The flow properties of soft matter originate from the structure of the material and the evolution of that structure due to deformation. Structural changes due to crosslinking or crystallization will often cause dramatic changes in mechanical properties to the point where the material transitions from a viscous liquid to a viscoelastic solid. This solidification often occurs during processing, and it is therefore critical to measure and characterize the relationship between structure and rheology during solidification. Since crosslinking and crystallization processes are sensitive to temperature and flow history, simultaneous measurements of chemical information, structural information, and rheology are needed for accurate structure-property correlations. To this end, an instrument for combined Raman spectroscopy and rheology has been developed to probe the relationship between mechanical properties and the underlying structure that drives solidification. The utility of the instrument is shown for three solidification processes that are relevant to soft matter industrial processing: epoxy-amine curing, ultraviolet curing of a 3D printing resin, and isothermal crystallization of a high-density polyethylene. Raman spectra are analyzed using either simple intensity ratios or chemometric techniques to identify and quantify changes in vibrational modes that are related to structure, bond formation, or molecular conformation. In cases where the Raman spectra can be used to quantify a degree of crystallinity or crosslinking, rheo-Raman spectroscopy can provide a direct measure of rheology-structure relationships.

Rheo-Raman spectroscopy for crosslinking and crystallization in polymers

ACS Spring 2022

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