(653g) The Effect of Flow-Induced Fluctuations on Crystal Nucleation in the Polyethylene Melt

In the classical nucleation theory (CNT), homogeneous nucleation of polymer crystals from the melt occurs with equal rate throughout the melt. Under a strong flow field, fluctuations in the local order of the melt are both stronger and longer lasting than those that are present in the absence of flow. In this study, we determine the propensity for these fluctuations to predispose some parts of the melt to nucleate crystals more readily. Using non-equilibrium molecular dynamics, we subject a melt of linear C₁₅₀H₃₀₂ molecules to steady shear and uniaxial extensional flow, and then induce nucleation by quenching below the melting temperature. Based on various measures of the local melt order, correlations are drawn between the fluctuations present in the melt prior to quenching and the resulting nucleation behavior. At weak strain rates where the flow field has little effect on the crystallization kinetics, the nucleation rate is insensitive to the initial fluctuations inherited from the melt prior to quenching. As the strength of the flow field increases, however, it becomes increasingly likely that the fractions of the melt with high initial order nucleate first. This result suggests that nucleation undergoes a transition from a homogeneous Poisson point process in the quiescent melt to a non-homogeneous Poisson point process under an applied flow field. We discuss the consistency of this behavior with theories of flow-enhanced nucleation in polymers that are based on precursors rather than CNT.