Analysis of the heterogeneity of dry cellulose fibers by small-angle x-ray diffraction

Abstract

We describe a technique for analysis of the specific internal surface area (SISA) of dry cellulose fibers by small-angle x-ray scattering, making it possible to rapidly evaluate the change in the polymer matrix — microvoid interface for cellulose-based materials when exposed to modifying factors. We demonstrate that treatment of cotton fibers with sodium hydroxide solutions, even under conditions ensuring exhaustive polymorphic conversion of the cellulose, leads to a decrease in SISA by only a factor of 1.16. This phenomenon is due to preservation of the fibril structure of the fibers upon mercerization. We show that dispersal of cotton fibers as a result of acid hydrolysis when microcrystalline cellulose is obtained leads to an increase in the specific internal surface area of the material by a factor of 1.32. The SISA of cotton is 7.6 times higher than the analogous parameter for viscose rayon fibers, for which during their synthesis the morphological structure of the natural material (and consequently also its microporous structure) is completely broken down. The heterogeneity of the physical structure of cellulose fibers and the presence of a number of fibril formations in them lead to formation of different size voids (pores) and consequently an internal surface [1]. The indicated factor, along with the content of the amorphous phase of the polymer, is a parameter determining the reaction of cellulose fibers with reagents during dyeing and final finishing. It is important to note that by no means is there always a clear relationship between the dyeability of cellulose fibers and their ordering [2]. This fact indicates that we need to take into account the porous structure and the specific internal surface area (SISA) of cellulose materials when evaluating their reactivity. Study of dry cotton fibers by the method of sorption of inert gas vapors has shown that their SISA lies within the range from 0.3 to 0.8 m 2 /g [3], which is consistent with the low pore content, where the pore volume is only 3% to 6% of the total volume of the polymer [2, 4]. Higher values of the indicated parameter for natural cellulose fibers (1.7 m 2 /g) are given in [1]. We must note that in such studies, the low temperature of the sorption experiment (—196°C) may have a significant effect on the measured parameters. In order to study the internal surface area of heterogeneous (porous) materials, a method is also used that is based on analysis of the nature of small-angle x-ray scattering, the theory of which has been developed for an ideal system of two phases with constant electron density and a dramatic change in the indicated parameter at the phase interface [5, 6], which is not always satisfied for polymers and fibers. Furthermore, the algorithm for calculating the interfacial surface area from small-angle x-ray scattering data includes a parameter depending on the radiation collimation conditions and the shape of the scattering particles, which are complicated to take into account. This study is devoted to solving the indicated problems for analysis of cellulose materials. As the objects of investigation, we used boiled fabrics made from cellulose fibers: viscose rayon, cotton, and cotton treated with 23% NaOH solution at 20°C for 60 seconds. We also analyzed microcrystalline cellulose (MCC) obtained from cotton fibers by acid hydrolysis [7].