Atomistic simulations and theory of composition-dependent crystal nucleation in polymer blends

We apply united-atom molecular dynamics (MD) simulations to quantify the composition-dependent crystal nucleation in polymer blends. We blend high-density polyethylene oligomers (PE) with small amounts of impurity polymers. By quenching polymer melts to 300K, we allow polymer blends to undergo isothermal crystallization. We show that in strongly phase-separated blends, crystal nucleation occurs in the PE domain with a rate identical to that in pure samples. In well-mixed blends, however, impurity polymers with weak nematic interactions, such as isotactic polypropylene (iPP) and cis-1,4-polybutadiene, impede the formation of nematic precursors and result in a PE nucleation rate that decreases with increasing impurity volume fraction. To quantify the role of impurity polymers in nematic precursor formation, we employ a mean-field theory to compute the nucleation barrier of nematic order in polymer blends. We predict that a mismatch in nematic coupling parameters can drive the impurity polymers out of the critical nematic nucleus, and in turn, enhance the free energy barrier for nucleating nematic precursors. The theoretical prediction agrees well with the composition-dependent nucleation rates in our MD simulations.