Coarse-Grained Simulations of Flow-Induced Nucleation in Semicrystalline Polymers

We perform kinetic Monte Carlo simulations of flow-induced nucleation in polymer melts with an algorithm that is tractable even at low undercooling. The configuration of the noncrystallized chains under flow is computed with a recent nonlinear tube model. Our simulations predict both enhanced nucleation and the growth of shish-like elongated nuclei for sufficiently fast flows. The simulations predict several experimental phenomena and theoretically justify a previously empirical result for the flow-enhanced nucleation rate. The simulations are highly pertinent to both the fundamental understanding and process modeling of flow-induced crystallization in polymer melts.

Figure 1

A sheared nucleus with protruding amorphous chains.

Figure 2

(a) Transient and quasistatic nucleation rates for a $Z=25$ monodisperse melt with ${ϵ}_B=1.9$ and ${\mu }_S=1.9$ under startup shear at $\dot{\gamma }{\tau }_e=0.1$; inset contains GLaMM model predictions for shear stress ($G_e$ is the shear modulus). (b) Master curve of nucleation rate against stretch ratio ($\dot{ϵ}$ is the extension rate); inset shows the master curves for varying ${ϵ}_B$. (c) Steady-state nucleation rate measurements against shear rate for an industrial polydisperse isotactic polypropylene melt [27] compared with projected simulation results.

Figure 3

Simulated nucleus aspect ratio at point of nucleation against shear rate for a 2% high-molecular-weight blend ($\alpha =5.0$, ${\mu }_S=2.5$, sheared for $120{\tau }_e$); inset shows experimental orientation data on a bimodal blend sheared for 20 sec [8].