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Alterations in Cage Structure with Addition of Polymer I. Adding small amounts of polymers to dense suspensions suppress local colloid “cage” coherence.

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Presentation on theme: "Alterations in Cage Structure with Addition of Polymer I. Adding small amounts of polymers to dense suspensions suppress local colloid “cage” coherence."— Presentation transcript:

1 Alterations in Cage Structure with Addition of Polymer I. Adding small amounts of polymers to dense suspensions suppress local colloid “cage” coherence II. Adding large amounts of polymers compresses cage (locally densify) Corresponds to Glass Melting GelFluid I II S(q*) c p /c p *  c R g /R PRISM Expt c p /c p * Local structure arrested Intermediate fluctuations suppressed Enhanced low angle scattering Insensitivity of gel structure at ALL measurable length scales to c p /c p *,  c q*q* Microstructure of Gels R g /R=0.078 c p /c p * = 0.6  c = 0.4 Hard Spheres No added polymer Lines : PY Traversing vertically at  = 0.4 Comparisons of Equilibrium Structure with PRISM LINES: PRISM R g /R=0.078 Excellent agreement with theory Fluid Structure Confocal microscopy (r-R ij )/R ij g ij (r) 1 1 # of nearest neighbors Fourier Transform Ultra small angle x-ray scattering neutron scattering coherence of local cage S(0) = dimensionless osmotic compressibility S ij (q) qR ij 1 q* ~ 2  /D q*q*  IsIs IiIi Scattering Theory Liquid State Integral Equation Approach p = monomer; s = sphere C pp C ps C ss g ps g ss g pp D =2R site-site density pair correlation function polymer self- structure factor Hard core colloid-colloid interaction p-p interaction depends on solvent quality Repulsive polymer segment-sphere interaction g ij (r) S ij (q) Fluid Structure and Thermodynamics PRISMPRISM Schweizer and Curro (1987) pp “polymer  random coil” u  (r) Athermal good Ideal Summary Equilibrium Theory and experimental measurements of S(q) agree. Non monotonic variation of “cage” coherence with increasing polymer concentration. Non-Equilibrium Scattering experiments suggest formation of heterogeneous regions as gel forms Ensemble-averaged structure is independent of strength and range of attraction after gelation. The UNICAT facility at the Advanced Photon Source (APS) is supported by the Univ of Illinois at Urbana- Champaign, Materials Research Laboratory (U.S. DOE, the State of Illinois-IBHE-HECA, and the NSF), the Oak Ridge National Laboratory (U.S. DOE under contract with UT-Battelle LLC), the National Institute of Standards and Technology (U.S. Department of Commerce) and UOP LLC. The APS is supported by the U.S. DOE, Basic Energy Sciences, Office of Science under contract No. W ENG- 38. Funding: DOE-BES via UIUC MRL NSF-NSEC (RPI-UIUC-LANL) S(q) ~ (qD)   ~ -3.5 at high  c Unphysical fractal exponent at high  c Fluctuations insensitive to c p /c p * and R g /R Porod-like exponents suggesting compact clusters with fractally rough surfaces? Departure from Porod-like decay occurs at intermediate length scale qD ~ c p /c p * = 0.6  c = 0.4 R g /R=0.078 clusters of size  c ~ 3-5 D Scattering At Small Angles Ensemble-averaged inter-particle spacing is “frozen” after gelation Experiments fall out of agreement with theory due to non-equilibrium transition local structural arrest suppression of intermediate scattering Signature of the Gel Not near spinodal --Why the upturn in S(q) at small q’s ? Clusters and Voids Debye-Bueche Analysis: Total Scattering Intensity : S(q)=S D-B (q)+S c (q) (inter-cluster) (intra-cluster) q q* Gel cc Voids Clusters  eff >  bulk vv Experimental System 100 nm Colloid: D = 100 ± 5 nm Sterically Stabilized Silica (Hard Spheres) Polymer: Polystyrene (near theta state) in decalin Gel Fluid-Crystal Fluid-Fluid No added polymer Gel Boundary Phase Diagram Aggregated Clusters Gel R g /R=0.078 The arrows indicate the path taken in X-ray scattering measurements. Introduction Colloid-Polymer mixtures are used in a number of industrial and scientific applications to achieve desired material properties. Understanding the effects of microscopic interactions provides tunable paramaters to manipulate the macroscopic properties such as the flow behavior governed by the underlying phase transitions. Suspensions can undergo both equilibrium and non-equilibrium phase transitions depending on the strength and range of these interactions. Strong short-ranged attractions often give rise to gelation whereas weaker longer ranged attractions result in equilibrium fluid-fluid or fluid-crystal transitions. Here we present a comprehensive study of the microstructure of the gel phase of a well characterized colloidal suspension by Small Angle X-ray Scattering as the gel boundary is traversed. The location of the boundary is controlled by carefully tuning the strength and range of interparticle interactions by adding non- adsorbing polymer. Fumed Silica Gel WeakStrong Hair Gel (Cosmetics) Toothpaste (Hygiene) Microstructure of Dense Colloid Suspensions and Gels S. Ramakrishnan, S.A. Shah, Y.-L. Chen, K.S. Schweizer, and C.F. Zukoski, Department of Chemical and Biomolecular Engineering, University of Illionois at Urbana-Champaign, IL 61801, Jan Ilavsky Department of Chemical Engineering Purdue University, IN 47097, Pete R. Jemian, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, IL 61801


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