Cellulose esters, conformational changes

Macroscopic and microscopic conformational changes of cellulose esters in CDC13 solution have been examined as a function of temperature and concentration.120 The temperature dependence of the T, values for the carbonyl and ring carbons of CTA in CDC13 shows a remarkable change in segmental motion at 53°C. This apparent transition, which is observed neither in solutions of CTA in (CD3)2SO nor in solutions of cellulose propanoate and cellulose butanoate in... 

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. 

Cellulose derivatives pack as sheet-like structures, for example, cellulose triacetate or butyrate as well as some ethers, e.g., trimethyl cellulose. Regio-selective derivatives of these mentioned esters or ethers placed at C2 and C3 with various other groups at C6 may belong to the same conformation class, that is, their conformation is determined by the substituent at positions 2 and 3. Many cellulose derivatives crystallize as rod-like helices of various kinds, e.g., 3/2,8/5, and these structures are also found in solvent built-in crystals. It should be mentioned that several cases are known where a change in conformation occurs if a polar solvent is built into the interstitial spaces between the polysaccharide chains. The sheet-like cellulose triacetate conformation converts to a rod-like 8/5 helix with nitromethane present. 

Another example of curvature of van t Hoff plots relates to irreversible changes in the conformation of carbamate-derivatized amylose and cellulose CSP, which was observed for the normal-phase separation of the enantiomers of a dihydropyrimidinone acid and methyl ester. The apparent conformational change was thermally induced and depended upon the polar component of the mobile phase. The irreversible change in the conformation of... 

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