Strong influence of branching on the early stage of nucleation and crystal formation of fast cooled ultralong n-alkanes as revealed by computer simulation

Abstract

Molecular dynamics simulations were performed on a series of ultralong n-alkanes in order to shed light on the early stage processes of the crystallization from the melt at high undercooling. The study contemplates a linear (C191H384) and two symmetrically branched [C96H193CH(CH3)C94H189 and C96H193CH(C4H9)C94H189] ultralong n-alkane samples. These systems can be considered as good models for the study of the mechanism of polymer folding at the early stage of crystal formation. First of all, it is observed that the short branch provokes a delay of the nucleation as compared to the linear chain. Additionally this process is further delayed as the branch length increases. To follow the process we compute the time evolution of the structure factor S(q) as the material is undercooled. It is obtained that both crystal thickness and chain packing are dramatically affected by the presence of the branches. The crystallinity, expressed as the mass ratio of the ordered structure to the total amount of material, also decreases as the length of the branch increases. The observed folding in the linear system is closer to the irregular model of folding at least for the early stages of crystal nucleation and formation. The length of the branch clearly determines its inclusion or not in the nuclei and consequently it disturbs in a different way the formation of the crystals.