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Morphology and melting of truncated single crystals of linear polyethylene

par Hocquet, Stephane;Dosière, Marcel;Thierry, Annette;Lotz, Bernard;Koch, Michel Hj;Dubreuil, Nicolas ;Ivanov, Dimitri
Référence Macromolecules, 36, 22, page (8376-8384)
Publication Publié, 2003-11

Article révisé par les pairs

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Résumé :

The thickness of (110) and (200) sectors in truncated single crystals of linear polyethylene grown from dilute n-octane solution at 95°C was measured by atomic force microscopy (AFM) in tapping mode. The (110) sector was found to be 1.1 nm thicker than the (200) sector. This can be explained by a somewhat smaller tilt angle of the chains with respect to the basal plane of the crystal. The tilt of the chains in (110) and (200) sectors, obtained by electron diffraction, is 22° and 30°, respectively. This implies that the length of the stem, i.e., the length of the straight part of the chains between two consecutive folds, is identical in both sectors of the truncated single crystals. The melting of individual LPE truncated single crystals was revisited by AFM. The melting temperatures of the (200) and (110) sectors are 124.9 and 125.9 ± 0.3°C, respectively. The lamellar thickness distribution calculated from AFM pictures clearly indicates that reorganization into thicker lamellae takes place during heating. The melting of mats of filtration of truncated LPE single crystals has also been investigated by time-resolved small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The melting curve of LPE single crystals generally exhibits two endotherms. The low and the high melting temperature endotherms are characteristic of single crystals formed from dilute solution and reorganized thicker lamellae during the heating ramp, respectively. Time-resolved SAXS measurements data indicate that two populations of lamellar crystals coexist between 122 and 128°C. The maximum value observed for the lamellar thickness of the recrystallized material is 30 nm, i.e., twice the initial lamellar thickness of single crystals obtained from dilute solution. There is good agreement between the thicknesses of the crystalline layers obtained from the SAXS data and the values calculated from the Gibbs-Thomson relationship using experimental values of the peak temperature of the two endotherms.