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Isobaric state diagrams of various binary n-alkane mixtures have been derived from DTA-measurements. They can be described by means of proper thermodynamic relations which are essentially taking into consideration the disparity in chain lengths. The discussion of the thermodynamic parameters is elucidating the physical situation in the longitudinal boundaries of extended mixed crystals such that stability conditions in binary n-alkane systems can be developed. Occurrence of eutecticai crystallization in the binary mixtures can fairly well be predicted dependend upon the relative disparity in chain lengths.

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Thermodynamics of eutectoid copolymers is based on the assumption that the co‐units are eutectic units. At low concentration of one of these units (non‐crystallizable units, nc‐units), crystallizable sequences (c‐sequences) are formed. Segregation of the eutectic units leads to lamella‐shaped extended c‐sequence crystals, the thickness distribution of which is controlled by the c‐sequence length‐distribution (“structure‐c‐sequence length relationship”). According to thermodynamics the lamellar crystals have to melt selectively and consecutively with respect to their mean thickness. For eutectoid copolymers it is, therefore, possible to deduce the c‐sequence length‐distribution from a thermodynamic analysis of the whole melting process. A computer‐aided method of analyzing differential scanning calorimetry measurements is applied to various semicrystalline copolymers (low‐density polyethylene (LDPE), vicinal‐chlorinated polyethylene, copolymers with OCH2 units). Theoretical structure parameters (crystallinity, mean crystal thickness) and small‐angle X‐ray scattering (SAXS) ‐pattern computed from the c‐sequence length‐distribution are shown to be in good accord with the experimental data. The universal features of the crystallization behaviour of eutectoid copolymers are emphasized. The influence of topochemical reactions onto the distribution of the eutectic co‐units of copolymers containing oxymethylene units is discussed.

Wide angle X-ray scattering experiments with LDPE at various temperatures from 20~ to 106~ have been exploited with respect to lattice parameters, degree of crystallinity, and dimensions of microparacrystals and their lattice defects. The density of the crystals decreases within the measured temperature range in steps from 0,998 9 103 kg m -3 to 0,952 . 103 kg m -3. Above 90~ we detected microparacrystals with a great extension in direction of the b-axis. At low temperatures additional microparacrystals exist, which are characterized by a prominent extension perpendicular to the 110-planes. The lattice defects discussed for the two kinds of microparacrystals show pronounced differences. The coherent scattering regions of the crystals in direction of the a-, b-, and c-axes are discussed within the framework of a structure model of the lamella, which leads to the conclusion that the molecular axis of the chain (c-axis) is inclined to the lamellae surface by 45 ~ Schliisselw6rter: Polyethylene, Wide angle X-ray scattering, Temperature dependence of 94~ -106~