A rod-to-coil conformational transition has been demonstrated for polydiacetylene, 4-butoxy-carbonyl-methylurethane (4BCMU) in solution. The transition can be induced by changing either the temperature or the quality of the solvent. The light scattering and spectroscopic data as a function of polymer concentration have shown that the transition is a single chain (intramolecular) phenomenon. However, because of the large end-to-end length (L≂1.2 μm) of the fully extended polymer, the dilute limit is not reached until concentrations below 10−5 g/cm3. At higher concentrations evidence of cluster growth and aggregation are observed prior to gelation which occurs above a critical concentration c0≂5×10−4 g/cm3. This cluster growth occurs as a result of the rod-like conformation of the individual molecules, but it is not the cause of the transition. The large increase in scattering intensity (at fixed polymer concentration) on going from coil to rod follows directly from the change in dielectric constant due to the spectral shift of the π–π* absorption; no significant increase in molecular weight is implied by the data. A theoretical model of the transition has been developed in which the ordered rod-like conformation is the low temperature phase. Conformational kinks (to a coil phase) cost energy through interruption of the π-electron delocalization and through the breaking of H bonds between R groups. Nevertheless, the increase in entropy associated with the many degrees of freedom of the coil-like conformation is sufficient to lead to the observed transition.
The birefringence of two common nematic liquid crystals were measured in the millimeter wave range, and were found to be in the range of 0.19–0.22 at room temperature. Using liquid crystal electro- and magneto-optical effects, we present the first experimental data of a novel liquid crystal millimeter wave electronic phase shifter. Our experiments show that liquid crystals could play an important role in future millimeter wave device technologies.
We have determined the effect of hydrostatic pressure on the electronic heat-capacity coefficient y of transforming Nb3Sn through measurements of the superconducting transition temperature T"the temperature derivative of the upper critical field near T"and the residual resistivity. We find that y and the bare density of electronic states are suppressed by pressure. Results are discussed in terms of a pressure-dependent d-band occupancy.
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