Modified Raman Microscopy Technique Reveals Molecular Orientation Patterns

Researchers at the National Institute of Standards and Technology (NIST) have developed a way to measure the 3D orientation of molecules within a material made of polymers. The measurement technique — based on broadband coherent anti-Stokes Raman scattering (BCARS) microscopy — will enable scientists to identify molecular orientation patterns that produce the mechanical, optical, and electrical properties that they seek for optimized materials to be used in medical devices and other items.

Though molecules in some materials line up in a regular, repeating pattern, and molecules in others point in random directions, the molecules in many advanced materials used in medicine, computer chip manufacturing, and other industries arrange themselves in complex patterns that dictate the material’s properties. Scientists have traditionally lacked good ways to measure molecular orientation in three dimensions at a microscopic scale, preventing them from fully understanding why some of these materials of interest behave the way that they do.

“Despite the ubiquity of three-dimensional (3D) anisotropic materials, their 3D molecular alignment cannot be measured using conventional two-dimensional (2D) polarization imaging,” the researchers said.

BCARS is used to identify a material composition. The technique — which was developed at NIST about 10 years ago — works by shining laser beams at a material, causing its molecules to vibrate and emit their own light in response. To measure molecular orientation, NIST research chemist Young Jong Lee added a system for controlling the polarization of the laser light and mathematical methods for interpreting the BCARS signal. The technique measures the average orientation of the polymer chains within 400-nm regions, along with the distribution of orientations around that average — in other words, the method enabled researchers to observe details of the polymer molecules as small as 400 nm.

These measurements allow scientists to identify molecular orientation patterns that produce material properties, including those that they desire for certain applications.

“Understanding that structure/function relationship can really speed up the discovery process,” Lee said. This will help researchers to optimize the materials used in arterial stents and artificial knees, for example. The orientation of the molecules on the surface of those devices helps determine how well they bond with muscle, bone, and other tissues. It can also help with additive manufacturing; scientists using polymers for 3D printing seek to tailor polymer qualities including strength, flexibility, and heat resistance. Polymer 3D printing supports applications in the electronics, automotive, and aerospace industries, among others.

To advance materials science, researchers at NIST developed a way to measure the 3D orientation of molecules within a material made of polymers. In the illustration, the pin-like forms represent polymer chains, with the color indicating average angle off the vertical plane and the size of the pinhead representing the distribution of orientations around that average. The background image shows the raw data, produced by broadband coherent anti-Stokes Raman scattering (BCARS). Courtesy of Y.J. Lee/NIST.

The measurement technique may also be used to optimize the polymer-based ultrathin films used in semiconductor manufacturing. Further, according to the researchers, it could be used to measure microscopic structures of other complex synthetic materials, as well as biological materials.

The research was published in Journal of the American Chemical Society (www.doi.org/10.1021/jacs.2c10029).