Molecular chain conformations

Changes in the conformation of the different forms of cellulose have been investigated157,158,195,196 by use of Raman spectroscopy. Celluloses I and II were found157 to have different, and distinct, molecular-chain conformations. No assignments of the frequencies were proposed, but the correlation between the spectra and the structure of celluloses was discussed. The major differences in the Raman spectra were observed below 800 cm-1, in the... 

To elucidate the phase structure in detail it is necessary to characterize the molecular chain conformation and dynamics in each phase. However, it is rather difficult to obtain such molecular information, particularly of the noncrystalline component, because it is substantially amorphous. In early research in this field, broad-line H NMR analysis showed that linear polyethylene crystallized from the melt comprises three components with different molecular mobilities solid, liquid-like and intermediate molecular mobility [13-16]. The solid component was attributed to molecules in the crystalline region, the liquid component to... 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

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. 

Figure 2 Characterization of molecular chain conformations by integer matrices derived from interatomic distances. The molecular chain is a 10-atom random walk with excluded volume interaction. The matrix is a 2D descriptor defined by the distances and one external parameter, the number of levels k into which the range of distances is divided. Note that the swollen, strand-like conformation is characterized by a matrix with constant diagonal bands.

CP/MAS NMR approach has been described to characterize molecular chain conformations and crystal structures of native and regenerated celji lose samples in the dry and hydrated states. First, C isotropic chemical shifts in the solid state are correlated to torsion angles cj) and vp in the 3"l 4-glycosidic linkage and X about the exo-cyclic C5-C6 bond, respectively. 

The surfactant can block the polar groups of the polymer, disturbing the intra- and intermolecular bonds [73-75], This results in alteration of the molecular chain conformation and of the intermole-... 

As described above, high-resolution C NMR spectra are obtainable also for solid samples, combining techniques such as DD, MAS and CP. It is, then, possible to investigate the molecular chain conformation characteristics of the solid state via the chemical shift of individual carbons. Furthermore, if one measures simultaneously the relaxation of C magnetization, one also expects to obtain information on molecular chain dynamics. This section describes the relaxation phenomenon. 

Next, we have examined the relationship between the chemical shifts and the molecular chain conformation. The versatility of chain conformation of cellulose molecules is expressed in three torsion angles as shown in Fig. 14 (j> and xp, rotations around the /3-l,4-glycosidic linkages and the rotation of methylol side groups around the C5-C6 bonds. In order to find out the relationship between the chemical shifts of cellulose samples and the torsion angles, the... 

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