Sorption properties of TEMPO-oxidized natural and man-made cellulose fibers

Abstract

Cotton and lyocell fibers were oxidized with sodium hypochlorite and catalytic amount of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO), under various conditions. Water-insoluble fractions, collected after TEMPO-mediated oxidation, were analyzed and characterized in terms of weight loss, aldehyde and carboxyl contents, and sorption properties. Aldehyde and carboxyl groups were introduced into the oxidized cotton up to 0.321 and 0.795 mmol/g, and into the oxidized lyocell up to 0.634 and 0.7 mmol/g, respectively, where weight loss was generally lower than 12% for cotton and 27% for lyocell. Oxidized cotton and lyocell were shown to exhibit 1.55 and 2.28 times higher moisture sorption than the original fibers, respectively, and water retention values up to about 85% for cotton and 335% for lyocell, while iodine sorption values of oxidized fibers were lower up to 35% for cotton and up to 18% for lyocell than the original fibers.

Introduction

The polysaccharides are the most abundant carbon-based polymers on the surface of the earth. For millennia, they have provided mankind with a functional, low cost and renewable raw material. By conservative estimates, well over 10¹² tons of cellulose, starch and related biomaterials are generated on our planet each year due to natural processes (Hon, 1996, Schurz, 1999).

Chemical modifications of polysaccharides are interesting routes for preparing new type of polymers on the basis of cellulose, with advances in performance and application, so-called “biopolymers for the future” (Chang and Robyt, 1996, Hon, 1996, Schurz, 1999, Tahiri and Vignon, 2000). The three hydroxyl groups of the cellulose molecule responsible for sorption properties (moisture, water, dye sorption capabilities, etc.), can undergo chemical reactions common to all primary and secondary groups, such as esterification, nitration, etherification and oxidation. The oxidation of cellulosic material is a quite frequent procedure in cellulose chemistry, and the oxidation is the only process that renders cellulose bioabsorbable in man (Stilwell, Marks, Saferstein, & Wiseman, 1997). For specific applications of cellulose fibers in medical devices such as absorbable hemostatic agents and absorbable adhesion barriers, drug delivery matrix, scaffold in tissue engineering, sensors, etc., it is necessary to obtain cellulose fibers with improved sorption properties, while the rate of bioabsorption can be adjusted by the degree of oxidation (Hon, 1996, Stilwell et al., 1997).

In the recent decade, catalytic oxidation using water-soluble and stable nitroxyl radicals such as 2,2,6,6-tetramethylpyperidine-1-oxy radical (TEMPO) under aqueous conditions has become one of the interesting routes to introduce carboxyl functionalities into polysaccharides (Chang and Robyt, 1996, Dang et al., 2007, de Nooy et al., 1994, de Nooy et al., 1995, Saito and Isogai, 2006, Saito et al., 2005, Tahiri and Vignon, 2000). The TEMPO-mediated oxidation of cellulose was described to cause selective oxidation at C6 of the anhydroglucose units to carboxyl groups via the intermediate aldehyde stage. The nitroxyl radical affects the oxidation from the alcohol to the aldehyde oxidation state, while the hypobromide generated in situ from hypochloride and bromide performs the further oxidation of the aldehyde to the carboxylic acid (Chang and Robyt, 1996, Saito et al., 2006). The advantages of TEMPO-mediated oxidation are: high reaction rate and yield; high selectivity; catalytic process, just modest degradation of polysaccharides through out the process, and only inexpensive NaClO and NaOH are consumed as the oxidation proceeds (Saito et al., 2006). On such way introduced functionalities can be either used directly because of their special properties, e.g. hydrophilization and improved sorption properties by introduction of carboxyl groups, or they can be used as reactive “chemical hooks” for further chemical modification.

According to the literature (Chang and Robyt, 1996, Dang et al., 2007, de Nooy et al., 1995, Saito and Isogai, 2005, Saito et al., 2005) TEMPO-mediated oxidation of cellulose pulp is widely investigated, while TEMPO-mediated oxidation of cellulose fibers has been investigated in considerable smaller scope (Saito & Isogai, 2006). In this paper, we chose cotton and lyocell fibers for obtaining information about influence of TEMPO-mediated oxidation on properties of natural and man-made cellulose fibers. After TEMPO-mediated oxidation of cellulose fibers under various conditions, collected water-insoluble fractions were analyzed and characterized in terms of weight loss values, aldehyde and carboxyl contents. The changes in sorption properties of cotton and lyocell fibers were evaluated by determination of moisture sorption, water retention power and iodine sorption value. On the bases of the results obtained in this study, an attempt has been made to explain the individual roles of introduced functionalities and changed fiber structure in the sorption properties of the oxidized cotton and lyocell fibers.