Rigidity Percolation Dictates Rheological Hysteresis Regime in Polypropylene during Crystallization and Melting

Understanding structure-property relationships during polymer crystallization and melting has been limited by challenges in the simultaneous measurement of crystallinity and rheological properties. Consequently, rheological models overlook the fundamental asymmetry between crystallization and melting processes. Here, we use simultaneous rheology and Raman spectroscopy to directly measure rheological behavior as a function of crystallinity. We find that polypropylene's rheological behavior can differ significantly between crystallization and melting at identical crystallinity values depending on thermal pathway. Using a generalized effective medium (GEM) model, we show that the onset of hysteresis aligns with the calculated percolation threshold. We quantify hysteresis through a normalized hysteresis parameter ΔG̃ and show that the maximum value of ΔG̃ occurs at the percolation threshold calculated by the GEM model for systems that have achieved complete space filling. Finally, we identify two hysteresis regimes: one prior to percolation with limited hysteresis and one after percolation with large hysteresis values. Mechanically, these regimes reflect the structural differences between the semicrystalline components and pure melt state: the former represents a suspension of "softening spheres" while the latter constitutes a softening network.