#
# arev:/usr/local/unicat/unireq/beamreq/req00137.txt
# UNICAT Member Beam Time Request  #137

# created Fri Sep 22 13:05:07 CDT 2000
#

collaboration:
	Yes

collaborator+Pete:
	on

contact:
	czukoski@uiuc.edu

days:
	3

description:
	Microstructure in Suspensions Experiencing Depletion Interactions
	
	The addition of soluble, nonadsorbing polymers to suspensions induces attractions between particles.  The attractions arise due to the exclusion of polymers from the volume between particles in close proximity and thus are referred to as depletion interactions. Recent theoretical advances allow prediction of suspension microstructures. We have initiated USAXS experiments on the UNICAT beam line to test these models.  These preliminary studies were used to establish the parameter space where high quality USAXS data could be gathered.  In the proposed research these results will be expanded to a broader parameter space.  In particular we are interested in characterizing suspension structure as the polymer approaches its theta point and changes in microstructure that occur as the suspension approaches and passes through a gel transition. 
	
	Our initial studies demonstrate that USAXS microstructural data and models are in good agreement for small scattering angles.  At larger angles, the predictions and models are significantly different. By working with hard sphere suspensions of 100 nm octadecyl silica in toluene and cyclohexane where the structure factors are well understood we will refine out experimental technique to ensure that structure factors gathered are of the highest possible quality.  Polystyrene will then be added to the suspensions and changes in microstructure characterized as a function of particle volume fraction, polymer concentration and molecular weight, and suspension temperature.
	
	 In cyclohexane, polystyrene has a theta point of ~35 C.  As the theta point is approached, large fluctuations in polymer density occur.  These are predicted to greatly magnify the strength and range of depletion interactions.  Microstructural evidence for this effect will be explored.  
	
	With increasing polymer concentration, suspensions of particles are observed to pass through a fluid/fluid phase transition driven by the depletion interactions.  However, as the polymer radius of gyration drops to a small fraction of the particle size, rahter than showing two distinct phases, suspensions gel. In the proposed study we will explore microstructural origins of this transition.
	
	Our initial studies for the basis of a publication that is in preparation demonstrating the agreement of USAXS and light scattering determination of suspension compressibilities in depletion systems.  These independent measurements compare favorably with model predictions.
	

equipment+required:
	Thermostatable SAXS cell holder

experiment:
	Microstructure in Suspensions Experiencing Depletion Interactions

foreign+nationals:
	Subramanian Ramakrishnan
	Ali Shah
	Narendra Dixit

hazards:
	The samples are suspended in small volumes (<1ml) of organic solvents.  
	No waste will be generated in these experiemnts and there will be limited evaporation as this alters the polymer and particle concentrations. 

name:
	Charles F. Zukoski

nonmembers:
	

station:
	33ID-D

unacceptable+dates:
	Nothing before Oct 31.
	November 12-17



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