X-Ray Scattering

Crystals of Small Molecules

• A single crystal of low molecular weight material gives the most detailed x-ray diffraction patterns, from which the structure of the molecular can be reconstructed.

• Consider instead a powder pattern, obtained with many very small crystals that are randomly oriented in the sample. The discrete spots characteristic of the single crystal now become rings instead, PC Fig. 7.16.

• A densitometer scans the intensity of the scattered radiation along a line stretching radially from the transmitted beam, PC Fig. 7.17.
  • Rather sharp lines are seen with fairly large crystals
  • Smaller crystals give broader lines
  • A melted (amorphous) sample exhibits a very broad line (or halo), and sometimes weaker secondary lines. Some residual local order persists in the amorphous state.

Example of polyethylene

Semicrystalline polymers exhibit amorphous and powder features simultaneously, PC Fig. 7.18.

• Low molecular weight n-alkanes can form perfect crystals, with sharp lines.

• The amorphous halo is seen when the alkane is raised above its melting temperature.

• Semicrystalline polyethylene exhibits the amorphous halo, with sharp lines (but perhaps broader than in the n-alkane) superimposed.