Trans-gauche crystalline phase

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

The chemical shift is determined by the relatively local electronic structure. One of the most important parameters which affect chemical shift is conformation. As mentioned in the section about crystalline and amorphous phases, a typical example for the conformational effect on the chemical shift is the chemical shift difference between the crystalline and amorphous phases of polyethylene. In the crystalline phase, polyethylene takes the all trans-zigzag conformation, while, in the amorphous phase, a rapid transition between the trans and gauche conformations takes place. As a result, the chemical shift of the amorphous phase is the average of the trans and gauche conformations. 

In some polymers, the crystalline phase shows a different C lineshape as compared to that of amorphous. For these, there is a chanee to observe lineshape changes due to recrystallization. In PET, the C resonance of the methylene carbon in the trans-conformation is narrower than that in the ganc/ie-conformation, and appears at lower frequency. Thus, it is possible to determine the relative amount of these conformers. Upon annealing PET/ Vectra-A, Tang et al. [103] found that the relative amount of the gauche-conformer is reduced greatly, and its linewidth becomes narrower. Another example can be found for Nylonb. The recrystallization of the a-crystalline... 

Most of the reported structural transformations of PC lipid bilayers with respect to environmental conditions such as temperature, pressure, pH, etc., are associated with isomerizations of the constituent lipid molecules (mostly 14—24 carbon acyl chains). For instance, the transition from the ordered gel to fluid liquid crystalline phase, called the main phase transition, has been related to the melting of the hydrocarbon chains in the gel phase, phospholipids with all trans alkyl chains are present, whereas in the disordered liquid crystalline phase the most populated conformational states correspond to gauche forms in the alkyl chains. 

ETIR analysis provided a direct method for the evaluation of the amount of alpha and beta form crystalline phases in syndiotactic PS, although both contain chains in the same conformation (trani-planar) (186). Syndiotactic PS was found to exhibit two distinct vibrational peaks in the Raman spectrum. The presence of long il-trans sequences gives rise to a peak at about 773 cm", whereas trans/gauche conformations result in a separate peak at 798 cm. With increasing levels of crystallinity, the integrated intensity of the 773 cm grows at the expense of the 798 cm. The relative area... 

Based on the values of hyperfine coupling constants due to fl-protons, the steric configuration of the atoms in the stable radical must be as illustrated in Fig. 5.5. It is well known that the conformation of the polyethyleneoxide chain in its normal crystalline phase is of trans-trans-gauche type i.e., a 7 -helical structure as illustrated by the left structure in Fig. 5.5. Since this conformation is in good agreement with that of the stable free radical represented by the right structure in Fig. 5.5., it can be assumed that the 7 -helical structure of the polyethyleneoxide chain still exists in irradiated UPEOC. The values of the line width AH j, and hyperfine coupling constants due... 

Figure 16.4 Illustration of the influence of the polymer chain conformation and the crystalline structure on the dipole moment and ferroelectricity of PVDF (left) polymer chain takes a//-trans conformation, resulting in a net dipole moment for the polymer chain. These polymer chains are packed into a ferroelectric structure /p phase a —8.58 A, b —4.9 A and c = 2.58 A along chain direction) in which the dipole moments line up (right) polymer chain fates trans-gauche conformation, resulting in a zero net dipole moment. These polymer chains are packed into a non-ferroelectric structure (a phase a = 4.96 A, b 9.64 A, and c= 4.62 A along the chain direction) in which the dipole moments cancel each other out.

Various authors have developed methods based on Raman spectroscopy for determining crystallinity. In some polymers, specific vibrational bands have been attributed to the crystalline phase. In others, the ratio of the intensities of trans conformation vibrations to gauche conformation vibrations has been used. Still others have used the width of a particular vibrational band to indicate crystallinity. The last method is based on the effect of the local environment on the frequency of the band. In a random environment, many different local environments exist and the band is broadened by these variations. In a crystal, the local environment is the same for all of the chain segments. Then all of these segments have (nearly) the same frequency and the band is narrow. For a band that is sensitive to the local environment, a narrow band indicates high crystallinity and a broad band indicates low crystallinity. 

However, the determination of the volume fraction of the crystalline phase in a polymer using the mid-infrared spectra faces difficulties. The latter are controlled mostly by molecular structure and conformation of macromolecules, and the usual estimation of the crystallinity degree from the absorbance ratio of the bands corresponding to traits and gauche conformers is not perfectly correct, because the disordered regions of the polymer also contain the trans conformers. 

---