PicotBenoit Effect in Polymer Solutions

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Picot-Benoit Effect in Polymer Solutions
Greg Beaucage- Dept. of Chemical and Materials
Engineering, University of Cincinnati. Sathish
Sukumaran - MPI, Mainz Germany. Jan Ilavsky
Purdue University/UNICAT, Argonne
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UNICAT Experiment
Use Extended q-range Bonse-Hart Camera at
APS/UNICAT To Bridge Static LS/SAXS Gap, 0.0001
Å-1 1 µm
Polystyrene in Cyclopentane at 23C Close to
q-solvent Dilute, Semi-dilute, Concentrated
Wide range of molecular weights 1,000, 668,
483, 100, 35, 2.5 kg/mole
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Abstract Several authors have noted that
excess intensity at low angles is observed in
small-angle x-ray and neutron scattering from
polymer solutions. This scattered intensity
beyond Lorentzian or Debye descriptions of the
polymer coils themselves can occur at size-scales
larger than an extended polymer chain indicating
heterogeneities on large scales. Several
authors, Bansil, Picot/Benoit, and Korberstein,
have characterized these large-scale fluctuations
and attempted to develop a theoretical basis for
these observations based on critical phenomena.
In addition to static measurements using SAXS and
SANS dynamic light scattering measurements in
semi-dilute solutions indicates a long time
relaxation that may be related to these large
scale structural features. To date there is no
good theoretical description of this phenomena
despite it seeming to be a fundamental feature of
polymer solutions. At the UNICAT USAXS facility
we have completed a series of USAXS experiments
at extremely small angles aimed at understanding
the temperature, concentration and molecular
weight dependencies of these large-scale
structures. The resulting monotonic and
reproducible behavior in temperature and
molecular weight supports a thermodynamic basis
for this Picot-Benoit effect. More complicated
dependencies in concentration were also observed.
Bansil indicates Picot-Benoit Effect is due to
proximity of phase separation in Semi-Dilute
Solutions Boue indicates it may be a shear
effect We looked well above and well below the
entanglement molecular weight Looked at dilute,
semi-dilute and concentrated solutions Looked at
theta and marginally good solvents (temperature
scans)
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UNICAT USAXS Camera
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Prior Efforts
Pinhole camera could not directly observe size of
domains.
Pinhole SANS data again does not have sufficient
resolution for size
"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-31-109-ENG-38."