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 degreesC < T-c < 108 degreesC and 108 degreesC < T-c < 110 degreesC, respectively. This is in contrast with polydisperse polyethylene where {110}-bounded lozenges form only at low T-c. Crystals grown from 1-phenyldecane at the highest T-c 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 v(s), 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.