Disordered conformations crystalline polymers

The recommendations embodied in this document are concerned with the terminology relating to the structure of crystalline polymers and the process of macromolecular crystallization. The document is limited to systems exhibiting crystallinity in the classical sense of three-dimensionally periodic regularity. The recommendations deal primarily with crystal structures that are comprised of essentially rectilinear, parallel-packed polymer chains, and secondarily, with those composed of so-called globular macromolecules. Since the latter are biological in nature, they are not covered in detail here. In general, macromolecular systems with mesophases are also omitted, but crystalline polymers with conformational disorder are included. 

Polymers showing crystalline mesomorphic modifications, characterized by chains in disordered conformations, are exceptions to the equivalence principle. Examples of these cases are shown in Section 2.6. 

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene (PTFE), /ra .s-1,4-poly (1,3-butadiene), and cis-1,4-poly(isoprcnc). 

Moreover, we have already shown in the Section 2.6 that chains characterized by disordered conformation may crystallize. The presence of disorder in polymer crystals is a typical feature, so that the real crystalline forms are generally different from the ideal models. These disordered modifications can,... 

The isotactic polyolefins prepared using a Ziegler— Natta catalyst form a helical conformation in the solid state (crystalline regions).11 38,42 This helical structure persists in solution, but because of fast conformational dynamics, only short segments of the helix exist among disordered conformations. When an isotactic polyolefin is prepared from an optically active monomer having a chiral side group, the polymer shows the characteristic chiroptical properties which can be ascribed to a helical conformation with an excess helicity.12,43-46 The chiroptical properties arise in this case predominantly from the helical conformation of the backbone. 

Wide-angle X-ray scattering techniques can provide direct information on key features such as crystallinity, preferred orientation, phase identification and compositional analysis.. zfs jjoj-g detailed analysis can yield details of local chain conformations and packing arrangements in both crystalline and disordered polymers. ... 

More in general, solid mesophases not only include crystalline forms of polymers containing a large amount of disorder in the conformation of chains and long-range order in the position of chain axes as in condis crystals, but also crystalline polymers characterized by disorder in the lateral packing of conformationally ordered chains [13,14]. 

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene [10-12,49-54], cis-l,4-poly (isoprene) [100-102] and trans-1,4-poly( 1,3-butadiene) [8,9,55-58]. In these cases, disorder does not destroy the crystallinity because of the similar shape of the various conformational units. The occmrence of cases of conformational isomorphism of the first kind demonstrates that a polymer chain can remain straight, as if it was constrained to run inside the walls of a tight cylinder, while its conformational freedom remains of the same order of magnitude as that in the melt. 

IR and Raman spectra are particularly sensitive to the configurations and conformations of the polymer molecules. The variations in the conformational and configurational structures are reflected in specific observable frequencies. Consequently, it is possible to detect and quantify the amounts of the conformational isomers whether they be crystalline, liquid crystalline, or disordered (amorphous). The vibrational spectroscopic approach to structural elucidation in polymers relies on the comparison of vibrational spectra of polymers containing specific conformational structures (incorporated into the polymer during polymerisation or by thermal or chemical modification), with spectra of models (polymers and small molecules) containing similar structures presumed to be present in the polymers. 

The nearly 1000-fold difference in the values of of the crystalline and amorphous carbons in PE permit their separate observation. However, for semi-crystalline polymers with rigid amorphous phases characterized by high glass-transition temperatures, Tg, such a clean separation may not be possible. Although the chains in a glassy polymer sample are disordered, they may not be sufficiently mobile to rapidly sample all potentially accessible conformations. As a consequence, each amorphous conformational environment will contribute a different solid state chemical shift, producing a broad envelope of resonances for the disordered chains. Warming the semi-crystalline sample above Tg will... 

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