Polarization modulated infrared spectroscopy: A pragmatic tool for polymer science and engineering

Abstract

In the area of polymer crystallization, the most widely used techniques to quantify structure, morphology and molecular orientation are fundamentally based on light or X-ray scattering and absorption. In particular, synchrotron X-rays are used for detailed studies on the semicrystalline structure in polymeric materials. The technical requirements for such techniques, especially when high spatial resolution is essential, make the application of X-ray diffraction not straightforward. Direct information on the chain orientation in different semicrystalline morphologies requires rather complex sampling and analysis procedures. Surprisingly, a simple yet versatile technique based on infrared spectroscopy is hardly applied in the field of polymer crystallization. By modulating the polarization of the incident light, local anisotropy can be studied in real time on a submolecular length scale. In this article, we provide the relevant details of the polarization modulated infrared microspectroscopy technique for the study of semicrystalline materials from an engineering perspective. We demonstrate the essence of the method using as model systems spherulitic and transcrystalline morphologies and present its applicability to polymer/fiber composite technology and the study of injection-molded parts. The results provided in the present work serve to illustrate the applicability of this informative technique in the field of semicrystalline polymer science.

The Role of Molar Mass in Achieving Isotropy and Inter-Layer Strength in Mat-Ex Printed Polylactic Acid

“…With the Fourier transform of these two interferograms, one obtains the normalised differential absorbance spectrum, or so-called polarisation modulated dichroic difference (PMDD), defined by Equation (1): where I II is the intensity at the detector when the incident light is polarised parallel to the reference axis of the sample, and I ⊥ the intensity at the detector when the light is polarised perpendicular with respect to the sample. For more details about this set-up, readers can refer to [ 34 , 35 ]. By changing the two orthogonal polarisation states and obtaining the differential absorbance spectra, it is possible to determine the local orientation of dipoles, and therefore the local chain conformation.…”

Infrared Linear Dichroism for the Analysis of Molecular Orientation in Polymers and in Polymer Composites

“…Measurements are usually performed with the use of a linear polarizer, but they can be significantly improved by the introduction of polarization modulation technique where a photoelastic modulator, placed in the beam of light, alternates the polarization state of the incident radiation between directions parallel and perpendicular to the stretching axis, thus allowing a direct measurement of the dichroic difference ΔA [ 18 , 19 ]. This technique allows the detection of very low dichroic effects observed at small extension ratios or in polymers that display low levels of orientation due to their high chain flexibility like poly(dimethylsiloxane) (PDMS) [ 20 ].…”

Super-resolved 3D mapping of molecular orientation with vibrational techniques

“…IR spectroscopy with linear polarization can provide new and useful information about orientation of molecules 9 . When a sample has anisotropic structure, changing the polarization of incident light in a microscopic investigation can reveal additional information.…”

“…The in-plane direction of transition dipole moments of vibrational bands can be described by the azimuthal angle (𝜓), when a linear polarizationbased method is applied. The IR 9 and Raman 10 two-polarization (2P) methods were used to examine orientation in polymeric samples for decades, however, with significant limitations. Next, the fourpolarization method (4P) has been proposed, which enabled the study of the in-plane organization of molecules in complex systems.…”