ZUSAMM ENFASSU NG :Hochdruckpolyathylene ( 361R. Kuhn. H. Kromer und G. RoBmanith from those of LDPE-samples with similar melt indices and densities which were polymerized under constant conditions, e.g. in a stirred autoclave reactor (type A). Thus, R-type products have narrower molecular weight distributions and wider distributions of shortand long-chain branching than A-type samples and show decreasing branching with increasing molecular weight. Moreover, there are differences in the structure of long-chain branching and in the molecular shape between R-and A-products. The results of meas.urements may be interpreted assuming that the A-type molecules have tree-like branched structures and approximately globular shapes whereas R-type molecules show comb-like branching and elongated rod-like shapes. The differences in the molecular structure lead to different supramolecular structures -e.g. narrower distributions of densities in spite of wider short-chain branching distributions with R-products as compared with A-products-and to different physical and technological properties of the two LDPE-types synthesized in different ways. The results may be interpreted in terms of reaction kinetics. By a new modified autoclave process the molecular structures and the physical and technological properties of low density polyethylenes may be varied within wide limits.

Low density polyethylenes (LDPE) made by the known high pressure processes show significantly different molecular structures. When the reaction conditions are variable, e.g. in a tubular reactor (resin T) or in a system of two autoclaves with a lower 9 . J .temperature m the first reactor (resin A~'), the polymer shows a narrower molecular weight distribution, but wider distributions of long-chain and short-chain branching compared with a polymer produced at constant temperature and under practically ideal mixing in a stirred autoclave reactor (resin A). The polymer Tdisplays a decrease in long-chain and short-chain branching with growing molecular weight and differs from sample A in the type of long-chain branching and in the molecular shape. A-type molecules show tree-like branching and nearly globular shapes whereas the T-type molecules are characterized by comb-like branching and consequently have more extended (rod-like but flexible) conformations. These structures may be interpreted in terms of reaction kinetics. The differences in molecular structure lead to changes in the morphology. For example, the bulk density distributions of the polymers Tand A~:" are narrower than that of polymer A although the latter has a much narrower short-chain branching distribution. The morphology (e.g. crystallinity and crystal size) is dominated by the tendency of short sidechains to accumulate in the amorphous phase and by the limited mobility of the molecules in the melt during crystallization. The proportion of short side-chains incorporated in the crystalline phase ranges from about 20% at high molecular weights to about 7% at low molecular weights for the resins A and T. The physical and technological properties are closely related to molecular structure and morphology. They may be optimized by selecting suitable polymerization conditions, e.g. by use of a new two-autoclave process.

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Low density polyethylenes made by the known high pressure processes show significantly different molecular structures. The physical and technological properties are closely related to molecular structure and morphology. For example, the adhesive strength of a laminate consisting of an ozone treated low density polyethylene film and an aluminum foil depends strongly on the synthesis conditions. The molecular structures and dilute solution properties of many fully aromatic thermotropic liquid crystalline copolyesters can now be determined using the new solvent 3,5-bis(trifluoromethyl)phenol. A typical random copolyester was fractionated by precipitation from solution, and the fractions were studied in detail by viscometry, integrated and dynamic light scattering, and size exclusion chromatography. From the structure data thus obtained the molecular mass distribution and the persistence length were calculated. Polymer blends, block copolymers, and graft copolymers can be characterized by fractionation procedures using demixing solvents in an ultracentrifuge and subsequent analysis of the fractions.