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

Abstract

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 < 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.

Analysis of crystal habits bounded by asymmetrically curved faces: Polyethylene oligomers and poly(vinylidene fluoride)

“…As mentioned in Section 1, nontruncated lozenge shape with more or less curved {110} edges and with apex angles different from those crystallographically predicted have been observed experimentally in extended-chain crystals of long alkanes C 198 H 398 and C 162 H 326 when grown from solution in 1-phenyldecane [16]. The value of the acute angle 4 was found to increase above the crystallographic value of 67 with decreasing crystallization temperature, reaching 84 at 92.4 C, i.e.…”

“…[15], these are the conditions where growth is least affected by self-poisoning. Furthermore, by changing the solvent to 1-phenyldecane or methylanthracene, for the first time nontruncated {110}-bounded crystals with curved {110} faces were observed [16,18]. As mentioned in Section 1, similar crystals appear also in PVDF [10,11].…”

“…As mentioned in Section 1, similar crystals appear also in PVDF [10,11]. The unusual asymmetric curvature of {110} faces has led us to propose that the rates of right-and left-moving steps (kinks and antikinks) are different [16,19]. In this section we build the whole shape of a PE/ alkane single crystal with nontruncated curved {110} faces.…”

“…at the lowest supercoolings e e.g. C 198 H 398 at 108 C or C 162 H 326 at 105 C [16]. As discussed in Ref.…”

“…The above observations motivated us to propose a modification of FrankeSeto equations by assuming that the rates of propagation of left and right steps on the growth surface are different [16]. In the preceding paper [19] we solved the appropriate differential equation of the secondary nucleation model with moving face boundaries, using a simple square lattice model.…”