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Modern Polymer Spect..

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102 3 l,i'brationcil <strong>Spect</strong>ra as n Probe of Structural Order<br />

300G i<br />

-<br />

2Got<br />

3000<br />

r<br />

7<br />

250{ -<br />

1500<br />

l 5 O 0 L =<br />

loook<br />

Figure 3-1. Dispersion curves of single-chain trans planar polyethylene. (a) v/p; (b) i$/k.<br />

As an example, in Figure 3-1 we report the dispersion curves of a single chain of<br />

polyethylene in v, and k space. The chain of trims-planar polyethylene -(CW?),,- is<br />

generated by application to a single CH2 group of the operator $(n. d/3). It follows<br />

that the crystallographic repeat unit contains two CH? groups. The physics and the<br />

numbers are not changed. The dispersion curves in Figure 3-lb are obtained by just<br />

halving Figure 3-la at .n/2 [53].<br />

As an example of a more complex polymer chain we quote the case of polytetrafluoi-oethylene<br />

-(CFZ)n- for which one can identify the rototranslational<br />

operator $(2n/15, d/15) which describes a polymer chain made up by z = 15 CF2<br />

units coiled in a helix with t = 7 turns. In cases such as polytetrafluoroetliylene the

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