Picot-Benoit Effect in Polymer Solutions

1. Picot Benoit Effect was Thought to occur in semi-dilute regime for moderate molecular weights Preliminary Studies had Limited Statistics New Camera Program to Enhance Resolution of Weak Signals Fit is based on the Picot/Benoit Structure being “Composed” of The Polymer Coils 30 mg/ml PS in Cyclopentane Temperature Dependence of Rg,2 (Cluster Size) in Cyclopentane USAXS instrument at UNICAT Dilute PS in Cyclopentane USAXS instrument is too compact to photograph well Previous Studies Tc CP -35°C Tb CP 50°C Tq PS/CP 22°C DOA initially Increases then Plateaus/decays In Temperature G2 ≈ NV2Dr2 483 kg/mole R0 = 465 Å Rg,1 = 190 Å(Calculated) Rg,2 ≈ 1,000 - 3,000 Å “Cluster” displays Porod Scaling: Dense domains with Smooth Surface 5 to 15 coils in diameter For a 3-d domain 100 to 3000 coils per “blob” Cluster is larger than an extended coil Data Shown is Low MW Dilute DOA ≈ 1/M0.6 Picot- Benoit Gaussian Coil 2q Picot-Benoit SAXS/SANS and DLS (Slow t) 0.3 – 1.0 m 0.6 m ~25 m ~25 m sample Static SALS Log I 10 15 We Observe Porod Domains That are Modeled as Composed Of Coils (this is needed for good fit) -5/3 I0 We wish to quantify this effect Benoit Proposed that this was Most important at high concentrations And might be related to shear history “Cluster” is associated with the critical point but does not follow correlation length scaling in temperature. Cluster grows with temperature Log q Bansil used Debye-Bueche (didn’t have the Guinier Knee) at low-q due to instrumental limitations in prior work H. Benoit, C. Picot Pure Appl. Chem. 12 (1966) 1271. X-rays in Max Planck Institute For Polymer Research “Blob” is larger for higher concentration http://www.uni.aps.anl.gov/~ilavsky/sas.htm 11 Standard 1-D collimation setup*slit-smeared geometry (a.k.a. Bonse-Hart**) 2 • Post-measurement desmearing calculationJ.A. Lake; Acta Cryst 23 (1967) 191-194 • Typical slit-length ~ 0.05 Å-1 * G. G. Long, A. J. Allen, J. Ilavsky, P. R. Jemian, and P. Zschack, in CP521, Synchrotron Radiation Instrumentation: 11th US National Conference, P. Pianetta and H. Winick, Eds., AIP, College Park, 183 (2000). ** U. Bonse & M. Hart, Appl. Phys. Lett.7, 238 (1965) and Zeit. f Phyzik189, 151 (1966). 7 12 8 13 4 16 Summary: -Synchrotron Bonse-Hart cameras can offer a new window into major unresolved issues in polymer solutions -The excess low-q scattering first noted by Picot and Benoit can be resolved as 3d domains composed of clusters of polymer coils (data can not be fit with other models) -These clusters are intimately tied to the coil structure as evidenced by the molecular weight and temperature dependencies -This does not appear to be a phenomena limited to semi- dilute solutions, sheared solutions, or molecular weights above the entanglement molecular weight 9 14 Scattering Model is based on a Cluster of Coils Sharp interface at size scales of observation of the cluster 2 level structure Primary, Gaussian Coils Secondary, Porod Domains Composed of Primary G. Beaucage, J. Appl. Cryst. 29, 134 (1996) 30, 4158 (1997) 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 5 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 23°C Close to q-solvent Dilute, Semi-dilute, Concentrated Wide range of molecular weights: 1,000, 668, 483, 100, 35, 2.5 kg/mole 1 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) 6 UNICAT USAXS Camera 3 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."