Flow enhanced nucleation of polyethylene in biaxial flows

What is it about?

Many polymers around us are semicrystalline in nature, meaning that they crystallize only incompletely as they solidify into the products we use. This semicrystallinity of polymers is the source of their remarkably mechanical properties like toughness and strength. Polymers crystallize faster and into different semicrystalline structures when they are subjected to flow. Biaxial flows involve stretching or extension of material in two directions and compression in the third. Such flows are very common in industrial polymer processing but our understanding of their effects on crystallization is largely empirical. We use molecular simulations to model polymers under these flows. Specifically, we use these simulations to calculate the enhancement in crystal nucleation rates to occurs for polyethylene in biaxial flows. We test and compare different analytical models for nucleation rates using our simulation data on biaxial flows. We find that the stress in the material during flow is not a good indicator of nucleation rates. On the other hand, we can measure the molecular level orientation of polymer chains during flow and find that nucleation rates indeed correlate well with this orientation. We quantified this correlation by fitting a model based on molecular level orientation to nucleation rates, not only for biaxial flows but also for available data from uniaxial and shear flows. The model provides a good description of flow enhanced nucleation for all cases, showing that molecular level orientation is an important determinant of nucleation rates for diverse set of flow conditions.

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Photo by BoliviaInteligente on Unsplash

Photo by BoliviaInteligente on Unsplash

Why is it important?

Biaxial flows are industrially relevant for a variety of polymer processing operations such as blown film extrusion, calendaring, heat sealing of packages, etc. Thus, many products that we use are influenced by polymer crystallization under these conditions. In this work, we improve understanding and provide a quantitative analytical model for the enhancement of nucleation rates under biaxial flows. This model can be used in a multi-scale approach to model many important industrial processes, which in turn can be used to make decisions about process parameters and to minimize product defects.