Trans conformation polyethylene

The potential energy shows secondary minima after rotation through 120°. These are called gauche conformations, and for polyethylene they are on the order of 4 kJ mol higher in energy than the trans conformation. 

The infrared absorption spectra of the same polymer in the crystalline and amorphous states may differ because of the following two reasons (i) Specific intermolecular interactions may exist in the crystalline polymer which lead to sharpening or splitting of certain bands and (ii) Some specific conformations may exist in one but not the other phase, which may lead to bands characteristic exclusively of either crystalline or amorphous material. For example in polyethylene terepthalate), the 0CH2CH20 portion of each repeat unit is restricted to the all trans-conformation in the crystal, but... 

First, let s revisit chain conformations. We ve mentioned that although there is a minimum energy conformation, one where all the bonds are trans in polyethylene, for example, a statistical distribution of conformations will be found in the melt. Upon cooling, however, ordered structures are formed as a result of crystallization (for reasons we consider later). So, the initial questions we want to answer are first, what is the shape or conformation of the chains in the crystal and second, how are they arranged relative to one another ... 

The other extreme conformations shown are ones in which the chain substituents are staggered. The latter are lower energy conformations than eclipsed forms because the substituents on adjacent main chain carbons are further removed from each other. The lowest energy form in polyethylene is the staggered trails conformation. This corresponds to the planar zigzag form shown in another projection in Fig. 4-1. It is also called an nW-trans conformation. This is the shape of the macromolecule in crystalline regions of polyelhylene. 

The difference in the bond polarity of C-H and C-F affects the relative stability of the conformations of the two polymer chains. Crystallization of polyethylene takes place in a planar and trans conformation. At extremely high pressure, PTFE can be forced into such a conformation. " Below 19°C, PTFE crystallizes as a helix with 0.169nm per repeat distance it takes 13 carbon atoms for a 180° turn to be completed. Above 19°C, the repeat distance increases to 0.195 nm, which means that 15 carbon atoms are required for a 180° turn. At above 19°C, the chains are capable of angular displacement, which increases above 30°C until reaching a melting point (327°C). 

Figure 1.1. The carbon-carbon backbone of a polyethylene chain in its extended planar a -trans) conformation (a) and its kinked, out-of-plane cis) conformation (b).

Since the backbone of polyethylene molecule is composed of a chain of tetrahedral carbon atoms, the molecule in aW-trans conformation has a linear zigzag structure (like corrugated sheets) shown in (I). (The dotted lines denote bonds below and the wedge signifies bonds above the plane of the page.)... 

The conformation of a polymer in its crystal will generally be that with the lowest energy consistent with regular placement of structural units in the unit cell. Helical conformations occur frequently in polymer crystals. Helices are characterized by a number fj where / is the number of monomer units per j number of complete turns of the helix. Thus, polyethylene could be characterized as a li helix in its unit cell with an -trans conformation. The arrangement of the molecules in the polyethylene crystal structure is illustrated in Fig. 2.8. 

In Table 7.3 is shown the chemical shift of polyethylene as a function of temperature [20]. The chemical shifts for the crystalline phase are independent of temperature, but those for peak A are dependent on temperature. At -120°C, the chemieal shifts for the crystalline and amorphous phases are about 33 ppm and 32 ppm, respectively. The peak for the crystalline phase arises from the erystalline part of the orthorhombic form. The chemical shift of the amorphous peak is about 32 ppm which is in the range of the tran -conformation. The methylene carbons seem to take a trani-conform-ation in the nonerystalline phase in this temperature range because of the... 

The fully extended planar zigzag trcms conformation) is the minimum energy conformation for an isolated section of polyethylene or paraffin hydrocarbon. The energy of the trans conformation is about 800 cal/mol less than that of the gauche form. Consequently, the trans form is favored in polymer crystal structures. Typical polymers that exhibit this trans form include polyethylene, poly(vinyl alcohol), syndiotactic forms of poly(vinyl chloride) and poly(l, 2-butadiene), most polyamides, and cellulose. Note that trans conformation is different from the trans configuration discussed in Section IV.A. 

A chain of four carbon atoms has three rotational isomers, those of the bond C2—C3. A chain of five carbon atoms has 3 rotational isomers—the three possibilities of bond C2—C3 being combined with the three possibilities of bond C3—C4, Consequently, a polymer chain of n carbon atoms has rotational isomers. Only very few of these contain regularly repeating sets of bond rotations, such as the sW-trans conformation that occurs in the polyethylene crystal, and the repeating sequence tg tg tg. .. that occurs in the monoclinic crystalline form of isotactic polypropylene (Fig. 3.19). The overwhelming majority of conformations are irregular, and we now consider how to generate typical members of this set. 

According to Rule No. 1, polyethylene chains tend to form the most stable aU-trans conformation in the crystal. In practice, however, such an extending of chain conformation could not easily be realized. The crystallization process often chooses the metastable folded-chain conformatirHi to form the lamellar crystals. 

Fig. 8.3. Schematic drawing of a polyethylene chain backbone in all-trans conformation, showing the bond length 6,the bond angle rand the structural composition of a monomer unit with the molar mass Mu.The torsion angle 6 between the first bond of a monomer unit to the first bond of the next monomer unit is shown along the connecting bond of the monomers...

Figure 1.7 Polyethylene terephthalate in the crystalline trans conformation (a) and in the gauche conformation present in amorphous regions (b). (After Grime and Ward [5])...

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