New habits in branched polyethylene single crystals

Abstract

Faceted polymer single crystals obtained from solution have been examined for decades. Most of the studies have been performed with linear polyethylene. It is well known that solution-grown linear polyethylene single crystals may exhibit different and often complex morphologies and habits. However, the effect of molecular architecture (i.e., short chain branching) in the morphology of the single crystals remains practically unexplored. At the highest crystallization temperature investigated, the shape of the crystals is lozenge-like, but with curved and slightly truncated {110} faces. As the crystallization temperature decreases, strong changes in the width-to-length ratio and, consequently in the characteristic angles of the single crystal occur. Interestingly, the single crystals obtained at the lowest crystallization temperature explored exhibit a nearly square shape. This phenomenon has not yet been observed in linear high molecular weight polyethylene, for which the characteristic lozenge habit with straight {110} faces is retained as crystallization temperature decreases. The application of the Shcherbina and Ungar approach to fit the unusual shapes of the branched PE single crystals obtained in this work requires a strong decrease in the ratio of the rates of propagation to the right and to the left of the growth edge. This result is probably linked to different steric conditions of chain attachments into non-equivalent niches induced by the presence of branches.

Predicting experimental results for polyethylene by computer simulation

“…In these studies, interesting phenomena were observed including a decrease in the trans/gauche ratio and crystal sizes with SCB, branch inclusion in the crystals for systems containing the shorter branches, exclusion of longer branches from the crystals and inhibition of crystallization for systems with more than 50 SCB per 1,000 C atoms. All these phenomena are in fairly good agreement with experimental observations in single crystals of SCB PE [244,259]. However, a more realistic description of the systems that explicitly takes into account solvent molecules is not an easy computational task.…”

“…It should be noted here that quite similar results for the temperature dependences of linear PE lattice parameters can be obtained using quasi-harmonic lattice dynamics coupled to molecular mechanics FFs [256,257]. [121][122][123], () EH copolymers, Otegui et al [243] and EH single-crystals, Vega et al [244,259]. Closed symbols represent computer simulations of lattice parameters including branching [255] Less studied through computer simulations have been the impacts of SCB on orthorhombic lattice parameters.…”

“…On the contrary, it was found that methyl branches can be partially included in the crystals. On close inspection of the semicrystalline state through the local bond-orientational parameter, we can define the crystalline stem length and overall crystallinity of the models: both magnitudes decrease with SCB content, as in experiments [244,259]. Recently, the effects of both different types and contents of precisely placed SCB and chain length (up to C 10000 and polydisperse systems) have been explored [290,291].…”