Rheological hysteresis in semicrystalline polymers during crystallization and melting

Semicrystalline polymers, like polypropylene, are known to exhibit history dependent properties. This history dependence puts additional emphasis on the polymer processing pathways used in manufacturing; processing conditions must be designed with final product properties in mind. Here we show that semicrystalline polymers do not melt in the same way that they crystallize, and this difference has direct effects on the rheological properties of a material; it is possible to have the same material at the same crystallinity behave differently, simply due to the direction that the semicrystalline state was approached (heating or cooling). We simultaneously measure small-amplitude rheological behavior and crystallinity of polypropylene during temperature sweeps using rheo-Raman spectroscopy. We observe that for a given temperature rheological properties are dependent on if the polymer is crystallizing or melting; the polymer exhibits thermal-rheological hysteresis. Interestingly, and counterintuitively, we also observe that a semicrystalline polymer can exhibit different rheological properties at the same crystallinity depending on whether the polymer is crystallizing or melting; the polymer exhibits crystallinity-rheological hysteresis as well. We then apply a generalized effective medium model for crystallization processes, finding that the percolation process from a spanning semicrystalline network contributes to the observed hysteresis.