3D Molecular Orientation Imaging

Summary

Conventional polarization-based microscopy has been extensively used to investigate molecular alignment occurring ubiquitously in various synthetic and natural materials. However, 2D-projected anisotropy images cannot provide out-of-plane angles of molecules. Over the last ten years, the NIST Biomaterials group has developed a novel theory and experimental methods for measuring 3D orientation angles using 2D polarization imaging acquired with transmission IR and coherent Raman microscopy. This rapid, non-invasive imaging technique helps understand the molecular-level structure of highly anisotropic and spatially heterogeneous materials.

Description

We have developed 3D orientation measurement techniques by analyzing polarization-dependent molecular vibrational signals based on infrared (IR) absorption [1,2] and coherent anti-Stokes Raman scattering (CARS) [3]. The two imaging modalities are complementary to each other: IR imaging broadly applies to any pair of non-parallel IR transition dipole moments, but its spatial resolution is in the several-micrometer scale [4,5], whereas CARS imaging can provide sub-micrometer resolution but requires more complicated data analysis [6]. These new complementary imaging methods will provide previously unavailable molecular orientation information that can bridge the gap in the relation of [process–structure–property]. This innovative optical metrology will advance plastic manufacturing, 3D printing, shape memory polymers, medical implants, tissue engineering, advanced packaging, and other material sciences and industries.

Figure 1. 3D orientation angle images of a polycaprolactone film deformed by shearing force [3]. The azimuthal angle (y) and the axial angle (q) images of an interface region of the quiescent and shear deformed film.

Figure 2. Polarization-controlled coherent Raman microscopy can map 3D molecular orientations with sub-micrometer resolution. Imaging the 3D orientation of polymer chains allows for addressing the lamellar twisting mechanism in polyethylene spherulites [6].

Credit: Young Jong Lee

PUBLICATIONS

[1] Y. J. Lee, Concurrent Polarization IR Analysis to Determine the 3D Angles and the Order Parameter for Molecular Orientation Imaging, Opt. Express 26, 24577 (2018). https://doi.org/10.1364/OE.26.024577

[2] Y. J. Lee, Determining 3D molecular orientation from polarization-IR spectra: tutorial, J. Opt. Soc. Am. A 42, 102 (2025). https://doi.org/10.1364/JOSAA.542283

[3] Y. J. Lee, Determination of 3D Molecular Orientation by Concurrent Polarization Analysis of Multiple Raman Modes in Broadband CARS Spectroscopy, Opt. Express 23, 29279 (2015). https://doi.org/10.1364/oe.23.029279

[4] S. Xu, J. Rowlette, Y. J. Lee, Imaging 3D molecular orientation by orthogonal-pair polarization IR microscopy, Opt. Express 30, 8436 (2022). https://doi.org/10.1364/OE.449667

[5] S. Xu, C. R. Snyder, J. Rowlette, Y. J. Lee, Three-Dimensional Molecular Orientation Imaging of a Semicrystalline Polymer Film under Shear Deformation, Macromolecules 55, 2627 (2022). https://doi.org/10.1021/acs.macromol.1c02036

[6] S. Xu, Y. Jin, Y. J. Lee, 3D orientation imaging of polymer chains with polarization-controlled coherent Raman microscopy, J. Am. Chem. Soc. 144, 23030 (2022). https://doi.org/10.1021/jacs.2c10029