Polarized Raman Spectroscopy Strategy for Molecular Orientation of Polymeric Fibers with Raman Tensors Deviating from the Molecular Frame

Leo Svenningsson,Lars Nordstierna

doi:10.1021/acsapm.0c00762

Abstract

Polarized light is frequently used to identify molecular anisotropy in polymers, biological systems, and other materials. The influence of the Raman tensor by polarized light reveals not only the chemical structure but also information on the morphology of polymers. The orientation distribution of molecules in polymers has previously been determined for systems with the principal axis components of the Raman tensor parallel to the molecular frame. In many cases, the Raman tensor principal axis is not parallel to the molecular chain axis. Therefore, the orientation of the Raman tensor, relative to the molecular chain axis, is crucial if accurate information about the molecular orientation distribution is sought for. This work presents a strategy for separating the Raman tensor orientation angles from the molecular orientation angles for polymeric samples with fiber symmetry. Composite polymeric materials often experience signal overlap in the X-ray scattering wide-angle region, where the anisotropy is often resolved. While X-ray scattering investigates intermolecular distances, Raman spectroscopy resolves chemical information, and anisotropy, by the influence of Raman scattering. The quantitative principles presented here may aid in the evaluation of anisotropy in such composite materials.

Highlights

Polarized Raman spectroscopy is an essential technique to measure molecular anisotropy in materials, with the most accessible method being the measurement of fractional scattered intensity I⊥/I∥
For determining the molecular orientation distribution in polymeric fibers, there is a handful of different experimental pathways, which analyze the nature of the Raman scattering tensor.[1−9] These methods are all based on Bower’s seminal work on molecular orientation distribution measurements and all of which share one important limitation
A set of reference frames and their relations are visualized in Figure 2 with the PAS of the Raman scattering tensor, an arbitrarily chosen molecular frame (MF), the DF of the fiber, and the laboratory frame (LAB)

Summary

■ INTRODUCTION

Polarized Raman spectroscopy is an essential technique to measure molecular anisotropy in materials, with the most accessible method being the measurement of fractional scattered intensity I⊥/I∥. For determining the molecular orientation distribution in polymeric fibers, there is a handful of different experimental pathways, which analyze the nature of the Raman scattering tensor.[1−9] These methods are all based on Bower’s seminal work (from 1972) on molecular orientation distribution measurements and all of which share one important limitation. This work presents a general model to determine the molecular orientation distribution when the Raman scattering tensor does not coincide with the MF. Such cases are useful when analyzing the crowded spectra, which is typically a situation for composite materials. The orientational effects of electrically conducting polymers have been of increasing interest.[18−20]

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES