Polyethylene molecules

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

Fig. 5.9. The three-dimensional appearance of a short bit of a polyethylene molecule.

For most practical purposes a polymer may be defined as a large molecule built up by repetition of small, simple chemical units. In the case of most of the existing thermoplastics there is in fact only one species of unit involved. For example the polyethylene molecule consists essentially of a long chain of repeating —(CH2)—(methylene) groups, viz. 

If the horizontal arm is rotated about its axis, the other arm will form a cone of revolution. On the polyethylene molecule, the bent wire is similar to the carbon backbone of the chain with carbon atoms at positions 1, 2 and 3. Due to the rotation of the bond 2-3, atom 3 may be anywhere around the base of the cone of revolution. Similarly the next bond will form a cone of revolution with atom 3 as the apex and atom 4 anywhere around the base of this cone. Fig. A.5(b) illustrates how the random shape of the chain is built up. The hydrogen atoms have been omitted for clarity. 

The ease with which a polymer will form into crystalline regions depends on the structure of the molecular chain. It can be seen, for example, that if the polyethylene molecule has a high degree of branching then it makes it difficult to form into the ordered fashion shown in Fig. A.9. Also, if the side... 

Metal alkyl is liable to react with nonsaturated ends of polyethylene molecules ... 

The term polyethylene separator is somewhat misleading, since this separator consists mainly of agglomerates of precipitated silica, held within a network of extremely long-chained, ultrahigh-molecular weight polyethylene molecules. The raw materials, precipitated silica (Si02 — about 60 percent), ultrahigh-... 

Even in the absence of flow, a polymer molecule in solution is in a state of continual motion set forth by the thermal energy of the system. Rotation around any single bond of the backbone in a flexible polymer chain will induce a change in conformation. For a polyethylene molecule having (n + 1) methylene groups connected by n C — C links, the total number of available conformations increases as 3°. With the number n encompassing the range of 105 and beyond, the number of accessible conformations becomes enormous and the shape of the polymers can only be usefully described statistically. 

Writing the stmctural formula of a macromolecule such as polyethylene with thousands of atoms would be veiy time-consuming and tedious. Fortunately, the entire structure of a polyethylene molecule can be represented by simply specifying its repeat unit, as shown in Figure 13-2. 

C13-0084. Compute the molar mass of a polyethylene molecule that has 744 monomer units. 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

Figure 1.2 Polyethylene molecule with a short chain (butyl) branch...

Figure 1.3 Polyethylene molecule with a long chain branch...

We can create thermoplastic polymers by chain growth or step growth reactions. In either case the polymer chains consist of a string of monomer residues, each of which is attached to two other monomer residues. The polyethylene molecule shown in Fig. 1.1 is an example of a thermoplastic polymer made via chain growth polymerization, as shown in Fig. 1.7,... 

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]).

Optical Isomerism of (D-L-Isomerism) and Tacticity of Polymers Optical isomerism has its origin in the way different substituents occupy positions on an asymmetric carbon atom in a polymer molecule. For example, polyethylene molecule has fully saturated carbon atoms as shown in the following chemical formula ... 

Because of the longer residence times, this type reactor can make polyethylene molecules of higher molecular weights that also have high melt temperatures. Both HOPE and LLDPE are produced this way. 

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... 

As shown originally by Malcolm Dole, polyethylene molecules may be cross-linked when subjected to high-energy radiation. These three-dimensional network polymers may be represented by the structure shown in Figure 1.5. 

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). 

FIGURE 3.22 A polyethylene molecule is a very long chain of — CH2—CH2— units. Each C atom is sp3 hybridized. 

Figure 12. The setting angle of polyethylene molecule in the unit cell (41)...

Polymer molecular weight and molecule size. You are asked to compute the maximum possible separation between the ends of a high density polyethylene molecule with an average molecular weight of 100,000. 

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...

With this approach the inside of a blown container is exposed to fluorine in such a way that hydrogen atoms on the polyethylene molecule may be replaced by fluorine. Thus polar groups are introduced into the thermoplastic and make... 

Here, again, we start from compressible SCFT formalism described in Section 2.2 and consider a model system in which bulk polymer consists of "free" matrix chains (Ny= 300) and "active" one-sticker chains (Na= 100). Flory-Huggins interaction parameters between various species are summarized in Table 1. This corresponds to the scenario in which surfactants, matrix chains, and functionalized chains are all hydrocarbon molecules (e.g., surfactant is a C12 linear chain, matrix is a 100,000 Da molecular weight polyethylene, and functionalized chain is a shorter polyethylene molecule with one grafted maleic group). The nonzero interaction parameter between voids and hydrocarbon monomers reflects the nonzero surface tension of polyethylene. The interaction parameter between the clay surface and the hydrocarbon monomers, Xac= 10 (a = G, F, A), reflects a very strong incompatibility between the nonpolar polymers and... 

Figure 6.1 Schematic representation of high-density (HDPE), linear low-density (LLDPE), and low-density (LDPE) polyethylene molecules.

---