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Handbook of Size Exclusion Chromatography and Related ...

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4.5 Other Cellulose Derivatives<br />

Besidescelluloseacetates,thepreviouslydescribedcellulosederivativesaremade<br />

tostudythecelluloseitself.Inthissection,SEC<strong>of</strong>etherderivativesmadeforsome<br />

given applications are mainly reviewed. These derivatives are heterogeneous; not<br />

only with respect to the types <strong>of</strong> substituents, but also because most <strong>of</strong> them are<br />

only partially substituted to attain the desired properties. SEC conditions used<br />

during the last decade for characterization <strong>of</strong> ionic <strong>and</strong> nonionic cellulose ethers<br />

are shown in Tables 7<strong>and</strong> 8, respectively.<br />

Examples <strong>of</strong> ionic cellulose ethers are carboxymethyl cellulose (CMC),<br />

mixed derivatives such as carboxymethyl hydroxyethyl cellulose (CMHEC), <strong>and</strong><br />

amphoteric cellulose derivatives (140–142) such as carboxymethyl-2-diethylaminoethyl<br />

(CM-DEAE) cellulose. Examples <strong>of</strong> nonionic organic ethers that<br />

recently have been characterized by SEC are methyl cellulose (MC), hydroxyethyl<br />

cellulose (HEC), hydroxypropyl cellulose (HPC), ethyl(hydroxyethyl) cellulose<br />

(EHEC), hydroxypropyl(methyl) cellulose (HPMC), <strong>and</strong> benzylated pulps. In<br />

addition, different types <strong>of</strong> hydrophobically modified CMC (HMCMC), have been<br />

studied by SEC (143-147).<br />

A common feature <strong>of</strong> partially derivatized cellulose is the tendency to form<br />

supermolecular structures in solution (148). This has been attributed to a<br />

nonr<strong>and</strong>om aggregation caused by an uneven derivatization along the cellulose<br />

chain, that is, blocks <strong>of</strong> less substituted chain segments. Commonly used mobile<br />

phases for SEC characterizations <strong>of</strong> cellulose ethers are aqueous saline or buffers.<br />

For polyelectrolytes, such as CMC, a high ionic strength <strong>of</strong> the mobile phase has<br />

the advantage <strong>of</strong> reducing the hydrodynamic volume, thereby reducing the effect<br />

<strong>of</strong> heterogeneity <strong>of</strong> the ionic groups along the polymer as well as reducing the<br />

viscosity <strong>of</strong> the sample (149). The relative viscosity <strong>of</strong> injected samples as<br />

compared to the mobile phase should be below 1.5 to obtain peak shapes <strong>and</strong><br />

retention times that are independent <strong>of</strong> sample concentrations (150,151). On the<br />

other h<strong>and</strong>, too high salt concentrations promote hydrophobic interaction between<br />

the sample <strong>and</strong> the stationary phase. Addition <strong>of</strong> methanol to the mobile phase is<br />

commonly practiced to circumvent associations <strong>of</strong> nonionic derivatives <strong>of</strong> medium<br />

polarity (Table 8).<br />

Sodium CMC is the most widely used cellulose ether. The most commonly<br />

used type <strong>of</strong> CMC has a DS <strong>of</strong> 0.65–1.0 (152), <strong>and</strong> is soluble in water. It has a<br />

wide range <strong>of</strong> utilization, for example, as an emulsion stabilizer, thickener, sizing<br />

agent, <strong>and</strong> binder. Water-insoluble CMC with a DS <strong>of</strong> less than 0.4 <strong>and</strong> crosslinked<br />

water-soluble CMC are used as superadsorbents <strong>and</strong> ion exchangers. Rinaudo <strong>and</strong><br />

co-workers (153) concluded that the charge density <strong>of</strong> CMC does not change with<br />

Mw in the range 40,000–550,000 <strong>and</strong> that a DS between 1.0 <strong>and</strong> 2.9 neither<br />

influences the refractive index increment dn/dc nor the K <strong>and</strong> a parameters in the<br />

Mark–Houwink relationship. The latter means that the universal calibration<br />

© 2004 by Marcel Dekker, Inc.

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