Influence of the carbohydrate composition on the molecular weight distribution of kraft pulps

The molecular weight distribution (MWD) of hardwood kraft pulps and softwood kraft pulps dissolved in lithium chloride/N,N-dimethyl-acetamide (LiCl/DMAc) have a different shape. Two fairly well separated distributions of hardwood kraft pulps are obtained by size exclusion chromatography whereas the MWDs of softwood kraft pulp is more complex. Since these two pulps contain different types of hemicelluloses, the neutral carbohydrate composition of different cuts of the MWDs was examined. The results indicate that the two distributions observed for hardwood kraft pulps originate from cellulose and xylan, respectively. In contrast, the hemicellulose of softwood kraft pulps elute over the entire molecular weight range. The carbohydrate analysis of the collected fractions shows that glucose is the major component in all fractions. The relative concentration of mannose decreases during elution, i.e. with decreasing molecular weight (M) whereas galactose, xylose and arabinose increases during elution, i.e. with decreasing M. The coelution of cellulose and glucomannan may be due to association properties, which gradually decrease with increasing amount of galactose substituents. The difference in elution behaviour between hardwood and softwood kraft pulp xylans may be due to different amounts of glucuronic acid groups and/or how these groups are distributed along the main chain. It is thus more difficult to interpret differences in the MWDs of softwood kraft pulps than of hardwood kraft pulps. (C) 2000 Elsevier Science Ltd. All rights reserved.