Chapter 20
Downloaded by RUTGERS UNIV on December 30, 2017 | http://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch020
SANS Studies of Polymers in Organic Solvents and Supercritical Fluids in the Poor, Theta, and Good Solvent Domains 1
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Y. B. Melnichenko , E. Kiran , K. Heath , S. Salaniwal , H. D. Cochran , M. Stamm , W. A. Van Hook , and G. D. Wignall 3
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Solid State Division, Oak Ridge National Laboratory*, Oak Ridge, TN 37831 Department of Chemical Engineering, University of Maine, Orono, ME 04468 Max Planck Institut für Polymerforschung, 55021 Mainz, Germany Chemistry Department, University of Tennessee, Knoxville, TN 37996
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We demonstrate that semidilute polymer solutions in supercritical fluids (SCFs) reproduce all main features of the polymer behavior in organic solvents which is indicative of the intrinsic similarity between the thermodynamic properties of polymers in SCFs and in the far sub-critical liquids. Using small-angle neutron scattering, we studied the effect of temperature and pressure on the phase behavior of polystyrene (PS) in organic solvents as well as of poly(dimethylsiloxane) (PDMS) in supercritical carbon dioxide (SC C0 ). The radius of gyration R of polymer chains in both organic solvents and in SC C 0 is invariant during both temperature and pressure quenches down to the critical point of demixing. The limit of infinite polymer miscibility (the Θ condition) of PDMS - SC C 0 solutions may be reached by varying the pressure (P =52±4 MPa at the density of SC C 0 pco2 = 0.95 g/cm ) and or the temperature (Τ = 65±5 °C). A sharp crossover between the critical and mean field behavior in PDMS - SC C 0 is similar to that in solutions of PS in cyclohexane, and at T> Τ , P>P the solutions reach the good solvent domain where R expands beyond the unperturbed dimensions R (Θ) at the Θ condition. 2
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* Managed by Lockheed Martin Energy Research Corporation under contract DEAC05-96OR-22464 for the U. S. Department of Energy. © 2000 American Chemical Society
Cebe et al.; Scattering from Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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Introduction In organic solvents, it is well known that the radius of gyration, R (i.e. the r.m.s. distance of scattering elements from the center of gravity) of polymer molecules depends on the sign and magnitude of the interactions between the chain segments and the molecules of the surrounding liquid. In "good" solvents, the dominating repulsive forces between the segments (excluded volume effects) work to expand the R , and the second virial coefficient (A ) is positive. In less favorable solvents, the pairwise attractive and repulsive segment interactions may compensate at the "Flory" or "theta temperature" (Τ ), where A =Q, and R corresponds to the dimension of a volume-less polymer coils, "unperturbed" by the excluded volume effects. By definition, Τ is the critical solution temperature (Tc) for a polymer with the molecular weight Mw = and thus corresponds to the threshold of unlimited polymer - solvent miscibility. One of the first significant applications of SANS was to confirm Flory's prediction that polymer chains would adopt such random-walk configurations in the condensed (amorphous) state [1]. In the poor solvent regime (Τ
