Birch xylan

Hardwood xylans and xylans of annual plants may contain up to 7% O-bound acetyl groups. Seven out of ten xylose residues in native hardwood xylan are acetylated on C-2 and/or C-3 (10). Because of the possible migration of O-acetyl groups between 2- and 3-positions during and after isolation of hemicellulose components, it is difficult to determine their original distribution in nature (11). The ratios reported for 2-, 3-, and 2,3-positions of acetyl groups in birch xylan have been 2 4 1 (3) and 2 2 1 (10) and in bracatinga xylan 3 3 1 (12). 

Recent studies have revealed certain interesting features in the structure of a birch xylan (Fig. 3-16). The unit next to the reducing xylose end group is D-galacturonic acid, linked to a L-rhamnose unit through the C-2 position. The rhamnose unit, in turn, is connected through its C-3 position to thje xylan chain. 

Fig. 3-16. Structure associated with the reducing end group of birch xylan (Johansson and Samuelson, 1977).

Hansson, J.-A. (1970). Sorption of hemicelluloses on cellulose fibres, Part 3. The temperature dependence on sorption of birch xylan and pine glucomannan at kraft pulping conditions. Sven. Papperstidn. 73,49-53. 

Johansson, M. H., and Samuelson, O. (1977). Alkaline destruction of birch xylan in the light of recent investigations of its structure. Sven. Papperstidn. 80, 519-524. 

Enzymic hydrolysis of the birch xylan previously referred to also gave a series of five acidic oligosaccharides, three of which (5) were fully identi-fied as 4-0-Me-a-D-GpA- (1—>2) -/3-n-Xylp- (1 ) -/3-D-Xylp-( 1—>4) -/3-D-Xylp and its two higher x>-xylo polymer homologs. The nature of these oligosaccharides again proves the mode of attachment of the acid side-chains. 

Evidently, the indiAddual xylan molecules do not contain the same relative number of acid side-chains, and it is to be expected that these groups will have a more decisive influence on the solubility of the polysaccharide than will its molecular weight. Similar variations in uronic acid content of birch-xylan fractions have been reported by other investi-gators. ... 

Hardwood hemicelluloses are represented by xylans and a small proportion of glucomannans. Hardwood xylans are linear polymers, constituted of [l,4]-linked xylanopyranosyl units that constitute the main skeleton. Every tenth D-xylanopyranosyl unit is substituted by a 4-0-methyl-D-glucuronic acid residue, linked to the birch xylan chain by [1,2] linkages, that has been found to retard the alkaline peeling reaction. Analysis of partially hydrolyzed xylan indicates that 4-0-methyl-D-glucuronic acid units linked to the C2 position are randomly distributed along the birch xylan backbone [9]. 

The positive-ion MALDI mass spectrum obtained for the oligo- and polysaccharides obtained from a hardwood (birch) xylan after mild acid hydrolysis is shown in Figure 7 (upper spectrum). This spectrum consists of several partially over-lapping series of peaks, which were identified on the basis of their exact molar masses as corresponding to neutral xylosaccharides with DP up to 30 and acidic xylosaccharides with one or two 4-0-MeGlcA substituents. 

Figure 4. Baseplane unit cell projection of white birch xylan hydrate. The unit cell is indexed as hexagonal a-b 9.16 A c (fiber axis) = 14.85 A. The circle represents the position of the water column. (By permission from ref 5)...

The softening behavior of hemicelluloses in a humidity scan of 0.1%RH/min is exemplified in Figure 2 for the birch xylan at 60°C at a load frequency of 1 Hz. With increasing moisture content in the material the mobility of the polymer chains increased, evident as a drop in the storage modulus. 

Figure 2. The relative storage modulus and the mechanical damping, tan d, as functions of relative humidity for birch xylan at IHz and 60°C. Scanning rate 0.1 %RH/min. The method of determining the softening humidity is indicated.

Figure 3. The softening point of the birch xylan at various temperatures shown in a diagram of the logarithm of the loading frequency versus the relative humidity. The lines are based on a linear regression of the data for each...

Figure 4. Water sorption isotherms obtained for birch xylan during absorption at different temperatures.

Figure 7 shows Arrhenius plots for the softening at different moisture ratios for the birch xylan obtained by this procedure. At higher moisture ratios the slope of the Arrhenius curve is lower. From such a slope an apparent activation energy, AHa, for the softening process can be calculated as ... 

Figure 6. Illustration of the procedure for converting softening points as a function of moisture ratio to an Arrhenius plot, showing the logarithm of the load frequency versus the reciprocal of the softening temperature at a moisture ratio of 16% in the birch xylan. The left part of the figure is a partial view of...

Figure 7. Arrhenius plots for the glass transition of birch xylan at different moisture ratios. The values are derived from the results in Figure 5.

Figure 9. The relative elastic modulus of glucomannan from Amorphophallus Konjac at 50°C and birch xylan at 60°C as a function of relative humidity at a load frequency of 1 Hz, in humidity scans of 0.1 %RH/min.

Figure 10 shows the derived values of the glass transition temperature for the birch xylan and the glucomannan as a function of moisture ratio compared with data for the transition temperature for softwood hemicelluloses taken from literature (4). In the xylan, the glass transition temperature at 50°C and 1 Hz occurred at 76 %RH, which corresponded to a moisture ratio of 22%, whereas for the glucomannan at 50°C and 1 Hz it occurred at 65 %RH, corresponding to a moisture ratio of 16%. 

The various cellulose substrates and the polypropylene membrane were autoclaved in the presence of a NaOH solution of birch xylan (pH 10). Weight increases were found for all substrates except the PP membrane after exposure to the xylan solution for 3 h. Table 1 summarizes the amounts of xylan retained... 

The xylan was commercial birch xylan (4-0-methylglucuronoxylan fix>m Roth, 14 400 g/mol, DP 100). It contains 7.8 % a-D-methylglucuronic acid side groups and is alkali soluble. NaCl, NaOH and HCl were all of analytical grade. Solutions were prepared in doubly distilled water (Millipore). The mica was muscovite mica (Electron Microscopy Sciences, FT.Washington). 

In this preliminary study, extracted willow xylan was not immediately available for use in blending studies. As a result, native, partially acetylated birch xylan and a synthetically acetylated elm xylan were obtained from the sample collection of Professor T. E. Timell, an emeritus faculty member at our institution. Blends of these polymers with cellulose esters and bacterial polyesters were typically prepared by mbcing solutions of the respective polymers dissolved in either dimethylformamide (DMF) or water followed by evaporation in a vacuum oven at 10S°C yielding a thin film. A thermoplastic bacterial co-polyester (poly(hydroxybutyrate)) containing 30% hydroxyvalerate content) was purchased from the Aldrich Chemical Company (cat. no. 28,248-0). The glass transition temperature (Tg) of each polymer and blend was determined using a TA Instruments 2920 differential scanning calorimetry (DSC) instrument. 

The higher the molar mass of the mannan, the better was its stabilizing effect. Deacetylated GGMs, in turn, were poor stabilizers. A birch xylan was also an excellent stabilizer, while neutral and cationic starches were not very effective. However, in another similar study birch xylan was found to have only a minor stabilizing effect [52]. 

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