MORE ON THE SCATTERING FUNCTION OF HELICAL WORMLIKE CHAINS

The scattering function of the helical wormlike (HW) chain, especially in the range of large magnitude k of the scattering vector, is reinvestigated. First, the scattering function for atactic poly(methyl methacrylate) with the fraction of racemic diads f(r) = 0.79, for which the HW model parameters were already determined, is evaluated for the HW chain having discrete scatterers on its contour, and it is shown that the Kratky plot obtained does not exhibit the second maximum and minimum as observed for the syndiotactic chain (with f(r) = 1.0). This conclusion is also consistent with Monte Carlo calculations on the basis of the rotational isomeric state model. Then the effect of chain thickness on the scattering function actually observed by small-angle X-ray scattering is evaluated on the basis of the cylinder and touched-spheroid models for electron distribution. The results show that the scattering function depends strongly on the model for the electron distribution in the range of kd greater-than-or-similar-to 7, where d is the diameter of the cylinder. Therefore, it is dangerous to construct the scattering function for the contour from the one actually observed in this range, as has often been done by the use of an approximate formula for the correction of chain thickness.