Hemicelluloses (or heteropolysaccharides) can also be used as a raw feedstock for numerous polymeric biomaterial applications, but thus far they have not received as much attention as cellulose or startch polysaccharides despite their abundance and structural diversity.32In many lignocellulosic refining processes to obtain cellulose, hemicelluloses are partly degraded. Hemicelluloses are hydrophilic components of the cell wall and they can be extracted from plant-material either by water and/or alkaline media. Hemicelluloses are classified according to their structure, compris­ing monomers (D-xylose, D-mannose, D-galactose, L-arabinose and D-glucose) forming xylans, mannans, galactans, arabinans and p-glucans, respectively, in the polymer main chain. In addition, most hemicelluloses possess side groups of 1-2 monosaccharide units and also acetyl groups. The most abundant hemicelluloses are xylans and mannans. While xylans are the most common hemicelluloses and considered to be the major noncellulosic cell wall polysaccharide component of an — giosperms (e. g., hardwood, grasses, and cereals), mannans (galactoglucomannans) are the predominant hemicelluloses in softwoods.

Recently, the incorporation of xylans and mannans for biodegradable films has been explored. They have the potential to be blended with other polymers or mixed with nanoparticles to achieve desirable properties.33

Xylan consists of р-D-xylopyranose units, linked by (1-4)-bonds, and different side groups depending on the plant source. The degree of polymerization (DP) of native cellulose is ten to one hundred times higher than that of hemicellulose. For instance, the DP of wood cellulose was reported to be 10,000 while the DP of wood arabino glucuronoxylan is about 100 and that of glucuronoxylan is about 200. In hardwood xylans, seven out of ten xylose units in the backbone contain an O-acetyl group at C-3 or C-2 (Fig. 14.7). The acetyl groups cannot be found in softwood xylans.

Alternating D-glucopyranosyl and D-mannopyranosyl units attached by P-(1—4) bonds characterize galactoglucomannans and glucomannans. The ratio of glucose to mannose in hardwood glucomannans can vary between 1:2 and 1:1 based on the wood species. Softwood galactoglucomannans can be basically categorized into two fractions with different galactose contents that contain D-galactopyranosyl units at­tached by a-(1-6) linkages to the backbone mannose chain. Figure 14.8 illustrates the structure of O-acetylated galactoglucomannan.

In terms of properties, hemicelluloses have the ability to absorb large amounts of water. They are highly hydrophilic due to the numerous free hydroxyl groups in their structure. Depending on the hemicellulose type, the solubility in water varies ac­cording to the degree of substitution: the higher the degree of substitution, the more water-soluble is the hemicellulose. The degree of polymerization also influences the solubility of hemicelluloses. In general, long chains are less water-soluble.34

Differently from cellulose, hemicelluloses do not have crystalline domains and they present low degrees of polymerization compared to cellulose, which probably lower their chemical and thermal stability compared to cellulose.