume fractions φ eff , 17,24 or the analysis of the φ effdependence of the shear modulus (G p ) in highly concentrated microgel suspensions based on rheological studies 10 and diffusing wave spectroscopy, 15 respectively.…”

Structure of marginally jammed polydisperse packings of frictionless spheres

“…While real-space experiments are rather * Frank.Scheffold@unifr.ch straightforward in two dimensions [16], they are much more difficult to carry out in three dimensions [17], particularly when other physical properties, such as mechanical strength or internal dynamic modes, need to be studied as well. For this case scattering techniques, often in combination with mechanical shear measurements, have long been methods of choice to study soft disordered materials in the bulk [21][22][23][24]. Although scattering methods are highly appropriate for soft systems, certain dynamic scattering methods can be highly sensitive to the rattlers in polydisperse systems, e.g., leading to ensemble-average mean square displacements that appear to relax more rapidly, compared to what would be predicted from macroscopic rheology measurements using the generalized Stokes-Einstein relation (GSER) [25] of passive microrheology.…”