X-ray scattering by edge-dislocations in the SA phase of mesomorphic side chain polyacrylates

Abstract

The X-ray diffraction patterns of mesomorphic side chain polymers in the SA Phase present diffuse streaks in shape of « butterfly wings ». We show that this diffuse scattering may be due to the presence of edge dislocations. On the basis of a previous description of edge dislocations within the framework of the elastic continuum theory of the SA fihase given by De Gennes, we have calculated the Fourier transform of the deformation field. Optical diffraction experiments on sketches of defects have also been made to reproduce the X-ray scattering patterns. Both methods show that this diffuse scattering may indeed be due to the presence of edge dislocations, Their density may be roughly estimated to some 108/cm~. The size of their cores should be only a few Angstr6ms, From the decay of their elastic deformation field, a typical length A = (K/B)~'~m1.5A can be obtained which shows that the elastic constant B of compression of the layers should be about twoorders of magnitude larger in the polymeric SA Phase than in the conventional one.