Effect of modification of powdered chitosan on its amorphous-crystalline and adsorption-porometric characteristics
Abstract
The subject of the study was a high molecular weight powdered crab chitosan that does not dissolve in aqueous media, has a high sorption and film-forming ability and is a promising raw material for the production of adsorbents, flocculants, and film and membrane materials. The aim of the work was to identify the effect of chitosan modification by swelling in water vapor or reprecipitation from solutions with alkalis followed by freeze-drying to amorphous-crystalline and adsorption-porometric characteristics. For the study, granulometric, X-ray diffraction, sorption-structural analysis methods were used.
The structure and properties of the initial chitosan and its modified forms are studied in detail. It is shown that the investigated high-molecular crab chitosan is a partially crystalline polymer. Using the method of low-temperature sorption of liquid nitrogen, adsorption isotherms for the initial air-dried and lyophilized chitosan dried after equilibrium swelling with water vapor were obtained. Sorption and porometric characteristics are calculated.
It was found that the powdered chitosan studied is a mesoporous material with a range of pore radii from 1.5 to 45 nm. The distribution of the integral and differential values of the volume and the specific surface of pores along the radii is analyzed. A comparative analysis of the obtained results showed that reprecipitation from the solution and a freeze-drying method of chitosan lead to a rearrangement of its supramolecular structure with a decrease in the degree of crystallinity from 0.5 to 0.1-0.22. As a result of amorphization and loosening of the polymer matrix, the proportion of fine pores and the internal specific surface increases, the number of primary adsorption centers increases and their availability. It was found that the freeze-drying of the re-precipitated chitosan leads to a significant increase in its adsorption capacity.
The use of the proposed methods for modifying chitosan allows us to significantly intensify the ad- ventive processes when it is used as sorbents and flocculants, to accelerate its solvation in the preparation of solutions, and also opens up possibilities for regulating the selective transport characteristics of the chitosan separation membranes.
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