Rheo-Raman Microscope for Soft Materials

NIST researcher Anthony Kotula using the new rheo-Raman microscope, a three-in-one instrument that enables researchers to measure the flow properties of a material along with its microscopic structure and composition. (Source: D. Anderson / NIST)

NIST researcher Anthony Kotula using the new rheo-Raman microscope, a three-in-one instrument that enables researchers to measure the flow properties of a material along with its microscopic structure and composition. (Source: D. Anderson, NIST)

An inno­vative three-in-one instrument that allows scientists to correlate the flowa­bility of soft “gooey” materials such as gels, molten polymers and bio­logical fluids with their underlying micro­structure and compo­sition has been developed by scientists from the National Institute of Standards and Techno­logy NIST and Thermo Fisher Scientific. Simul­taneous measure­ments yield a clearer picture of how structural make-up and flow behavior during processing dictate the macro­scopic pro­perties–such as strength, hardness, or electrical conductivity – that make so-called soft materials desirable for certain products or appli­cations.

A novel combination of off-the-shelf instruments, the new research tool, the rheo-Raman micro­scope, integrates: a Raman spectrometer, which shines a laser on the sample and measures the tiny portion of scattered light that reveals vibrational energy levels of molecules in the sample, providing the equivalent of molecular finger­prints showing how atoms are arranged; a rotational rheo­meter to track and measure how a liquid, suspension or slurry flows in response to stress – or, put another way, the degree to which the sample deforms; and an optical micro­scope that collects polarized light reflected from a sample to increase contrast, enabling measure­ments of a specimen’s structural features at micro­scopic scales.

”The new instru­ment is designed for multi­tasking,” said Anthony Kotula, a NIST materials scientist. “It allows you to trace the evolution of micro­structure across a range of tempera­tures and to do it in one controlled experiment rather than in two or three separate ones. It provides insights that would be very difficult to obtain through measure­ments made one at a time.” Homing in on the flow behavior is especially important, because it is intimately coupled with the micro­structure and ultimate pro­perties, Kotula explained.

Soft materials share features of liquids and solids. They range from plastics to liquid crystal displays and from contact lenses to bio­pharmaceu­ticals. For these “in-between materials,” even slight variations in processing con­ditions can alter internal structures and drasti­cally change material properties, which can open the way to improved per­formance or entirely new techno­logical appli­cations. As they report, the team used their prototype rheo-Raman micr­oscope to follow and measure changes before, during and after melting a cosmetic material composed of coconut and almond oils and about 10 other ingredients. They also present simul­taneous “melt” measure­ments taken on high-density polyethylene, which is used to make plastic bottles, corrosion-resistant pipes and many other items, as liquid molecules arrange and solidify into crystals.

Both demon­strations yielded a detailed, unfolding picture of how flow behavior and other phenomena during melting and crystal­lization correspond to changes in the shape and arrange­ment of molecules due to processing con­ditions. “Based on the possi­bilities for direct corre­lation between chemical, structural and mechanical pro­perties, we expect the rheo-Raman micro­scope to be criti­cally relevant to both academic and industrial interests,” the researchers concluded. “The rheo-Raman micro­scope is a general purpose instrument with lots of potential uses,” Kotula said. “At NIST, one of our first appli­cations pertains to 3D printing. We’ll use it to better understand how polymer crystal­lization proceeds during the layer-by-layer printing process.” (Source: NIST)

Reference: A. P. Kotula et al.: The rheo-Raman microscope: Simultaneous chemical, conformational, mechanical, and micro structural measures of soft materials, Rev. Sci. Instrum. 87, 105105 (2016); DOI: 10.1063/1.4963746

Link: Materials Science and Engineering Division, NIST, Gaithersburg, USA

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