The isothermal crystallization of linear polyethylene fractions was studied by means of simultaneous measurements of wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) employing synchrotron radiation. From WAXS the overall degree of crystallinity x, was determined. From SAXS, the scattering power Q was evaluated and compared with several different models for the crystallizing system. When the scattering arises solely from the supermolecular structures, such as spherulites, Q is proportional to x, . xcL . (1 -xCL), where xs is the fraction of material transformed into such structures and xCL is the degree of crystallinity within these structures. By comparing x, with Q it was possible to separate the primary and secondary crystallization and to show that secondary crystallization takes place within the lamellar stacks where the crystals become thicker and the amorphous layers become thinner. The process of recrystallization during annealing of crystalline samples was studied in the same way. Other models were treated in a similar manner.
Experimental equipment was constructed for simultaneous measurements of wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) using synchrotron radiation as a strong source of X-rays. The crystallization occurring during heating of an initially amorphous sample with constant heating rate was investigated. Poly(ethy1ene naphthalene-2,6-dicarboxylate) (PEN), poly(ethy1ene terephthalate) (PET) and the copolymer PEN-co-PHB (90:10), where PHB stands for poly(p-hydroxybenzoic acid), were used as materials. By comparing the change of the degree of crystallinity as measured by WAXS and DSC with the change of the SAXS power Q, it was shown that the primary crystallization, during which morphological units like spherulites axe growing, is followed by a secondary crystallization during which the degree of crystallinity within the lamellar stacks in the spherulites is increased. Assuming a two-phase system for the semicrystalline structure, consisting of crystals and amorphous regions, three different processes were shown to be responsible for this increase: The formation of new crystal lamellae within the already existing lamellar stacks, the thickening of already existing crystals at the expense of the amorphous regions, and the complete melting of crystals followed by formation of new, thicker, crystals.
In order to obtain detailed information on structural changes in polymers during crystallization and melting, it is necessary to perform real-time measurements of the change in small angle x-ray scattering simultaneously with measurements of the change in wide angle x-ray scattering, light scattering and enthalpy (by differential scanning calorimetry). Different experimental setups to perform such experiments are described. Some examples demonstrate the variety of information that may be obtained by means of these setups.
At the x-ray wiggler beamline BW4 a new instrument for small-angle x-ray scattering is available. It uses pinhole collimation thus allowing the use of position sensitive detectors for the data acquisition. It is equipped with a doubly focusing mirror and a double-crystal monochromator. The energy of the focused beam can be tuned in the range from 4 to 12 keV. Due to the high intensity of the wiggler source a geometry could be realized which extends the attainable range of scattering vector lengths markedly towards lower values compared with the previously existing instruments. Presently with the new instrument using 8 keV photons values down to 0.0016 nm−1 can be observed which corresponds to maximum correlation lengths of about 600 nm. This resolution is demonstrated, as an example, with measurements carried out on a polymer system.
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