Rod to coil transition and aggregation in soluble polydiacetylenes: Semidilute regime

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Abstract
Visible absorption, rheological measurements, polarizing light microscopy, and temperature–time dependent quasielastic light scattering was used to examine the dilute–semidilute properties of polydiacetylene solutions, specifically poly(4BCMU). A good (tetrahydrofuran) and poor (toluene) solvent were selected for examining solution properties over a broad range of solvent quality. In this particular instance, poly(4BCMU) dissolved in both the pure solvents and their respective mixtures. Based on the observation that dramatic color changes (yellow–orange–red) can occur with the hydrodynamic radius remaining invariant, it is quite likely that the color changes are due to a local modification in the conjugation length. That is, on a molecular level, a local stiffening of the segments occurs as the magnitude of the red absorption component increases. Interestingly, the stability of the solutions towards aggregation/phase separation also correlates well with the absorbance value of the red peak. Furthermore, careful thermal aging measurements confirm that as the aggregation process proceeds, the absorption spectrum remains unchanged, although the initially relatively broad spectral peaks do become significantly easier to resolve. As a result, an ‘‘intermediate’’ peak (due to an intermediate conjugation length) in the orange region is resolvable. Finally, polarized light microscopy confirms that the aggregates formed from thermally aged solutions containing an orange/red component are highly anisotropic, indicative of high chain alignment. Similar results are obtained from evaporating off the solvent from a dilute yellow solution. The implications of this self-alignment process are discussed.

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