Reversible optical anisotropy and induced rigidity in polydiacetylene films

Abstract

The soluble polydiacetylenes' -3) provide a unique system for the study of rod-like polymers. Because of the conjugated chemical structure, the electronic states are strongly coupled to the conformation of the polymer backbone; conformational changes are known to cause striking color changes in these polymer^^.^). The solubility is especially important, because it permits studies at concentrations ranging from the very dilute to the melt. For example, this solubility was crucial for a recent light scattering study" of very dilute polydiacetylene solutions that identified a single chain, rod-to-coil transition coincident with the red to yellow color change; the first example of this kind of transition in a conjugated polymer. The macroscopic properties of polydiacetylene in the melt and in the solid state have also been investigated5s7). These condensed phases often exhibit optical anisotropy which is not unexpected for systems of aligned rod-like polymers8). In addition, the same single chain transition found in dilute solutions has been identified in gels and in the melt 5*7, 9).Recent observations suggest a coupling between the single polymer chain conformation and the macroscopic state of the polydiacetylene. Plachetta et al. ' ) have noted the disappearance of the optical anisotropy coincident with the color change (indicative of the rod-coil transition) in the melt. In our laboratory, a gel-sol transition has been observed9) in a semidilute solution coincident with the rod-coil transition.In this communication, we report observations of a reversible anisotropic-isotropic transition in an aligned film of polydiacetylene 4BCMUa) also coincident with the rod-coil transition. The transition temperature is intermediate between the rod-coil transition temperature for a dilute solution of polydiacetylene 4BCMU and a single crystal of the solid. We relate these results to the concept of induced rigidity as put forward in the mean field model of nematic polymers developed by Pincus et al. lo -12). The anisotropic absorption of the aligned films is used to spectroscopically characterize the fully extended polydiacetylene 4BCMU conformation. Comparison of these results with solution spectra demonstrates that in the red solution phase the polymer is fully extended. a) Systematic IUPAC name of 4BCMU: dibutyl 4,19-dioxo-5,18-dioxa-3,20-diaza-10,12docosadiynedioate.

Dissolution and Spectroscopic Properties of the Polydiacetylene Poly(10,12-docosadiyne-1,12-diol-bisethylurethane)

“…In samples subject to large shear deformations the initial crystal structure is almost completely destroyed. [48,54] In particular the mechanical deformation of 4BCMU polymer by rubbing on a glass substrate produces thin, disordered films with a strong preferential alignment of the polymer chains in the direction of rubbing. [48] This also proved to be the case for 9BCMU and 9EU.…”

“…The spectrum of the 9BCMU film is similar to that reported for 4BCMU. [48] It has a more distinct absorption peak, with a maximum at 529 nm (18 900 cm -1 ), than is visible in the spectrum of 9EU. As already stated immersion of the rubbed films of 9BCMU in chloroform resulted in rapid dissolution.…”

“…[48] This is above the first solid-state transition of the polymer crystals. [42,55] Rubbed films of 9BCMU and 9EU were heated to 130 and 170 8C; these temperatures were chosen to be just above those of the solid-state and melting transitions of 9BCMU crystals, respectively.…”

“…Heating into the high temperature solid red-phase at 130 8C results in an annealing out of defects produced by rubbing, which is more pronounced for 9BCMU and 9EU than for 4BCMU. [48] This reflects the fact that all-trans sequences are favoured for the longer alkyl sequences even when the terminal moieties of the side-groups are very bulky. [45] However, while the annealing process produces blue-phase polymer in 9EU films it results in a more homogeneous red-phase polymer in 9BCMU films.…”

“…This conclusion is supported by the observation that mechanical deformation of 4BCMU polymer crystals by rubbing on a glass substrate produces an oriented film of red polymer, so that although the polymer chains are aligned the backbones and side-groups have been disordered. [48] Observations of the solvato-and thermo-chromism show that in general the 3BCMU, 3ECMU, and 3PU polymers have a yellow to blue transition while the 4BCMU, 4ECMU, and 4PU polymers have a yellow to red transition. [3,4,36] 6BCMU has similar behaviour to 4BCMU but for 9BCMU both red and blue forms can be produced.…”