(74b) Molecular Dynamics Simulation of Flow Enhanced Nucleation in Biaxial Flows

The fabrication of many polymer products involves melt phase processes like injection molding, blown-film extrusion and heat sealing. For semi-crystalline polymers, the effect of this flow on their final properties, especially crystallinity, is not trivial. Crystallization involves rearrangement of molecules into ordered regions, and flow aids in this process through reorientation of polymer chain segments. The amount of stretching and orientation that occurs has a dramatic effect on the kinetics and morphology of crystallization. This phenomenon is known as Flow Induced Crystallization (FIC). The earliest stage of FIC, known as Flow Enhanced Nucleation (FEN), is a subject of on-going research and is especially challenging because it occurs at a very small spatio-temporal scale. This scale makes definitive experimental measurement difficult. Thus, research on how response of the melt to flow affects crystal nucleation has led to a number of empirical models, each with its own physical interpretation. Literature has various studies for FIC or FEN under shear flows^1–5. Recently, uniaxial flows were also studied, and a new model for nucleation rate enhancement proposed⁶.

Biaxial flows are common in all of the industrial processes mentioned above, yet to date have not been studied for their effects on nucleation. Moreover, going from isotropic strain rates in the plane of extension (equibiaxial flows) to anisotropic strain rates (non-equibiaxial flows) provides an additional level of control over the stretching of polymers in flow. In this work, we use nonequilibrium molecular dynamics (NEMD) simulations to study flow-induced orientation in biaxial flows, and the effect of such orientation on FEN. The intensity of biaxial flow is characterized by a Weissenberg number, Wi, while the anisotropy in strain rate is characterized by a strain rate ratio, L. We probe the orientational order of Kuhn segments in the melt using a nematic order parameter, and find that nematic domains, observed previously in uniaxial flows⁷, are less likely to form in equibiaxial flows. Upon quenching, the resulting nucleation rates are used to challenge models previously proposed in the literature⁶, to determine which models are sufficiently robust to explain nucleation rate enhancement under general flow conditions. We find that the model based on orientation of Kuhn segments is able to capture the FEN effect in all of the biaxial flow results. Meanwhile, the orientation of the nuclei formed under flow are observed to orient themselves away from the direction of compression and in the direction of strongest extension. Thus, this work provides support for the dominant role played by the orientation of Kuhn segments on FEN, and demonstrates that a model based on Kuhn segment orientation works for a more generalized flow field than previously shown. It also provides insight into the chain and crystal stem orientation effects of biaxial flows that are common in melt processing.

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