Molecular conformation in the amorphous polymer

The reader will have noted that although we have made precise statements about the molecular conformation in the crystal, our description of molecular conformations in the amorphous fraction of crystalline polymers has been vague. There are two reasons for this. First, there is no precise physical technique available for determining the amorphous conformation. Second, statistical methods are inapplicable for the short length of the molecule which is constrained between the crystals for the long macromolecule in the completely amorphous state the position is quite different, as we now show. 

1 In recent years, however, neutron scattering has yielded evidence supporting the condusions outlined in this section. 

13 The rotation around C — C bonds which causes the polyethylene planar zigzag conformation to be changed into one of the enormous number of random conformations typical of the amorphous polymer. 

14 Rotational freedom in an idealized chain of four carbon atoms. 

The probability can be thought of physically in two ways, which appear at first sight different, but are in fact the same. Consider a cubic metre of 

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Most differences between crystalline and amorphous phases are attributed to the order and disorder of polymer chains. All polymer chains in a crystalline phase see neighbouring chains at the same distance with the same mutual orientation. In contrast, neighbouring chains in the amorphous phase exist at random distances and random mutual orientations. Therefore, there is room for molecular motion in the amorphous phase. As the molecular motion causes a conformational transition, both parameters correlate with each... 

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 the first section, up to Uhlmann s paper, we arc concerned with polymer melts in equilibrium. In dilute solution, the dominant restriction on randomness, apart from the very fact that the polymer molecule is a chain, is self-exclusion, and Paul Flory taught us how to cope with that many years ago. In the interior (and I emphasize interior) of the pure amorphous phase, mutual exclusion has a large effect on the total entropy, but its effect on molecular conformations is the relatively minor one of virtually cancelling the effects of self-exclusion. That knowledge we also owe to Flory. Hence a limited number of parameters suffice both to describe and explain the conformations in this case. Uhlmann s paper dismisses for us the aberrant nodular structures which have been proposed there only remains to ask how in certain circumstances the appearance of nodular structure can be produced. 

Fluoropolymers are semicrystalline polymers most do not exhibit glass transition in the conventional sense during which all crystalline structures are converted to the amorphous. The glass transitions of fluoroplastics have been described as molecular relaxation (conformational disorder) that takes place in the amorphous phase of the polymer. These temperatures are also called second order transitions their value depends on the technique and the frequency of energy addition to the polymer sample. Table 3.61 presents these temperatures and melting points of perfluorinated and partially fluorinated fluoroplastics. 

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