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Concentration fluctuations in polymer blends and their change after a temperature jump were studied by time-dependent small angle X-ray scattering experiments. Measurements were conducted on homogeneous mixtures of polystyrene and a partially brominated derivative. Structure factors in thermal equilibrium show the form given by the random phase approximation, thus enabling a direct determination of thez-parameter and the mean radius of gyration. The T-dependence of Z can be understood as the result of superposed enthalpic contributions and a free volume term. In the T-jump experiments, samples were quenched to temperatures near Tg. Relaxation occurs on the time scale of minutes and is nonexponential, becoming slower with time. Initial relaxation rates increase with increasing scattering vectors q in accordance with theoreticalpredictions.

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Zusarnmenfassung: Auf der Grundlage der Ergebnisse von R6ntgenkleinwinkel-Streuexperimenten und elektroenenmikroskopischen Untersuchungen an verzweigtem Polyethylen wird ein neues Modell zum Ablauf des partiellen Kristallisierens und Schmelzens entwickelt. Im Modell wird der Aufbau der Lamellarstruktur als sukzessive ,,Einschubkristallisation" beschrieben, die durch die Konzentration an nichtkristallisierf~higen Einheiten in den amorphen Bereichen gesteuert wird. Die Kinetik der Einschubkristallisation l~i~t sich durch dilatometrische Experimente verfolgen. Aus vergleichenden R6ntgenk-leinwinkel-und Ramanstreuexperimenten kann auf eine Anreicherung der Cobausteine an den Lamellenoberfl~ichen geschlossen werden. Summary:The results of small angle X-ray scattering experiments and electron microscopic investigations on branched polyethylene suggest a hew model of partial crystallization and melting. The formation of lamellar structures is described as a consecutive process ("insertion crystallization") controlled by the concentration of non-crystallizable units in the amorphous layers. Crystallization kinetics upon stepwise cooling is investigated by dilatometry. Comparison of low angle X-ray scattering and Raman spectroscopic results provides evidence for a accumulation of comonomer units at the lamellar surfaces.Schli~'sselwSrter: Polymere, Kristallisation.