Cellulose families, chain conformation

The allomorphs and derivatives prepared from cellulose I and II in solid state could be transformed into cellulose I and II, respectively. The memory phenomenon of the original crystal structure should be due to a structural characteristic (chain conformation, chain polarity or others) of an individual chain that is common within each family and kept through the change of crystal structure. There were direct irreversible conversions between corresponding cellulose esters, Na-cellulose and cellulose IV prepared from cellulose I and II just like that between I and II. Accordingly, the structural characteristic should be the cause of the structural irreversibility between the I and II families. 

By analogy with the IR spectra, the 1- C NMR spectra showed that there were common differences based on the cellulose families at signals related to the chain conformation. The results were given a more reasonable interpretation through our hypothesis than the proposal of parallel and antiparallel chain systems. 

In conclusion, we think that the cause of structural irreversibility between the cellulose I and II families is an irreversible transformation between the skeletal chain conformations in the families. Although we expect that further studies of cellulose will provide clearer details of chain conformation, it is not likely that it will be possible to completely solve the structures on the basis of the limited amount of X-ray data available. 

Several studies have been made of LB films of esters of naturally occurring polysaccharides. Kawaguchi et al. [242] formed long chain esters of cellulose which, however, could only be formed into multilayers by the horizontal lifting technique. Schoondorp et al. [243] studied LB multilayers of esters of amylose and showed that materials with short alkyl side chains have a helical conformation at the air/water interface and that this structure can be transferred into multilayers. As in the case of the isotactic polymethylmethacrylate, the helical structure appears to lead to an oriented structure in the LB film. These two families of materials are illustrated in Figure 5.9. 

Atalla and Van der Hart (11, 12) concluded, based on their Raman and NMR spectra, that the molecules in cellulose I and II have different conformations. Based on x-ray analyses, Sarko et al. (13i H) and Blackwell et al. (15, 16) both concluded that crystal structures of cellulose I and II were based on parallel and antiparallel packing, respectively, of chains that have similar backbone conformations. Sarko (17) concluded that the allomorphs in the I and II families were based on parallel and antiparallel chains, respectively. The irreversibility may arise from the increase in entropy when parallel packing is converted to antiparallel packing. 

Other workers (19-20) have interpreted these differences in the NMR spectra and other data in alternative ways. They believe that celluloses I and II have the same skeletal conformation but are packed in different lattices. In this theory, the differences within the cellulose I family are derived from the size of the unit cells. Valonia contains a larger 8 chain unit cell, whereas ramie contains a mixture of the 8 chain unit cell and the smaller Meyer and Misch unit cell. Therefore the interpretation of the NMR spectra remains controversial. 

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