Polyethylene zigzag form

The crystal structures of C2Ht—CO copolymers with QH4/CO ratios of 1, 1.3, 2.2 and 3.5 have been determined49,50). For the 1 1 copolymer an orthorhombic unit cell of dimensions a = 7.97, b = 4.76, c (fiber axis) = 7.57 A was observed 49). The main chain had a planar zigzag form. For copolymers with higher C2Ht contents, the fiber periods were essentially identical with that of polyethylene (c 2.54 A) 50). Also, the higher the C2H content, the shorter the a axis and longer the b axis. 

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 polyethylene molecule may be represented skeletally in a planar zigzag form as shown in Fig. 2.12, where I = 0.154 nm and 9 = 109.5°. In order to perform the calculation, the number n of backbone bonds is required. It can be obtained... 

Figore 2.9 Skeletal representation of polyethylene chain in planar zigzag form. 

Polyethylene chains form a zigzag shape due to the characteristic tetrahedral bond angle of 109.5°. The chain can bend and rotate within the region marked by the dotted line. 

Unlike polyethylene, which crystallizes in the planar zigzag form, isotactic polypropylene crgtaUizes in a helical form because of the presence of the methyl groups on the chain. Commercial polymers are about 90 to 95 percent isotactic. The amount of isotac-ticity present in the chain will influence the properties. As the amount of isotactic material (often quantified by an isotactic index) increases, the amount of crystallinity will also increase, resulting in increased modulus, softening poinL and hardness. 

Because of the larger fluorine atoms, PTFE assumes a twisted zigzag in the crystalline state, while polyethylene assumes the planar zigzag form. There are several crystal forms for PTFE, and some of the transitions from one crystal form to another occur near room temperature. As a result of these transitions, volumetric changes of about 1.3% may occur. 

The unit cell structure of polyethylene was first investigated by Bunn (20). A number of experiments were reviewed by Natta and Corradini (21). The unit cell is orthorhombic, with cell dimensions of a = 7.40, b = 4.93, and c = 2.534 A. The unit cell contains two mers (see Figure 6.5) (22). Not unexpectedly, the unit cell dimensions are substantially the same as those found for the normal paraffins of molecular weights in the range 300 to 600 g/mol.The chains are in the extended zigzag form that is, the carbon-carbon bonds are trans rather than gauche. The zigzag form may also be viewed as a twofold screw axis. 

Not all polymer chains can be crystallized only those with stereoregularity can. For example, polyethylene can be crystallized because there is a regular configuration inherent in the monomer. Polypropylene, on the other hand, can be crystallized only under certain conditions. The crystal structures of these two polymers are also quite different. The former is packed in a zigzag form, whereas the latter has helical content (not, of course, 100%). We select these two polymers to illustrate the crystal structure, if any, of synthetic polymers. 

Polyethylene can adopt a number of crystal forms depending on the method of preparation. The most common form is a centrosymmetiic orthorhombic form, space group Pnma = (number 62), with two formula units (CH2-CH2) in the unit cell rotated at 90° with respect to each other as shown in Fig. 10.1. Each chain adopts an all-tram planar zigzag conformation. This can also be described as a 2 helix with one complete turn for each two CH2 units. 

Structure [3, 4]. As described in Chapter 7, the chemical shifts of CH2 for paraffins and polyethylene in orthorhombic form are shifted by about 1 ppm when compared with those in the monoclinic and triclinic forms. Quantum chemical calculations reveal that the chemical shift difference is caused by a local difference of mutual orientation for the trans-zigzag plane in inter-molecular interactions in the orthorhombic form and the triclinic and monoclinic forms [5]. 

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. 

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