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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Absorption and scattering losses of visible radiation in nematic liquid crystals (LCs) were measured by an improved transmission technique. The absorption loss of the E-7 LC mixture was found to be ~1 order of magnitude larger than the scattering in the isotropic state. On the contrary, light scattering surpasses absorption by ~2 orders in the nematic state. Wavelength-dependent absorption and scattering losses of two LCs, E-7 and MBBA, were investigated at some laser wavelengths. Results indicate that the absorption tail of LCs decreases with wavelength as lambda(-2) and scattering as lambda(-4) as expected by theory.

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.

The gel-sol transition for the polydiacetylene 4-butoxycarbonylmethylurethane in toluene is studied. For gelation to occur, the polymer must be in its fully extended rodlike conformation. Specific evidence that the gelation is due to the percolation of rods is presented.PACS numbers: 61.25.Hq, 61.40.Km, 62.10, + sThe properties of rodlike macromolecules in solution have been a subject of interest in recent years. Both the microscopic properties of individual chains and the interactions between chains can lead to novel phenomena. Since such polymers are often conjugated, the rodlike conformation arises primarily from electronic derealization along the polymer backbone. Consequently, changes in conformation are inherently coupled to changes in electronic structure. Moreover, the rodlike conformation can be expected to lead to ordered liquid-crystalline phases, an area of considerable importance.On the other hand, relatively few studies have addressed the effects of interaction between rodlike macromolecules in solution. Doi and Edwards 1 discussed the dynamics of such solutions in the semidilute concentration (c) regime, L~3 « c« (Ld 2 )' 1 , where L and d are the length and diameter of the rod, respectively. The elastic properties of gels of rigid rods were discussed by Doi and Kuzuu 2 for {dL 2 )' 1 <>c« (d 2 L)" 1 . In both theories, the interaction between the rods was assumed to be a hard-core repulsion; the rods cannot pass through each other.Solutions of the polydiacetylene 4-butoxycarbonylmethylurethane (4BCMU) (see Fig. 1) in toluene present an opportunity to study some of these properties, 3 since a rod-to-coil transition has been identified by use of quasielastic light scattering. 4 The rod-to-coil transition has been studied 5 ' 6 as a function of temperature by monitoring the spectroscopy of the 7T-7T* absorption. A theory of this conformation transition has been proposed 5 in which the high-temperature (yellow) phase consists primarily of the cis structure, which is free to coil, whereas the low-temperature (red) phase consists of the trans structure, which is rodlike.In this Letter we present the results of a study of gels of polydiacetylene (4BCMU) in toluene. Our results demonstrate that for gelation to oc-cur, it is essential that the polymer be in its fully extended, rodlike conformation. The shear modulus was measured as a function of temperature and concentration. In addition, the critical concentration, below which no gel forms, was also measured. We find specific evidence that the gel-sol transition for this system is determined by the percolation of rods. This is the first report of detailed studies of the gel-sol transition with rodlike macromolecules.The gels were prepared by dissolving the polydiacetylene (4BCMU) in toluene at T ^ 80 °C. This formed yellow solutions (coils). Upon cooling, the polymer chains undergo the coil-to-rod transition, and the solution becomes a gel.The shear modulus was measured with a mechanical resonance technique. 7 A cylindrical bobbin is suspended coaxially i...