Asymmetric Curvature of {110} Crystal Growth Faces in Polyethylene Oligomers

Lamellar crystals laterally bounded by {110} faces only (nontruncated lozenges) are obtained from 1% octacosane solutions of alkanes n-dohexacontahectane (C162H326) and n-octanonacontahectane (C198H398) at the highest crystallization temperatures, i.e., 103 °C < Tc < 108 °C and 108 °C < Tc < 110 °C, respectively. This is in contrast with polydisperse polyethylene where {110}-bounded lozenges form only at low Tc. Crystals grown from 1-phenyldecane at the highest Tc are also nontruncated. However, while the crystals obtained from octacosane are faceted and rhombic, those grown from phenyldecane have their {110} faces asymmetrically curved at the obtuse apexes. This gives the crystals a leaf-shaped appearance, normally associated with {100}-bounded lamellae. To distinguish it from the established {100} bounded lenticular crystal morphology, the new habit is designated “a-axis lenticular”. The particular type of curvature found for {110} faces is explained qualitatively by assuming a different rate of propagation in the two opposite directions of a new layer of stems. The “sharp” step travels faster, at a rate vs, toward the acute apex, while the “blunt” step travels more slowly, at a rate vb, toward the obtuse apex. The asymmetry is due to the absence of a mirror plane bisecting the {110} growth face.