SMALL-ANGLE X-RAY-SCATTERING FROM DILUTE-SOLUTIONS OF SUBSTITUTED PHTHALOCYANINATO POLYSILOXANES

The dilute-solution properties of alkoxy-substituted phthalocyaninato-polysiloxanes are investigated by using small-angle X-ray scattering (SAXS). The scattering of rodlike particles is studied with special emphasis on molecular parameters important in real polymeric systems such as polydispersity, overall length, finite cross-section, and chain flexibility. Polydispersity and finite flexibility show a strong interdependence with opposite influence on the particle scattering factor. It is shown that especially in the limit of high persistence lengths (rod limit) polydispersity plays a crucial role in determining molecular dimensions by scattering techniques. The exact form factor for a polydisperse, circular cylinder is compared to the approximate form factor for infinitely thin rods corrected for a finite cross-section. Good agreement is found in the angular range from h = 0.05 to 0.1 angstrom-1. The Zimm analysis of the experimental data obtained from dilute solutions in heptane and toluene gives weight-average degrees of polycondensation typically between 50 and 160. While the fit of the radius of gyration as a function of the contour length only allows for the estimation of a lower limit of 1000 angstrom for the persistence length, the angular dependence of the scattered intensity agrees well with particle scattering functions of polydisperse, thick rods. Radii of gyration of the cross-section are determined to range typically between 10 and 25 angstrom while the polydispersity is in good agreement with a most probable distribution (M(w)/M(n) - 1 = 1). The experimental mass per unit contour length is well within the range of the theoretical values, indicating the absence of aggregation in the concentration regime investigated. Besides the conventional Zimm extrapolation, which is nonlinear for rodlike particles, several linear methods arising from the asymptotic scattering behavior of rodlike particles are discussed and applied to the experimental data after elimination of the cross-section factor. Data from both treatments show a good agreement and moreover confirm the rodlike chain conformation of the polymers.