TABLE OF CONTENTS Polydispersity is an important aspect of polymer rheology since the relaxation times of macromolecules are strongly dependent on molecular weight and its distribution. A useful model system that has been investigated to offer insights into the role of polydispersity is the bimodal distribution, where two narrow molecular weight chains of different sizes are blended together. If the two molecular weights are sufficiently well separated, mechanical measurements can offer a reasonable separation of the individual dynamics of the separate components. However, this separation is never perfect and becomes difficult to establish when the two molecular weights are too close together. Optical methods, when properly designed, can provide the capability of effecting such a separation, and spectroscopic techniques such as infrared dichroism and Raman scattering are particularly useful.
In this case study, which is extracted from reference 184, infrared dichroism is described as a means of separating the component dynamics in multicomponent polymer melts. What is necessary is the existence of distinct absorption peaks for at least one of the components. In the present problem, however, where two chains of identical chemistry but different molecular weights are mixed, there will not be any intrinsic differences in their absorption spectra. In this case it is necessary to label one of chains with a tag that will allow its presence in the blend to be revealed. For this purpose, deuteration of one of the chains is often used. This provides the labeled chain with...
