Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow

TL;DR

This study uses molecular dynamics simulations to explore how short polymer chains nucleate crystals under different conditions, revealing the effects of shear flow and chain length on the nucleation process.

Contribution

It identifies the molecular mechanism of crystal nucleation in short polymer melts and compares behaviors below and above the entanglement length under various shear conditions.

Findings

Entanglement does not affect the nucleation mechanism.

Chains align and straighten before density increases during nucleation.

High shear rates cause simultaneous chain alignment and straightening.

Abstract

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under quiescent conditions and under shear flow. We compare results for n-eicosane(C20) and n-pentacontahectane(C150), i.e. one system below the entanglement length and one above. Under quiescent conditions, we observe that entanglement does not have an effect on the nucleation mechanism. For both chain lengths, the chains first align and then straighten locally. Then the local density increases and finally positional ordering sets in. At low shear rates the nucleation mechanism is the same as under quiescent conditions, while at high shear rates the chains align and straighten at the same time. We report on the effects of shear rate and temperature on the nucleation rates and estimate the critical shear rates, beyond which the nucleation rates increase with the shear rate. We show that the viscosity of the system is not affected by the crystalline nuclei.