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

M.A. Stieger, J.S. Pedersen, P. Lindner, W. Richtering

Research output: Contribution to journalArticleAcademicpeer-review

210 Citations (Scopus)

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 Pinho(q) is extended by a model hard sphere structure factor S(q), and the average radial density profiles f(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 ¿ of the concentration fluctuations of the internal polymer network decrease with concentration, revealing the increasing compression of the spheres. Thus, the particle form factor Pinho(q) of the swollen PNiPAM microgels depends on concentration. The particle-particle interaction potential does not change significantly between 25 and 32 °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 RHS and volume fraction ¿HS are used. Microgels with different degrees of cross-linking and particle size resemble true hard sphere behavior up to effective volume fractions of feff <0.35. At higher effective volume fractions feff > 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.
Original languageEnglish
Pages (from-to)7283-7292
JournalLangmuir
Volume20
Issue number17
DOIs
Publication statusPublished - 2004

Keywords

  • aqueous polymer-solutions
  • block-copolymer micelles
  • volume phase-transition
  • core-shell colloids
  • contrast variation
  • hard-spheres
  • soft spheres
  • x-ray
  • dispersions
  • crystals

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