Amorphous Conformation

When the isopentane-quenched sample is annealed at 100 °C for 10 hr, only a 10% decrease in amorphous structures is observed (Figure 12). The crystalline vibrations show only a slight increase. The difference spectra at 78 K show the same behavior (Figure 13). A sample was also quenched in ice water for comparison with the isopentane quench method. The subtractions of the spectra of the two quenched specimens from the low-crystallized one are shown in Figure 14. Each technique produces a similar amount of amorphous conformations. The decreases in amorphous content by annealing are the same for both systems (Figure 15). 

Model constructions (pure polymer chain or polymer blends under periodical boundary condition and in an amorphous conformation). 

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. 

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... 

Infrared (IR) and Raman spectroscopies have been used for decades to routinely characterize polymeric and other materials. Vibrational Spectroscopy (qv), particularly Fourier transform IR (FTIR), has been used extensively to probe crystalline and amorphous conformations in a wide variety of polymers, as well as to determine a measure of the crystallinity of such materials. In the FTIR spectra of crystalline polymers, one or more absorption bands are often observed that disappear when crystallization is inhibited. Provided these bands can be genuinely assigned to 3-D crystalline order, and if the absorbance of this band in the specimen under examination is in the range for which the Beer-Lambert Law is applicable, then... 

B. Bom, H. W. Spiess, Ah Initio Calculations of Conformational Effects on NMR Spectra of Amorphous Polymers Springer-Verlag, New York (1997). 

Unlike other synthetic polymers, PVDF has a wealth of polymorphs at least four chain conformations are known and a fifth has been suggested (119). The four known distinct forms or phases are alpha (II), beta (I), gamma (III), and delta (IV). The most common a-phase is the trans-gauche (tgtg ) chain conformation placing hydrogen and fluorine atoms alternately on each side of the chain (120,121). It forms during polymerization and crystallizes from the melt at all temperatures (122,123). The other forms have also been well characterized (124—128). The density of the a polymorph crystals is 1.92 g/cm and that of the P polymorph crystals 1.97 g/cm (129) the density of amorphous PVDF is 1.68 g/cm (130). 

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

To understand the global mechanical and statistical properties of polymeric systems as well as studying the conformational relaxation of melts and amorphous systems, it is important to go beyond the atomistic level. One of the central questions of the physics of polymer melts and networks throughout the last 20 years or so dealt with the role of chain topology for melt dynamics and the elastic modulus of polymer networks. The fact that the different polymer strands cannot cut through each other in the... 

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

In some crystalline polymers chemical shift differences between crystalline and amorphous phases have been observed and interpreted and for several crystalline forms the signals to be attributed to nuclei in different conformational environments have been identified [111, 112]. 

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