ABSTRACT
Light hydrogen and deuterium atoms possess very different neutron scattering amplitudes, termed scattering lengths: bu = -0.374 X 10-12 cm and bD = 0.667 X 10-12 cm, respectively, though they have almost the same chemical properties. Such a large difference in scattering lengths gives rise to three distinct small-angle neutron scattering (SANS) methods, namely a labelling method (LM), a contrast enhancement method (CEM) and a contrast variation method (CVM). The LM enables the determination of chain conformations in the bulk state since a dilute solid solution of labeled molecules can be realized by dispersing the labeled molecules in the unlabeled matrix. The combination of SANS and intermediate-angle neutron scattering (IANS) with the labelling method gives information about the global and local conformations of polymer chains, respectively. Hence this method is very useful for solving various important problems in the physics of polymer solids. A partial labelling method is also important for investigation of the partial structure such as crosslinking in polymer networks. By means of the CEM heterogeneous structures due to density fluctuations such as long periods in semicrystalline polymers can be studied more easily. For example, the contrast enhancement is made for usual polymer materials by introducing a deuterated swelling agent into the amorphous or less ordered regions with lower density through absorption. The CVM reveals the internal structures of composite particles such as core-shell latex and many kinds of biological globular proteins without destroying the particles. Thus, by immersing the core-shell latex in the mixture of deuterated and hydrogenated media (usually the mixture of light and heavy water) and adjusting the fraction of the deuterated medium so as to match the scattering contrast of the shell part,
we can 'see' only the core part, and vice versa. This method is not further described in this chapter, however.
