Handbook of Size Exclusion Chromatography and Related ...

Concentration-sensitive detectors (104,105) as well as low-angle laser light
scattering (LALLS) detectors (106,107) have been used during the last decade.
Cellulose acetate is commonly produced by reaction with acid anhydride
usingacatalystsuchaszincchlorideorsulfuricacid(35).Inthissolutionprocess,
the derivative formed is dissolved in glacial acetic acid or dichloromethane. For
cellulose triacetate (CTA) (DS 2.8–3.0) the fibrous process is commonly applied.
This process uses perchloric acid to catalyse the reaction and anonsolvent of the
derivative to maintain the fiber structure. Cellulose diacetate (CDA) can also be
made by deacetylation of CTA.
The chemical and enzymatic reactivity is highly dependent on the DS of
the cellulose acetate. The depolymerization of cellulose is catalyzed by sulfuric
acid; that is, the latter does not only catalyze the acetylation reaction. By using
GPC-LALLS, Shimamoto et al. (107) found that the depolymerization reaction
is faster during the early stages of acetylation than for the fully substituted
derivative, and also that the depolymerization of CTA proceeds randomly
whereas the hydrolysis of cellulose does not. The degree of biodegradability is
also closely connected to the DS, the lower DS the more biodegradable the
cellulose acetate becomes (105).
Depending on application, the target DS is in the range 1.2–3. The
solubility depends on the DS but also on the distribution of substituents between
the three possible positions (108). CDA (DS 2.2–2.7) is soluble in acetone and
THF whereas CTA requires chloroform or dichloromethane for dissolution.
From light-scattering studies, various degrees of aggregation of dilute solutions
of CTA in m-creosol, tetraethane, and mixtures of dichloromethane–methanol
have been shown (109). SEC characterizations have been performed primarily
on cellulose diacetates and triacetates using THF (110,111), chloromethane
(112), or acetone (106) as solvent, but also polar solvents such as DMAc
(104,105) or N-methylpyrrolidone (NMP) (104) with or without addition of salt
have been used in the past (Table 5). Owing to the high viscosity of cellulose
acetate solutions of DMAc or NMP, the chromatography is performed at
elevated temperature.
A general problem encountered with SEC of cellulose acetate is the
presence of extra humps and/or shoulders on the high molecular mass range of
the main distribution and, in addition, a gel fraction (104,106,110–112).
Whereas the gel fraction may be found in solutions of cellulose acetate samples
from both cotton linter and wood pulps, the other anomalies are commonly only
observed in cellulose acetates from wood pulps. The observed prehumps
correlate with the hemicellulose content of the sample and can be reduced by
optimizing the reaction conditions during acetylation or removed by fractional
precipitation (110,111).
The extra peaks have also been attributed to ionic effects caused by sulfate
groups in the CDA solutions of acetone (106,113). The prehumps could only be
© 2004 by Marcel Dekker, Inc.