Polymers disordered chains

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

An amorphous bulk polymer contains chains that are arranged in less than a well-ordered, crystalline manner. Physically, amorphous polymers exhibit a Tg but not a T, and do not give a clear x-ray diffraction pattern. Amorphous polymer chains have been likened to spaghetti strands in a pot of spaghetti, but the true extent of disorder that results in an amorphous polymer is still not fully understood. 

Surface imperfections are often caused by damaging they are the most important source of cracks that are propagated throughout the material under influence of external or internal stresses. Volume imperfections are all kinds of structural disorder, such as dislocations in metals and crystalline polymers and chain ends in all polymers. 

The influence of chain conformation on mobility is clearly seen in the data for PDA-4BCMU. The mobility is highest for isolated, extended linear polymer chains produced at low concentration in the monomer crystal matrix and falls dramaticallyfor the disordered chains in a yellow solution, but is slightly... 

In a perfectly crystalline polymer, all the chains would be contained in regions of three dimensional order, called crystallites, and no glass transition would be observed, because of the absence of disordered chains in the sample. A perfectly crystalline polymer, on heating, would thus follow curve G-F-A, melting at to become a viscous liquid. 

Side-chain LCPs are, in a way, more subtle than their main-chain counterparts because, thanks to the flexible spacers, the polymer main chain (called backbone) still retains many degrees of freedom. In fact, there is a true balance between the natural disorder of the backbones and the liquid-crystalline order of the rod-like moieties. For instance, in the SmA phase, it was recently demonstrated by SANS that the backbone performs a 2-dimensional random walk between the smectic layers [22]. Therefore, these materials have poor mechanical properties but display a rich variety of mesophases, even at room temperature. This makes them more suitable for applications in displays and electro-optic devices. 

The free-volume concept has been mentioned in previous sections, but it is instructive now to consider this idea more closely and to draw together the various points alluded to earlier. Free volume, Vf, is defined as the imoccupied space in a sample, arising from the inefficient packing of disordered chains in the amorphous regions of a polymer sample. The presence of these empty spaces can be inferred from the fact that when a polystyrene glass is dissolved in benzene, there is a contraction in the total volume. This and similar other observations indicate that the polymer can occupy less volume when surrounded by benzene molecules, and that there must have been unused space in the glassy matrix to allow this increase in packing effidraicy to occur. 

In another way, ct-amino acid N-carboxyanhydrides are attractive monomers for the synthesis of controlled architectures (14AML378). Poly-phosphonohomoalanine is a pH-responsive polymer which undergoes consequent conformational modifications when pH goes from 7.4 to 1.0. At pH 1, circular dichroism experiments showed that the fuUy protonated polyacid was mainly present on a ct-helical form whereas at pH 7.4, electronic repulsions of the polyanionic system induced a disordered chain conformation (Figure 3). 

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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