Polyethylene helical structure

The fully extended trans-trans conformation of the polyether chains would require an increase of 3.5 A per monomer unit or 7 A increase in interlayer spacing for every integral increase in the value of n because of the bilayer arrangement.. We observe less than half this value. Polyethylene oxide has a helical structure in the solid state (10), the polymer chain having 7 monomer units in 2 turns of the helix... 

Table 6 Relative stabilities of different regular,helical structures of polyethylene and polyoxymethylene. Energy values in kcal/mol. ...

Figure 3.6. Zigzag structure of polyethylene and helical structure of polypropylene.

The conformation adopted by a molecule in the crystalline structure will also affect the density. Whereas polyethylene adopts a planar zigzag conformation, because of steric factors a polypropylene molecule adopts a helical conformation in the crystalline zone. This requires somewhat more space and isotactic polypropylene has a lower density than polyethylene. 

Such normal vibration analyses have been applied to the spectra of macromolecules to only a limited extent. In the first place, the only structure which has been analyzed in detail is that of the planar zig-zag chain of CHg groups, i.e., polyethylene. Neither substituted planar zig-zag chains nor the helical chain structures characteristic of many polymers [Bunn and Holmes (28)] have been submitted to such a theoretical analysis. In the second place, even for the case of polyethylene the answers are not in all instances unambiguous. Different assumptions as to the nature of the force field, and lack of knowledge of some of the force constants, has led to varying predictions of band positions in the observed spectrum. For the identification of certain modes, viz., those which retain the characteristics of separable group frequencies, such an analysis is not of primary importance, but for knowledge of skeletal frequencies and of interactions... 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

As pointed out above with relation to the data at 87 °C, the Tic of the crystalline-amorphous interphase is appreciably longer than that of the amorphous phase, suggesting the retention of the helical molecular chain conformation in the interphase. We also note that a Tic of 65-70 s for the crystalline phase is significantly shorter than that for other crystalline polymers such as polyethylene and poly-(tetramethylene oxide), whose crystalline structure is comprised of planar zig-zag molecular-chain sequences. In the crystalline region composed of helical molecular chains, there may be a minor molecular motion in the TiC frame, with no influence on the crystalline molecular alignment that is detected by X-ray diffraction analyses. Such a relatively short TiC of the crystalline phase may be a character of the crystalline structure that is formed by helical molecular chain sequences. 

In polypropylene, one methyl group replaces an hydrogen atom in the polyethylene repetitive unit this results in an intense structure located in the middle of the C-C (C2s) band. In isotactic polypropylene moreover, the bonding and antibonding structures of this band are split in accordance with theoretical calculations performed on isotactic polypropylene in an helical conformation (1, A4). 

Polymer Conformation and Crystallinity. Beyond the stereoregularity and tacticity, the geometrical conformation of the polymer chain in the solid material could influence its electronic structure, through a modification of its valence band molecular orbitals. Indeed, a few years ago, very characteristic band structures were calculated for T, G, TG, and TGTG polyethylenes ( ). More recently. Extended Huckel crystal orbital calculations showed that for isotactic polypropylene, a zig-zag planar or a helical conformation resulted in significant changes in the theoretical valence band spectra, supporting the idea that conformation effects could be detected experimentally by the XPS method ( ). 

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