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PROJECTS/PROGRAMS

Polymer Advanced Manufacturing and Rheology

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Summary

This project develops measurements and characterization techniques to overcome challenges in polymer processing, from recycled materials to advanced applications. We target critical national needs that support advanced manufacturing of polymers, focusing on two key areas: plastics reprocessing and additive manufacturing. For plastics reprocessing, we develop techniques to sort and characterize semicrystalline materials by both composition and crystallinity, as well as develop strategies to control crystallization during processing to transform post-consumer waste into a reliable domestic feedstock. For additive manufacturing, our research on polymer rheology and non-equilibrium kinetics provides the understanding needed to overcome technical barriers, ensure part quality, and accelerate commercialization. This work strengthens U.S. leadership in advanced manufacturing and in domestic supply chains.

Description

The manufacture of polymeric materials, whether from virgin or reclaimed sources, occurs under highly non-equilibrium conditions where temperature and stress fields evolve rapidly. The quality of the materials made from these processes depends not only on the composition of the starting material, but also the complex interplay between deformation and structure generated during the process. In this project, we develop the essential instrumentation and methodologies to address measurement challenges in composition and processing across the life cycle of polymeric materials.

Polyolefin Blend Rheology and Crystallization

Current recycling processes for polyolefins, mainly polyethylenes and polypropylenes, are unable to avoid contamination from waste streams that result in the formation of immiscible blends in post-consumer resins. Materials made from these unintentional blends have properties that are strongly dependent upon process history. This is due to the complex interaction between temperature, flow, composition, morphology, and crystallization kinetics. We address this complexity by leveraging in situ techniques like small- and wide-angle X-ray scattering and polarized optical microscopy to characterize the evolution of morphology and crystallization kinetics under controlled temperature and flow histories. These techniques are combined with ex situ electron microscopy and differential scanning calorimetry (DSC) measurements to develop a more comprehensive understanding of the process-structure-property paradigm in these materials.

Optical micrographs of spherulitic growth during crystallization
Optical micrographs of spherulitic growth during crystallization of mixed polyolefins

We also leverage hybrid characterization techniques developed in-house to characterize thermal transitions in semicrystalline polymers and polymer blends:

  • Rheo-Raman spectroscopy measures rheological properties simultaneously with Raman spectra, which can be used to quantify the degree of crystallinity in blends with chemical specificity. Our goal is to use these measurements to improve the strength of recycled plastics and better understand mechanical properties including tensile stress, compressive stress, thermal properties, and nanostructure. We have observed a strong dependence of composition on the crystallization process, a phenomenon we attribute to finite size effects, morphology and partial miscibility. Read more.
  • DSC-Raman spectroscopy correlates thermal transitions with structural or chemical changes in materials. This technique provides a direct relationship between Raman spectral features and properties like crystallinity or chemical conversion that is conventionally performed with calorimetry.
An animated GIF of two plots: one showing Raman spectra of a high density polyethylene, and the second showing a DSC trace of the same sample. A red dot moves along the DSC trace to indicate the temperature of the corresponding Raman spectrum. The Raman spectra show sharp peaks at lower temperatures indicating crystallinity, but when the temperature increases past the melting peak in the DSC trace, the sharp peaks in the Raman spectra rapidly decrease in magnitude until only broad peaks remain.
DSC-Raman measurements on SRM1475, a high density polyethylene
Credit: Chad Snyder

 

Time-gated Raman Spectroscopy for Advanced Characterization

Although conventional continuous-wave (CW) Raman spectroscopy is a powerful tool, it can suffer from a strong luminescence background in materials containing pigments or other additives. This background can often overwhelm the Raman scattering signal to make characterization impossible. One solution is to use a pulsed laser and time-resolved detector to perform time-gated Raman spectroscopy (TGRS), where the slower luminescence signal can be separated from Raman scattering. In this project, we leverage TGRS to characterize the composition of materials, including post-consumer plastics. We support this effort through the development of databases for plasticizers other additives used in commercial plastics. We are also examining the potential for quantifying additive content in post-consumer plastics as well as developing in-line TGRS monitoring capabilities.

Plot showing three Raman spectra measured using 532 nm continuous-wave excitation, 780 nm continuous-wave excitation, and time-gated Raman spectroscopy. The continuous-wave spectra show large backgrounds with small Raman peaks, but the time-gated result shows sharp, well-resolved Raman peaks. Inset plot shows the blue polypropylene piece that was measured.
Raman intensity vs. wavenumber measured with two continuous-wave (CW) lasers and via TGRS for a recovered piece of pigmented polypropylene
Credit: Anthony Kotula

Material Property-Based Strategies for Sorting Plastics

Novel strategies for sorting mixed plastics can help to improve the quality and composition of post-consumer resins used by manufacturers. Here, we focus on techniques that could be applied to separate plastics that are chemically similar, but can have dramatically different material properties, such as the common grades of polyethylene: high-density, low-density, and linear low-density. Our efforts include

  • Frictional sorting: The coefficient of friction of plastics varies strongly near (but still below) their melting temperature, which provides a potential strategy to sort polyolefins. Read more.
  • Magnetic levitation (Maglev): Magnetic levitation uses a magnetic field gradient to sort plastics with high precision based on the mass density of plastics in a paramagnetic salt solution. Our efforts include characterizing the magnetic properties of post-consumer plastics to assess separability and developing strategies for continuous sortation.

Fundamentals of Materials Extrusion Additive Manufacturing

In the common thermoplastic additive manufacturing process known as materials extrusion (MatEx), a solid polymer filament is melted, extruded though a rastering nozzle, welded onto neighboring layers and solidified.  We are developing a framework to measure and understand the physical processes that underly this 3D printing process for both amorphous and semicrystalline feedstock.  Our framework centers around three interrelated components: 1) characterization of characteristic material timescales governing rheology and crystallization, 2) in situ measurements of temperature (via infrared thermography), stress (via birefringence imaging), and crystallinity (via Raman spectroscopy or X-ray scattering) during MatEx, and 3) quantification of structure and mechanical properties that result from the competition between MatEx processing timescales and material timescales. 

A composite image with three columns. Left: gray bright-field image with faint lines. Middle: striped retardance map in orange and blue with color scale. Right: solid red orientation map with circular orientation guide, indicating angles in degrees.
Bright-field, retardance, and orientation map of material extrusion printed polylactide part
Credit: Anthony Kotula

Major Accomplishments

2025

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2016

2014-15

 

Bioscience, Biomanufacturing, Chemical engineering and processing, Molecular characterization, Manufacturing, Additive manufacturing, Process measurement and control, Materials characterization, Polymers, Nanomaterials and Spectroscopy
Created December 7, 2016, Updated September 17, 2025
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