Polyethylene chain molecule

Figure 8-13. Top The structure and translation period of the polyethylene chain molecule Bottom Symmetry elements of the polyethylene chain molecule.

A polymer such as polyethylene is a long-chain molecule with repetitions of the same monomer. Due to topological constraints, the crystallization process of polymer chains is expected to be different from that of simple molecules as discussed so far [160]. 

A hypothetical chemical structure in the interfacial area of the PMPPlC-treated composite [72] is shown in Fig. 10. The long-chain molecules present in PMPPIC interact with polyethylene leading to van der Waals type of interaction. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

Addition polymers are formed by the reaction of the monomeric units without the elimination of atoms. The monomer is usually an unsaturated organic compound such as ethylene, H2C=CHs, which in the presence of a suitable catalyst will undergo an addition reaction to form a long chain molecule such as polyethylene. A general equation for the first stage of such a process is... 

One of the most remarkable features of polymer crystallization is that such chain molecules can form lamellar crystals that contain heavily folded polymer chains. In experiments, the structural analysis of these lamellar crystals became possible when polyethylene single crystals were first prepared from a solution [100-102]. It was found that the orientation of the polymer chains... 

Figure 9. The end-to-end distance per skeletal bond n for regular conformations of polydimethylsiloxane and polyethylene network chains (12). Maximum extensibility rm of this chain molecule occurs at rm/n = 1.34 A.

Substance made of giant molecules formed by the union of simple molecules (monomers) for example, polymerization of ethylene forms a polyethylene chain, or condensation of phenol and formaldehyde (with production of water) forms phenol-formaldehyde resins. 

Nonpolar molecules such as ethane H(CH2CH2)H and polyethylene (CH2CH2) are attracted to each other by weak London or dispersion forces resulting from induced dipole-dipole interactions. The temporary dipoles in ethane or along the polyethylene chain are due to instantaneous fluctuations in the density of the electron clouds caused by constant motion of electrons about the nucleus with the homogeneity upset by similar electron movement about the other nucleus. The energy of these forces is about 2 kcal per mole of repeating unit in nonpolar and polar polymers alike, and this force is independent of temperature. These dispersion forces are the major forces present between chains in many elastomers and soft plastics. 

The present theoretical approach to rubberlike elasticity is novel in that it utilizes the wealth of information which RiS theory provides on the spatial configurations of chain molecules. Specifically, Monte Carlo calculations based on the RIS approximation are used to simulate spatial configurations, and thus distribution functions for end-to-end separation r of the chains. Results are presented for polyethylene and polydimethylsiloxane chains most of which are quite short, in order to elucidate non-Gaussian effects due to limited chain extensibility. 

Fig. 157. Packing of chain molecules in polyethylene and some of the polyesters. The chains, seen end-on, appear to be compact groups of atoms at P and Q.

Present experiences, however, seem to show that the made omissions are permitted. This can only be the consequence of the fact that chain molecules, in general, form long and extremely thin threads which are practically onedimensional. In fact, a polyethylene molecule of a molecular weight of about 150,000 has been compared with a human hair of one metre length (161). 

In conclusion, NMR spectroscopy on polyethylene in the melt implies the existence of a variety of segmental motions characteristic of long-chain molecules, but does not support the argument that the structure is not homogeneous. 

First, it is necessary to define the structure. The structure of a planar zig-zag polyethylene chain is shown in Fig. 2, together with its symmetry elements. These are C2 — a two-fold rotation axis, C — a two-fold screw axis, i — a center of inversion, a — a mirror plane, and og — a glide plane. Not shown are the indentity operation, E, and the infinite number of translations by multiples of the repeat (or unit cell) distance along the chain axis. All of these symmetry operations, but no others, leave the configuration of the molecule unchanged. 

In the preparation of polyethylene, ethene molecules (C2H4) are bonded together to form a long carbon chain, with two hydrogen atoms bonded to each internal carbon atom and three hydrogen atoms bonded to the end carbon atoms. Draw polyethylene and explain why the formation of polyethylene is an addition reaction. 

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