National Institute of Standards and Technology,
Keywords: rheology, Raman spectroscopy, polymer crystallization, epoxy
Summary:Polymer processing techniques often involve flowing and shaping a molten material until it solidifies. When and how that solidification occurs often dictates whether the material can be successfully processed, as well as the necessary processing conditions to get desired properties. Despite a clear need for measurement techniques to assess the relationship between rheology (flowability) and structure, these two properties are often measured separately, and the resulting correlation can be skewed by the sensitivity of solidification processes like crystallization or chemical crosslinking to process 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 relevant to advanced processing technologies.