Polyethylene molecular structure

Fig. 2.1. Schematic illustration of polyethylene molecular structure of various density ranges (Elias 1992). Top LDPE, radical polymerisation yields a number of (long) side ehains. Bottom HDPE, catalytic polymerisation gives rise to linear ehains with a small number of short branches. Both drawings in the middle illustrate LLDPEs produced by catalytic polymerisation with a-olefines. Small amounts of bntene-1, hexene-1 or octene-1 co-monomers lead to etlyl, butyl or hexyl side chains. Polymerisation in the gaseous phase produces chains arranged in a block-shaped fashion and distributed at various frequencies along the chaia Solution phase polymerisation provides a statistical random distribution along the whole chain...

Because of its exceptional symmetry, the polyethylene molecule fits so readily into a crystalline lattice, in spite of its lack of molar cohesion, that its tendency is to crystallize into spherulite structures on relaxation rather than revert to a disordered state. However, the degree and type of chain branching that interrupts the geometrical regularity of the polyethylene molecular structure causes a local... 

Three examples of the importance of GPC data in understanding the changes in polyethylene molecular structure due to modifications of ethylene polymerization catalysts will be discussed. 

There are at the present time many thousands of grades of commercial plastics materials offered for sale throughout the world. Only rarely are the properties of any two of these grades identical, for although the number of chemically distinct species (e.g. polyethylenes, polystyrenes) is limited, there are many variations within each group. Such variations can arise through differences in molecular structure, differences in physical form, the presence of impurities and also in the nature and amount of additives which may have been incorporated into the base polymer. One of the aims of this book is to show how the many different materials arise, to discuss their properties and to show how these properties can to a large extent be explained by consideration of the composition of a plastics material and in particular the molecular structure of the base polymer employed. 

So far the structure of polymers has been described with reference to the material with the simplest molecular structure, i.e. polyethylene. The general principles described also apply to other polymers and the structures of several of the more common polymers are given below. 

Performance of Polyethylenes in Relation to Their Molecular Structure... 

C13-0123. Draw the structures of polyethylene and the copolymer of butadiene and styrene, showing at least six repeat units for each polymer. On the basis of their molecular structures, explain why polyethylene is more rigid than butadiene-styrene copolymer. 

The general molecular structure of polyether-based polyurethanes is illustrated in Fig. 25.3 a). Typical polyether sequences include polyethylene glycol and polypropylene glycol. The length of the polyether sequences between urethane links can vary from one or two ether groups up to several hundred. As the length of the polyether sequences between urethane links increases, the polymer exhibits more of the properties normally associated with polyethers. 

FIGURE 1.2 Electronic and molecular structures of (a) polyethylene and (b) polyacetylene. 

Molecular solutions, 8 697 Molecular speciation/quantification, infrared spectroscopy in, 23 140 Molecular spectroscopy, 10 508 Molecular structure. See also Chemical structures Molecular formulas of linear low density polyethylene, 20 182-184... 

Polypropylene. A similar study on polypropylene is interesting because polypropylene has a molecular structure intermediate between polyethylene and polyisobutylene. An atactic polypropylene specimen was prepared by ether extraction and irradiated in a nitrous oxide atmosphere. The changes in gel fraction (insoluble in hot xylene) as a function of N-jO pressure are shown in Figure 6. Gel formation (cross-linking) of polypropylene is also promoted in the presence of nitrous oxide. 

At pressures of 13 GPa many carbonaceous materials decompose when heated and the carbon eventually turns into diamond. The molecular structure of the starting material strongly affects this process. Thus condensed aromatic molecules, such as naphthalene or anthracene, first form graphite even though diamond is the stable form. On the other hand, aliphatic substances such as camphor, paraffin wax, or polyethylene lose hydrogen and condense to diamond via soft, white, solid intermediates with a rudimentary diamond structure (29). 

DIAMINES AND HIGHER AMINES, ALIPHATIC. The aliphatic diamine and polyaminc family encompasses a wide range of multifunctional. mullireactivc compounds. This family includes ethylenediamine I EDA) and its homologues. the polyethylene polyamines (commonly referred to as ethyleneainines). the diaminoprnpancs and several specific alkancdiamines. and analogous polyamines. The molecular structures of these compounds may be linear, branched or cyclic, or combinations of these. 

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