Application of an external electric field induces birefringence in a solution of polydiacetylene (poly-4BCMU) in toluene due to orientation of the macromolecules. The field induced orientation in the red phase indicates an anisotropic polarizibility tensor characteristic of a rod-like conformation. The birefringence due to the polymer in its coil conformation (yellow phase) is extremely weak. The transient response of the anisotropic light scattering was studied in the red phase after switching the electric field on or off. We find a free rotational relaxation time of ∼0.1 s, consistent with the rotational diffusion constant expected for rod-like polydiacetylene macromolecules. These results rule out the large aggregate interpretation proposed to explain the color change transition. The field induced birefringence data independently demonstrate the existence of the rod-coil conformational transition for polydiacetylene 4BCMU macromolecules in solution.
It is suggested that the high-transition temperatures (T, s) observed in the superconducting oxygen-deficient perovskites may be a consequence of highly anharmonic potentials. Evidence is cited which demonstrates that the harmonic approximation fails badly for Cu + ions in a ligand field with octahedral symmetry.Since Bardeen-Cooper-Schrieffer theory and its strong-coupling extensions are based on the harmonic approximation, the reasons for their failure to describe the Cu-based perovskites can now be understood. These conclusions suggest a new interpretation for the recent results in oxygen-isotope substitution experiments. This new interpretation may be consistent with an electron-lattice mechanism for superconductivity.Surprising similarities between the heme group from hemoglobin, hemocyanin, and the oxygen-deficient perovskites are discussed. Based on these ideas, the criteria for reaching high T, s in Ni-substituted perovskites are discussed. It is also suggested that nonperovskite materials, which have nearly degenerate, multiple-valley potential wells, may be potential high-T, superconductors.
We report experimental results on several polydiacetylenes and a polythiophene in solution which demonstrate a change in charge state associated with solvatochromic and thermochromic transitions. The planar conformations are charged and the nonplanar conformations are neutral. When fields of ≊600 V/cm were applied to solutions of polymer in chloroform–hexane mixtures or toluene (i.e., the polymer is in the extended state), the polymers plated out on one electrode. We have measured the current as a function of time associated with the deposition of the polymer to determine the quantity of charge per polymer. The time dependence of the current had two regimes which can be associated with two types of charged material in the solution: the charged polymer, which plates out, and an unidentified charge transfer species which does not plate out. Polydiacetylenes 4BCMU and 3BCMU in solutions of chloroform/hexane became positively charged. Poly-3-hexyl-thiophene also became positively charged. PDA-9PA and PDA-92NA became negatively charged. We found a charge of 3×10−4 e/monomer for 4BCMU, 7×10−3 e/monomer for 3BCMU, and 4×10−2 e/monomer for PDA-9PA. Using this effect, we can demonstrate that both rod and coil segments coexist on the same polymer chain in the intermediate stages of the rod–coil transition. We suggest a mechanism which can qualitatively account for this unusual behavior. We also describe novel experiments in which an electric field was used to drive the polymers across a boundary between two nonmiscible solvents. When the polymer crosses the boundary, it changes color. The possibility of using the plating effect for precise fractionation by electrophoresis is suggested.
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