Polyethylene schematic structures

Fig. 10. Schematic structure models of the bulk-crystallized polyethylene samples. 1, II, and 111 indicate the crystalline, interfacial, and interzonal regions, respectively. Models A, B, C, D, and E express the molecular crystal, unpeeled crystal, disheveled unpeeled crystal, and lamellar crystals for medium and large molecular weight samples, respectively66), f and x designate the lamellar thickness and the extended molecular chain length, respectively...

Coordination catalysts allowed for the first time the copolymerisation of ethylene with other olefins such as 1-butene, 1-hexene or 1-octene, which, by introducing side branches, reduces the crystallinity and allows a linear low-density polyethylene to be produced at comparatively low pressures [136], Figure 2.3 shows schematic structures for the three polyethylenes, with the main features exaggerated for emphasis [46]. 

Figure 14.1 Schematic structure of fully stretched polyethylene.

Schematic structures of both a dimethyl polysiloxane and a polyethylene glycol liquid phase were given in Chapter 4. There is, however, one difference between packed column and capillary column liquid phases capillary column phases are extensively cross-linked. By heating the freshly prepared capillary column at high temperatures (without column flow) the methyl groups form free radicals which readily cross-link to form a more stable, higher molecular weight gum phase. There is even some chemical bonding with the silanol groups on the fused silica surface. These cross-linked and chemically bonded phases are more temperature stable, last longer and can be cleaned by rinsing with solvents when cold. Most commercial capillary columns are cross-linked.

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...

Ethylene ionomers consist of copolymers of ethylene and an organic add, such as methacrylic acid, the acid moieties of which have been neutralized to form a metal salt. The metal salts from neighboring chains tend to form clusters, such as the one shown schematically in Fig. 18.3. The net result is the overall structure shown in Fig. 18.2 g), in which the ionic clusters form weak crosslinks between adjacent chains. Ionomers also contain short and long chain branches, which are similar to those found in low density polyethylene. 

Figure 7.17 AFM image of polyethylene grown at 160 °C and subsequently crystalli/.ed during cooling on the surface of a planar CrOVSiOj catalyst. The left hand inset indicates schematically how polyethylene molecules fold into lamellar structures. The AFM image shows how these lamellae have a tendency to order locally. The right hand inset is a measurement at higher magnification in phase contrast, and shows that lamellae contain substructure, attributed to ordered and amorphous domains (courtesy of J. Loos and P. Thiine [48]).

All of these chemical species have importance in the production of polymeric materials. There are several shorthand techniques for writing down the structures of polymers. The carbon-based polymer molecules using the stick representation are made up of atoms connected by covalent bonds (represented here by the straight lines between the carbon and the hydrogen and the carbon-to-carbon molecules), as shown in Fig. 2.6. To reiterate, carbon is always tetravalent, having four covalent bonds, and a schematic of the paired electrons for two of the incorporated carbon molecules can be seen in the bottom of Fig. 2.6. Thus each stick represents two electrons. For the two highlighted carbon atoms in the polyethylene molecule of Fig. 2.6, the electron representation is shown, where there are four covalent bonds associated with each carbon and each bond is made up of two shared electrons represented by the black dots. This polymer molecule is made up of only carbon and hydrogen with no double bonds, and it represents a linear form... 

Figure 8.4. Schematic diagram showing the structure of a typical polyethylene glycol (PEG) conjugate and the chemical structure of PEG-asparaginase.

Fig. 1. Schematic diagram of polymer structures (a) linear (b) cross-linked and (c) branched, where LDPE — low density polyethylene and...

Crystallization in asymmetric diblocks with compositions = 0.35 and 0.46 was also investigated by Hamley et al. (19966). It was found that a lamellar structure melted epitaxially (i.e. the domain spacing and orientation were maintained across the transition) to a hexagonal-packed cylinder structure in the /PE = 0.35 sample. This is illustrated in Fig. 5.15, which shows SAXS patterns in the solid and melt states, with a schematic of the epitaxial melting process (Hamley et al. 1996a.b). The same epitaxial transition has been observed for a polyethylene oxide)-poly(buty)ene oxide) diblock (Ryan et at. 1997) vide infra). 

The structural hierarchy of melt-crystallised polyethylene is schematically displayed in Fig. 8. Crystal thickness and its controlling factors are discussed in Sect. 2. It includes one phenomenologically resolved issue, the initial crys-... 

Figure 1.13 Schematic of the molecular structure of different polyethylenes.

Fig. 7.20 AFM image of polyethylene grown at 160 °C and subsequently crystallized during cooling on the surface of a planar CrOx/Si02 catalyst. The left inset indicates schematically how polyethylene molecules fold into lamellar structures. The AFM image...

Figure 2.3 Schematic molecular structures of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE)...

Figure 3.3.5 (A) Chemical structure of sulfonated perfluorinated polyethylene (Nafion ). (B) Schematic illustration of the microscopic structure of hydrated Nafion membrane perfluorinated polyethylene backbone chains form spherical hydrophobic clusters. Sulfonic end groups interface with water-filled channels and mediate the migration and diffusion of protons. The channels are filled with water and hydronium ions. Figure adapted from [4].

In 1978 Union Carbide reported a special manufacturing process called Unipol that gave linear low-density polyethylene (LLDPE). Linear low-density polyethylene may contain small amounts of butene or octene as co-monomers. The structural differences between HDPE, LDPE, and LLDPE are shown schematically in Fig. 6.1. These structural features determine physical properties such as elasticity, crystallinity, melt-flow index, etc. of the resultant polymers. 

Fig. 4.17 The crystal structure of polyethylene (a) a perspective view of the chains and the unit cell and (b) a schematic plan view looking down the c-axis, with only the directions of the interatomic bonds shown, ((a) Reproduced by permission of Oxford University Press.)...

Fig. 11.4 Schematic of (a) an automotive floor composite (500-900gsm) comprising an upper tufted carpet structure, back-coated scrim, and lower, thermoformable low density polyethylene (LDPE) acoustic layer (b) a boot sideliner comprising a preformed composite faced with a textile.

Polyethylene is a polymer formed from ethylene (C2H4), which is a gas having a molecular weight of 28. The generic chemical formula for polyethylene is -(C2H4) -, where n is the degree of polymerization. A schematic of the chemical structures for ethylene and polyethylene is shown in Figure 1.4. 

The observable differences in polymer properties between high-pressure LDPEs and low-pressure LDPEs are caused by the linearity of the main polymer chains, the molecular weight distribution, and the type of chain branching. Figure 1 illustrates schematically the polymer chain structure of various polyethylenes. 

On crystallization of polyethylene at atmospheric pres sure, the structure typical for a semicrystalline polymer results. It usually consists of nanophase-separated crystalline/amorphous phase structures, as described in Chap. 5, and is represented by phase areas 7 and 8 of Fig. 6.1. A zero-entropy production path on heating, discussed in Sect. 2.4, permits to evaluate the free enthalpy distribution in areas 7 and 8, as shown in Fig. 6.3 [4] (see also schematics of G in Figs. 2.88 and 2.120). 

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