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Atomic Force Microscopy Explores Polymers Used in 3D Printing

Oct. 23, 2018
Details on how polymers used in 3D printing behave have relied on as much guesswork as science—until now.

Resins and other materials that react under light to form polymers, or long chains of molecules, are attractive for 3D printing parts ranging from architectural models to functioning human organs. But it’s been a mystery what happens to the polymers’ mechanical and flow properties during curing at the scale of a single voxel. (A voxel is a 3D unit of volume, the equivalent of a pixel in a photo.)

Now, researchers at the National Institute of Standards and Technology (NIST) have demonstrated a novel light-based atomic force microscopy (AFM) technique called sample-coupled-resonance photorheology (SCRPR). This technique measures how and where a material’s properties change in real time at the smallest scales during the curing process.

3D printing, or additive manufacturing, is lauded for its flexible, efficient production of complex parts, but it has the disadvantage of introducing microscopic variations in a material’s properties. Because software builds the parts as thin layers and then reconstructs them in 3D before printing, the physical material’s bulk properties no longer match those of the printed parts. Instead, the fabricated parts’ performance depends on printing conditions.

A 3D topographic image of a single voxel of polymerized resin, surrounded by liquid resin. NIST researchers used sample-coupled-resonance photorheology (SCRPR) to measure how and where the material’s properties changed in real time at the smallest scales during 3D printing and curing. (Credit: NIST)

NIST’s new method measures how materials evolve with submicrometer spatial resolution and submillisecond time resolution, which is thousands of times smaller in scale and faster than bulk measurement techniques. Researchers can use SCRPR to measure changes throughout a cure, collecting critical data for improving the processing of materials ranging from biological gels to stiff resins.

The new method combines AFM with stereolithography, the use of light to pattern photoreactive materials ranging from hydrogels to reinforced acrylics. A printed voxel may turn out uneven due to variations in light intensity or the diffusion of reactive molecules. 

AFM can sense rapid, minute changes in surfaces. In the NIST method, the AFM probe is continuously in contact with the sample. Researchers adapted a commercial AFM to use an ultraviolet laser to start the formation of the polymer (“polymerization”) at or near the point where the AFM probe contacts the sample.

The method measures two values at one location in space during a finite timespan. Specifically, it measures the resonance frequency (the frequency of maximum vibration) and quality factor (an indicator of energy dissipation) of the AFM probe, tracking changes in these values throughout the polymerization process. This data can be analyzed with mathematical models to determine material properties such as stiffness and damping.

The method was demonstrated with two materials. One was a polymer film transformed by light from a rubber into a glass. Researchers discovered that the curing process and properties depended on exposure power and time and were spatially complex, confirming the need for fast, high-resolution measurements. The second material was a commercial 3D-printing resin that changed from a liquid into a solid in 12 milliseconds. A rise in resonance frequency seemed to signal polymerization and increased elasticity of the curing resin. Therefore, researchers used the AFM to make topographic images of a single polymerized voxel.

Interest in the NIST technique has extended well beyond the initial 3D printing applications. Companies in the coatings and optics manufacturing fields have also reached out, and some are pursuing formal collaborations, according to NIST researchers.

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