Combining Rheology and Raman Spectroscopy to Analyze 3D Printing Polymers

Additive manufacturing is enabling rapid prototyping, especially in the medical industry, yet the process parameters’ relationship to the material properties of the extruded product are not well understood and difficult to study. NIST scientist Anthony Kotula and his team in the Polymer Process Group worked with Thermo Fisher Scientific to use a newly developed “Rheo-Raman Microscope” that simultaneously runs rheology and Raman measurements from a single measurement point.

“Today, what I’ll be showing you that we can use Raman spectroscopy and Rheology to better understand the 3D printing process,” Dr. Kotula says in this webinar. He walks the audience through the analysis of the medical polymer polycaprolactone (FDA-approved to be used in medical scaffolding implants) as it is extruded and cooled on the build plate.

Dr. Kotula begins by outlining the problem: the strength and performance of materials is directly related to the processing, and yet, with additive manufacturing, this is not well understood, or modeled, for many polymers. However, studying the structure-process-property relationship of the additive polymer material requires real-time process line measurements.  Specifically, two main measurements are needed:

• Raman spectroscopy to determine the effect of extrusion/deposition on crystallization kinetics and
• Raman + rheology measurements to determine the relationship between structure (crystallinity) and rheology.

“We’re going to perform these simultaneous Raman and rheology measurements using an instrument developed in collaboration with Thermo Fisher called the Rheo-Raman microscope,” Dr. Kotula said.  “This instrument allows for simultaneous measurements of rheology, polarized optical microscopy, and Raman spectroscopy of soft materials.”

For additive manufacturing in semicrystalline polymers, Raman spectroscopy and rheology can be used to understand the conformation and mechanical transitions during the process. The plan:

• Identify crystalline indicators in the Raman spectra of polycaprolactone
• Measure crystallization kinetics following extrusion from the printer nozzle
• Determine the relationships between crystallinity and rheology during isothermal crystallization

The Rheo-Raman microscopy was used to study the relationships between crystallinity and rheology during isothermal crystallization. According to Dr. Kotula, current mathematical models do not capture crystallinity-modulus relationships appropriately. The team determined that a model using low- and high- solid fraction calculations resulted in a “fantastic fit to our data,” Dr. Kotula said.

“This model works for all 5 temperatures that we study,” Dr. Kotula continued.  “We can start using this to characterize the crystalline process.”  Using this method, Raman can be used for quantitative crystallinity measurement in polycaprolactone.

During the filament extrusion process, a faster crystallization occurs at lower nozzle temperatures. Simultaneous rheology and Raman reveals that the cumulus-crystallinity property follows a suspension-based mathematical model.

If you work with additive manufacturing that is dependent on consistent and reliable material properties, this webinar will be fascinating and potentially useful for your future 3D printing additive manufacturing research.

Watch the webinar here.