The crystallization and melting of linear polyethylene studied by temperature-modulated SAXS and WAXD

Temperature-reversible and -irreversible morphological events could be separated in the case of linear polyethylene during quasi-isothermal crystallization by using simultaneous temperature-modulated synchrotron SAXS and WAXD. Crystallization and subsequent annealing was followed at 126 degreesC for 90 min while applying a temperature modulation with an amplitude of 1 degreesC and a period of 2 min. The crystal growth rate associated with the irreversible part of the crystallization decreases with increasing temperature in a cycle, The crystalline lamellae irreversibly thicken with time. The actual crystallite thickness, however, exhibits a superimposed modulation out of phase with that of the temperature modulation. Melting was studied during heating at 1 degreesC/min after cooling at 10 degreesC/min. A temperature modulation was superimposed with an amplitude of 2 degreesC and a period of 3 min. Once again temperature-reversible crystal thickness changes and irreversible crystal thickening could be observed.status: publishe

Gelatinisation related structural aspects of small and large wheat starch granules

Wheat starches from four European varieties (Charger, Estica, Skirlou and Soissons), and their small and large granule fractions were characterised on a structural level. Gelatinisation properties of starches and fractions thereof were compared. Isoamylase debranching revealed only limited differences between amylopectin chain lengths of the various starches and their fractions. Wide-angle X-ray diffraction, which showed predominant A-type crystallinity for all starches, revealed that large granules had systematically higher B-type and total crystallinity than small granules. Small-angle X-ray scattering (SAXS) indicated structural differences between amylopectins of distinct granule classes. At 42% moisture, the SAXS patterns of small granules were more intense than those of large granules, which, along with their higher gelatinisation peak temperature, suggested denser crystalline lamellae. Upon hydration from 42 to 66% moisture, small granules showed decreased larnellar repeat distances. As small granules displayed narrower helix-coil transition endotherms than large granules, shorter single stranded chains are envisaged to connect longer double helices to the amylopectin backbone in small wheat granules. (c) 2005 Elsevier Ltd. All rights reserved.status: publishe

Amylopectin molecular structure reflected in macromolecular organization of granular starch

For lintners with negligible amylose retrogradation, crystallinity related inversely to starch amylose content and, irrespective of starch source, incomplete removal of amorphous material was shown. The latter was more pronounced for B-type than for A-type starches. The two predominant lintner populations, with modal degrees of polymerization (DP) of 13-15 and 23-27, were best resolved for amylose-deficient and A-type starches. Results indicate a more specific hydrolysis of amorphous lamellae in such starches. Small-angle X-ray scattering showed a more intense 9-nm scattering peak for native amylose-deficient A-type starches than for their regular or B-type analogues. The experimental evidence indicates a lower contrasting density within the "crystalline" shells of the latter starches. A higher density in the amorphous lamellae, envisaged by the lamellar helical model, explains the relative acid resistance of linear amylopectin chains with DP > 20, observed in lintners of B-type starches. Because amylopectin chain length distributions were similar for regular and amylose-deficient starches of the same crystal type, we deduce that the more dense (and ordered) packing of double helices into lamellar structures in amylose-deficient starches is due to a different amylopectin branching pattern.status: publishe

Thermal behavior of poly(hexamethylene terephthalate) oligomers. I. Melting behavior and morphology of the crystalline phase

The thermal behavior of poly(hexamethylene terephthalate) (PHT) oligomers containing variable amounts of isophthalic (IPA) monomer has been investigated. Poly(hexamethylene terephthalate-co-isophthalate) 3400 was first studied. This polymer has a M̄n of 3400, is constituted by heterodisperse PHT units (crystallizable units) with a M̄n of 2000, contains an average number of 2.6 IPA per chain, and has carboxylic chain ends. DSC results have shown that this polymer exhibits multiple melting endotherms. These are described in terms of (i) the existence of two melting zones located on both sides of Tc and associated with two polymer fractions differentiated by their crystallizable unit length, (ii) different crystalline populations growing at Tc, (iii) a reorganization process. A simultaneous small (SAXS) and wide (WAXS) angle time resolved X-ray scattering study using synchrotron radiation has revealed that the different crystalline populations are characterized by different lamellar structures rather than by different crystalline lattices. Other PHTcoIs and PHTs differing from PHTcoI 3400 by their molecular weight and IPA content were also studied using DSC. IPA acts as a defect for crystallinity and causes Tm and ΔHm depression in the PHTcoIs. When the molecular weight of PHT increases, Tm increases and ΔHm decreases as usually reported in the literature. Finally, two crosslinked polymers (PHTcoI 3400 and PHTcoI 4700) were investigated. The insoluble fraction of crosslinked PHTcoI 3400 does not crystallize anymore while the one of PHTcoI 4700 does. This results from shorter crystallizable units and from the distance between the crosslinks of the former. © 1999 John Wiley & Sons, Inc.SCOPUS: ar.jSCOPUS: ar.jinfo:eu-repo/semantics/publishe

Crystallization Memory Effects in Polyethylene Blends above their Equilibrium Melting Temperature

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The Morphological Response of Semicrystalline Polymers to Small Oscillatory Temperature Perturbations

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