Calcium alginate gel fibers: Influence of alginate source and gel structure on fiber strength

Abstract

SYNOPSISThe low-deformation extension modulus was measured for Ca-alginate gel fibers prepared from alginates of different compositions and molecular weights. The modulus showed an increase with increasing molecular weight for alginates of the same composition and sequence parameters. Fibers prepared with an inhomogeneous polymer concentration profile showed higher moduli than did fibers with a homogeneous polymer concentration profile. The higher modulus observed for fibers prepared from an alginate with low Ca2+ affinity is tentatively explained as resulting from a higher degree of polymer chain alignment. INTRODUCTIONAlginates are a family of h e a r copolymers of p-Dmannuronate ( M ) and a-L-guluronate ( G ) with widely varying monomer composition and sequence. The alginates are well known for their gelling and viscosifying properties, 'y2 and they are also used for fiber spinning by extruding a sodium alginate solution into a calcium chloride spinning bath.Alginate fibers have several commercial or potential areas for use in such diverse areas as in pharmacy as wound dressings, 3-6 in food technology, for paper production, in the textile industry, and for immobilization purposes.13 Despite the diversity in the use of alginate fibers, not much is found in the literature regarding the physical properties of such fibers in relation to the chemistry of the material used. Alginates, which often are regarded as a defined polymer, are, in reality, a family of biopolymers rather than one polymer with defined physical and chemical properties.',' The physical and chemical properties of alginates depend on both the monomeric composition and sequence of the polymer, which vary greatly depending on the source of ~~ ~ * To whom correspondence should be addressed. ) , the G-centered triad frequencies ( F G G G , FMGM, and F G G M ) and the average number of consecutive guluronate residues in the G-blocks ( N G , l ) were determined from the 'H-NMR spectra recorded at 92OC on a Bruker 500 WM spe~trometer.'~''~ The molecular weight of the alginate samples was determined from intrinsic viscosity measurements using the Mark-Houwink-Sakurada equati~n.'~,'~ The intrinsic viscosity was measured in 0.1M NaCl in aqueous solution in a Micro-Ubbelohde viscometer, Schott-Gerate. The composition and molecular weight parameters are given in Table I. Alginate gels, when prepared by diffusing gelling ions into a Na-alginate solution, show a concentration inhomogeneity as illustrated in Figure 1. This concentration inhomogeneity can be controlled by a careful selection of alginate molecular weight, concentration of gelling ions in the gelling bath, and concentration of nongelling ions in the alginate solution and in the gelling bath.19920 Fibers with a homogeneous alginate concentration profile were made by extruding a 3% Na-alginate/0.2M NaCl solution through syringes with a diameter of 0.5 mm into a bath of 0.1 M CaC12/0.2M NaCl and winding them onto a drum with a diameter of 20 cm. Fibers with an inhomogeneous concentration profile...

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