Infrared polarimetry: gaining a molecular view of polymer viscoelasticity

Recent exciting advances have been made using infrared (IR) diode lasers to obtain sensitive and rapid measurements of molecular-level dynamics that control polymer material properties. Infrared diodes provide a bright, nearly monochromatic, polarized source that is excellent for use in polarimetry. Optical polarimetry is the measurement of the state of polarization of light. It is a powerful technique for studying molecular and microstructural anisotropy in a sample by measuring how the sample alters the polarization of a transmitted beam. In polymers, it is precisely such molecular anisotropy that underlies their viscoelastic properties. As theories are developed to predict the relaxation dynamics of individual parts of a polymer molecule, and of different species in polymer blends, there is increasing interest in experimental methods to distinguish these dynamics. Infrared polarimetry is extremely valuable in this regard, not only because of the direct relationship between the IR dichroism and molecular orientation, but also because it is amenable to labeling. The present paper reviews the basis of this technique and its application to study the effects of polydispersity on melt rheology, and the relaxation dynamics at different positions along a polymer chain. It concludes with an overview of research in progress and future problems to be tackled