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 (C 162 H 326 ) and n -octanonacontahectane (C 198 H 398 ) at the highest crystallization temperatures, i.e., 103 °C < T c < 108 °C and 108 °C < T c < 110 °C, 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 v b, toward the obtuse apex. The asymmetry is due to the absence of a mirror plane bisecting the {110} growth face.