Evaluating models for polycaprolactone crystallization via simultaneous rheology and Raman spectroscopy
Anthony P. Kotula, Kalman D. Migler
The crystallization of a polymer melt is characterized by dramatic structural and mechanical changes that have a significant impact on the processing conditions used to generate industrially-relevant products. Relationships between crystallinity and rheology are necessary to simulate and monitor the effect of processing conditions on the properties of the final product. However, separate measurements of crystallinity and rheology are difficult to correlate due to differences in sample history, geometry, and temperature. Recently we have developed a rheo-Raman microscope for simultaneous rheology, Raman spectroscopy, and polarized reflection-mode optical measurements of soft materials, which allows for quantitative crystallinity measurements through features in the Raman spectrum. In this work, we apply this technique to monitor the isothermal crystallization of polycaprolactone to probe the relationship between structure, crystallinity, and rheology. Both the crystallinity and the modulus vary over comparable timescales, but the birefringence increases much earlier in the crystallization process. We directly plot rheological parameters as a function of crystallinity to probe a range of suspension-based and empirical models relating the complex modulus to crystallinity, and we find that the previously developed models cannot describe the crystallinity-modulus relationship over the crystallization process. By developing a model based on the principles of gelation, we can fit the complex modulus over the crystallization range. The crystallization process is characterized by a critical percolation fraction and a single scaling exponent.