X-Ray Analysis of Characteristic Features of the Heterogeneity of Cellulose Fibers When Interacting With Water

Abstract

UDC 539.264:677.03 We have developed a procedure for quantitative analysis of small-angle x-ray scattering by cellulose fibers swollen in water, enabling a comparative estimate of the interfacial surface area between the crystalline and amorphous phases and also the specific surface area of the crystallites. We show that the specific interfacial surface area between the crystallites and the swollen amorphous phase of the polymer practically remains unchanged on going from cotton to less crystalline mercerized and viscose rayon fibers. Mercerization of cotton fibers, under conditions ensuring complete transition of cellulose I to cellulose II, leads to an increase in the specific surface area of the crystallites by a factor of 1.4, suggesting a decrease in crystallite sizes and an increase in the amount of crystalline phase per unit weight in the polymer. Breakdown of the morphological structure of the natural cellulose material when obtaining viscose rayon fibers leads to formation of a system of crystallite formations, the specific surface area of which is 1.82 times greater than the analogous parameters for cotton. In order to estimate the reactivity of cellulose fibers, determining their specific internal surface area (SISA) is very important both in the dry state and when the fibers interact with water, i.e., under conditions approaching real dyeing and finishing processes. Comparative analysis of the specific surface area of cotton fibers from sorption of nitrogen and water vapor [1] has shown that the values of the indicated parameter determined in the first and second cases differ by tens of times (respectively 1.7 and 137.0 m 2 /g). In the author’s opinion, the observed phenomenon is due to a significant increase in micropore content in the cellulose during sorption of water vapor. In this case, by micropores we mean the space between the polymer macromolecules appearing during absorption of water by the fibers. Below the compatibility limit, water sorption occurs via the amorphous regions of the cellulose, with formation of a single-phase system in them [2-4]. The ability of the amorphous phase of cellulose to form a molecular dispersion with water, characterized by new packing of the polymer and water molecules, has been noted in [5, 6]. In this case, the water and the polymer lose their individual physical and chemical properties. Consequently, the SISA value of cellulose fibers calculated from water vapor sorption isotherms cannot be considered as reliable. A method based on analysis of the nature of small-angle x-ray scattering is also used to study the internal surface area of heterogeneous materials [7, 8]. Study of the internal surface area of cellulose fibers by small-angle x-ray scattering is complicated when a system with water is formed. Analysis of the factors determining the appearance of small-angle scattering for swollen fibers has been presented previously in [9, 10]. However, the lack of procedures for normalization of the diffraction parameters for a polymer — solvent system does not permit quantitative studies of the characteristic features of the SISA for swollen fibers. This study was devoted to solving the problems indicated above.