Solutions of polydiacetylenes in good and poor solvents : a light and neutron scattering study

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Abstract
The properties of the P-3BCMU and P-4BCMU solutions in good and poor solvents have been investigated, by use of light and neutron scattering techniques. In good solvents, P-3BCMU and P-4BCMU dissolve with a yellow color and form semi-rigid coils in a trans isomeric state. Their internal structure is better described by a continuous model of a wormlike ribbon than by a model of discrete defects separating misoriented rigid platelets of variable lengths. At room temperature and for both polydiacetylenes, the longitudinal persistence length is found to be close to 160 Å and the mean square radial radius of gyration, which is related to the side extension of the macromolecules, is of the order of 60 Å2. The conjugated chain stiffness does not seem to depend on the presence or absence of Hydrogen bonds between neighboring side groups, as long as a small enough proportion of such bonds are formed. In poor solvents, specifically for red solutions of P-4BCMU in toluene, the chains form complex aggregated structures of large molecular weight. No evidence for red solutions of isolated rod molecule has been found, even for concentrations as low as 0.5 x 10-3 g 1-1 with high molecular weight P-4BCMU (Mw =1.67 x 106 g mole-1) or 0.5 gl -1 with low molecular weight P-4BCMU (Mw ~ 4.32 x 10 4 g mole-1). This strongly suggests that all experimental investigations on the blue and red solutions of P-3BCMU and P-4BCMU observe the aggregated structures only. However, it still remains an open question whether the observed change in the electronic structure is caused by aggregation and/or partial crystallization or whether the color change originates from an instantaneous coil to rod transition followed by subsequent aggregation of the rods. The aggregation process, as seen by small angle neutron scattering, is studied and discussed