Polymer crystallization is characterized by dramatic structural and rheological changes during the transition from disordered melt to a heirarchically-structured semicrystalline solid. Knowing the viscoelastic response of the sample at any point during the crystallization process is desirable for polymer processing, but these efforts are often frustrated by a lack of simultaneous rheology and crystallinity measurements to critically assess models. In this talk, simultaneous Raman spectroscopy and rheology measurements are used to validate a generalized effective medium model for polymer crystallization over at least two orders of magnitude in frequency. The model describes the crystallizing polymer as a suspension of semicrystalline domains dispersed in a melt phase - these domains grow to form a percolating network that eventually consumes the melt phase at the end of the primary crystallization process. A simple mathematical model system is used to show that the percolation transition in the effective medium model recovers the relaxation dynamics of a critical gel. The relaxation spectrum of the critical gel can be approximated as the power mean of the initial melt and final semicrystalline material. This model adequately describes rheo-Raman measurements of polycaprolactone crystallization and reveals the pitfalls in using purely experimental methods to determine the critical gel point in crystallizing polymers. From the generalized effective medium model, we can calculate the percolation fraction and power law relaxation exponent at the critical point, even when the measurement frequency range is dominated by shear thinning of the melt phase.

Effective medium model for the rheology of polymer crystallization

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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