Experimental Techniques for Characterizing Adsorbed Polymers

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Experimental Techniques for Characterizing
Adsorbed Polymers

• Brian Makowski

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Small Angle Neutron Scattering (SANS)
General Definition
A beam of neutrons is directed at a sample and
scattered by changes of refractive index.
Scattering occurs on a nanometer level and
involves interaction of neutrons with atomic
nuclei.
Neutrons of all atoms are of comparable size so
this method is favored over Small Angle X-ray
Scattering (SAXS).
www.hmi.de
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Small Angle Neutron Scattering (SANS)
Application to Adsorbed Chains
Recognized as most efficient way of probing
interfacial layers.
The amount of polymer at the surface, G, and the
thickness of the layer, h, can be determined from
the scattered intensity, I(q).
q2I(q) 2?(S/V)(np-ns)2G2(1-q2h2/a)
D distance between grafting sites
D (6.023G/M)-1/2
h thickness of layer
hD2 M
Auroy, et. al. Physical Review Letters. 1996,
66(6).
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Isothermal Titration Calorimetry
Similar to Differential Scanning Calorimetry
(DSC).
Adiabatic calorimeter with reference and working
cells.
Reference cell contains blank solution medium,
working cell contains dispersion of crystal
surfaces.
Polymer is injected and change in heat flow is
monitored as polymer adsorbs to crystal surface
(no sedimentation occurs).
Dimova, et. al. Langmuir. 2003, 19, 6097.
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Isothermal Titration Calorimetry
Calculations
Heat produced in working cell arises from
adsorption of polymer to surfaces.
The amount of polymer can adsorbed to the surface
can be calculated from the change in heat flow
where S surface site and P polymer
S P SP
define Cp polymer concentration Cfs free
surface area Csp occupied surface area Cs
concentration of crystal site
K Csp/(CfsCp)
Cs Csp Cfs
?h ?CspV?H
?Csp can now be solved for.
VinjCp0
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Infrared Spectroscopy
Stretching vibrations provide a direct measure of
the surface concentration as a function of the
bulk concentration. Reflected beam interacts with
molecules sitting at the surface. The spectra
obtained can also provide information on the
structure and orientation of molecules in the
surface layer.
Angle of reflectance 0-90.
Ren, et. al. Macromolecules. 1995, 28(1).
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Infrared Spectroscopy
Need to compare reflectance of sample with
reflectance of a known standard.

Number of chains at surface for a known standard
were calculated using a different method.
where N number of chains at surface As
reflectance of polymer from IR As0 reflectance
of reference from IR N0 number of reference
chains at surface
N As No
0.9Aso