Análise de propriedades de superfície de xilanas modificadas

Abstract

Xylans belong to the group of hemicelluloses and are the second most abundant biopolymer in the world, since they incorporate the cell walls of plant cells. They represent a large fraction in various types of agricultural base material, integrating about 20-35% and 5- 15% of the hemicelluloses present in hardwoods and softwoods, respectively. Xylans are complex linear polymers of low molecular weight and can be characterized by their high degree of amorphousness, due to the numerous ramifications and different functional groups present throughout its structure. The composition and structure varies according to the type of tree and the wood species, but the D-xylose monosaccharide fraction is the common core structure of xylan. Due to the recent energy crisis and the declining of non-renewable materials reserves, the need to search for alternative substitutes is increasing considerably. In this context, xylans have been the target of several lines of investigation, and certain derivatives of this material have already been used in several commercial applications. The present Master's Thesis consist on the inverse gas chromatography analysis of surface properties from chemically modified xylans, in order to refine the ability to form films. In collaboration with the research group of the University of Beira Interior and RAIZ - Research Institute of Forest and Paper, the chemical surface analysis of different xylans was performed: two did not suffer any chemical modification and four endure distinct processes of functional groups addition, in particular by carboxymethylation, esterification and reductive amination. The addition of these groups has the similar objective, which is modifying the solubility or the molecular weight of the polymer in order to improve film forming properties, since the natural xylan polymer exhibits several limitations with regard to this topic. The reference xylans X-4252 (a commercial birch xylan) and X-RAIZ (a lab eucalyptus xylan extracted at RAIZ) have a considerable apolar character, making them partially insoluble in water. In fact, for the dispersive component of the surface energy,46,2 mJ/m2 was found for the first one and 41,7 mJ/m2 for the second one. Both xylans have a dominant amphoteric behavior, but essentially more acidic than basic, since there is a considerable amount of OH and COOH groups in the xylan polymer chain. Concerning the modified xylans, it was only possible to carry out the IGC analysis for two of the four samples, since both xylan, which had undergone an esterification process, showed continuous noise levels in the equipment which precluded the surface analysis. Regarding the carboxymethylated xylan (X-CM) it was concluded that the addition of carboxymethyl groups caused an increase in the polarity of the xylan, since the value of dispersive component of surface energy of this xylan (= 30,12 mJ/m2) was considerably lower than the X-4252 xylan. The predominance of the Lewis acidic character over the basic one is not so significant for the X-CM xylan, since the carboxymethyl groups introduced have a basic characteristic. Regarding the results obtained for the aminated xlan (X-AMI), as regards to the dispersive component of surface energy, they have a certain similarity to the results obtained for the commercial xylan X-4252, however the specific component of the work of adhesion with acid probes increased considerably for the X-AMI xylan, as a result of introduction of amino groups. Finally, an additional study was conducted, whose goal is to assemble all information collected, in order to characterize the potential properties that modified xylans allow to the produced films.