Morphology and growth of extended chain crystals of polyethylene

Abstract

The morphology and growth of extended chain crystals of polyethylene is discussed on the basis of optical and electron microscopy. The spherulitic habit is shown to be caused by nucleation of new lamellae on the (001) surface of lamellae, branching of lamellae on their growth face, and thickening of lamellae. Spherulite centers have been identified down to the primary lamella. Analysis of intersections of lamellae during crystallization allowed the identification of intermediate stages of crystal growth. Fracture surfaces parallel to the growth face and the (001) face are discussed. "Ripple decoration" by surface recrystallization is developed as a tool for fracture surface study. Kink bands are described as growth defects. Bending of lamellae is shown to go by slip parallel t o the polymer chains. Ripple and drawn out fiber annealing is analyzed. .ill observations on extended chain crystals are used to propose a mechanism of extended chain crystal growth of polyethylene under elevated pressure. ZUSAMMENFASSUNG:Die Morphologie und das Wachstum von gestrecktkettigen Polyathylenkristallen werden an Hand von optischer und Elektronenmikroskopie diskutiert. Die sphirolithische Gestalt wird durch Keimbildung neuer Lamellen auf der (001)-Oberflache von Lamellen, Verzweigung von Lamellen an der Wachstumsoberflache und Dickenwachstum hervorgerufen. Der Sphirolithkern wird bis zur Primiirlamelle identifiziert. An ifberschneidungen von Lamellen wahrend der Kristallisation sind Zwischenstufen des Kristallwachstums festgestellt wordea. Uber Bruchflachen parallel zur Wachstumsoberflache und der (OOl)-Oberflache wird diskutiert. ,,Ripple decoration'' durch Oberflachenrekristallisation wird zum Studium der Bruchoberflachen verwandt. Kinkbander werden als Wachstumsdefekte beschrieben. Lamellendeformation erfolgt durch Gleiten parallel zu den Polymerketten. Das Tempern von ,,Ripples" und ausgezogenen Fasern wird untersucht. Alle Beobachtungen an gestrecktkettigen Kristallen werden benutzt, um einen Mechanismus fiir das Wachstum von gestrecktkettigem Polyathylen unter hohem Druck vorzuschlagen.

Lamellar Thickening Growth of an Extended Chain Single Crystal of Polyethylene. 1. Pointers to a New Crystallization Mechanism of Polymers

“…As to the origin of ECCs of polyethylene (PE), Uhlmann 9 and Takemura et al 10 maintained that ECCs are formed directly from the melt, while Wunderlich et al 11 and Bassett et al 12 insisted that ECCs are formed from FCCs through lamellar thickening. Note that they studied a stacked lamellar system, i.e., the secondary crystallization process.…”

Thermal Properties and SSA Fractionation of Metallocene Ethylene‐Oct‐1‐ene Copolymers with High Comonomer Content Cross‐linked by Dicumyl Peroxide or β‐Radiation

“…At high DCP concentration (4 phr), it can be observed how larger crystals disappear, as stated before by Narkis et al, [36] and the distribution is then centered at a crystal size that corresponds to a melting temperature of approximately 65 8C. Processes of crystalline reorganization [31,37] might be promoted by the effect of cross-linking at low DCP concentrations (0.5 phr) that might be responsible for the increase in crystal size reflected in the shift of high-temperature endotherms to higher temperatures. However, the total crystallinity decreases when comparing it with the data of the corresponding neat copolymer.…”

“…The low endotherm is associated with the melting of bundled crystals (secondary crystallization) formed by the shorter sequences that were excluded from the primary crystallization (longer crystallizable sequences). [31] The reversibility of the crystallization and melting processes in the copolymers was investigated by obtaining the degrees of crystallinity as a function of temperature (w c (T)) in a process of melting subsequent to crystallization during cooling at a constant rate (10 8C Á min À1 ), both of them preceded by a process of erasing the previous thermal history by heating at 150 8C for 5 min. The degrees of crystallinity as a function of temperature were obtained through the partial areas from DSC thermograms and subsequently converted into degrees of crystallinity through the appropriate scaling factors, depending on the mass of the sample used.…”

“…The degrees of crystallinity as a function of temperature were obtained through the partial areas from DSC thermograms and subsequently converted into degrees of crystallinity through the appropriate scaling factors, depending on the mass of the sample used. The melting enthalpy (DH m ¼ 293 kJ Á kg À1 ) corresponding to the 100% crystalline polyethylene reported by Wunderlich and Mellilo [31] was used. A comparison of the curves in Figure 5 clearly indicates that the crystallinities obtained from the heating and the cooling traces are identical to each other at low temperatures, and depart significantly from each other at higher temperatures.…”

Reversible melting of polyethylene extended‐chain crystals detected by temperature‐modulated calorimetry

“…24 Dilatometry, 25 standard DSC, 26 and electron microscopy of the melting process 26 were performed. The crystal growth involved initial folding followed by chain extension in the crystal, 27 and on crystallization under pressure above 300 MPa the growing crystal has a hexagonal structure 28,29 and represents a conformationally disordered mesophase (condis crystal). 30 On release of the pressure, there is an ordering transition to the common orthorhombic phase that is then analyzed as done in this research.…”