New habits in branched polyethylene single crystals. (July 2016)

• Record Type: Journal Article
• Authors: Vega, J.F.; Müller, A.J.; Martínez-Salazar, J.
• Is Part Of: European polymer journal. Volume 80(2016:Jul.)
• Page Start: 169
• Page End: 174
• Publication Date: 2016-07
• Subjects: Single crystals -- Habits -- Polyethylene -- Short chain branching -- Curved faces; Polymers -- Periodicals; Polymerization -- Periodicals; Polymères -- Périodiques; Polymérisation -- Périodiques; Polymerization; Polymers; Periodicals; Electronic journals; 547.705
• DOI: 10.1016/j.eurpolymj.2016.05.012
• Languages: English
• ISSNs: 0014-3057

Abstract

Graphical abstract: Highlights: Novel morphologies are observed in single crystals of branched polyethylene. As the supercooling increases the crystals adopt an almost square-like morphology. The shapes of the crystals are predicted by the Shcherbina and Ungar model. The results are rationalized by assuming a decrease in the step propagation ratio. This trend may be related to the steric conditions of chain attachment. 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 {1 1 0} 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 {1 1 0} 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.