Hemicelluloses molecular-weight determination

Although most polysaccharides may be acylated without degradation, acyl derivatives of xylans29 are often unsatisfactory for molecular-weight determinations owing to their poor solubility characteristics. Mixed esters of xylans143 are often superior in this respect, but there are obvious difficulties in the differential analysis of acyl groups. Hemicellulose nitrates have been used for measurements of molecular size,168 but xylan nitrates,29 like xylan acetates, are reported to have poor solubility characteristics. 

The results of molecular-weight determinations on hemicelluloses need to be interpreted in the light of comments made earlier (see p. 461) regarding the degrees of heterogeneity possible in polysaccharides of this type. 

Table V. Molecular weight determination of hemicellulose fractions...

The molecular weights of the hemicelluloses and their carboxymethylated derivatives were determined by GPC on a PL aquagel-OH50 column (3007.7 mm. Polymer Laboratories Ltd.). Detection was achieved with a Knauer differential refractometer. The weight of carboxymethylated hemicelluloses became lower than that of native hemicelluloses (26800 g.moP), as the Mw range from 12213 to 15312 g/mol. In addition, carboxymethyl hemicelluloses had a relatively low index of polydispersity (2.75-2.87), which indicated that the molecular weights of hemicellulosic derivatives were more uniform [72]. 

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