Some studies of crosslinking chitosan–glutaraldehyde interaction in a homogeneous system

https://doi.org/10.1016/S0141-8130(99)00068-9Get rights and content

Abstract

Chitosan dissolved in acetic acid reacted with glutaraldehyde solution, ranging in concentration from 0.10 to 25.0×10−2 mol dm3. The modified polymers were characterized by means of carbon, hydrogen and nitrogen elemental analysis, scanning electron microscopy, X-ray diffractometry, 13C nuclear magnetic resonance (NMR), infrared and Raman spectroscopies. The uptake of metallic cations in aqueous medium was checked through copper. The obtained data from 13C NMR, infrared and Raman spectroscopies evidenced the formation of an ethylenic double bond in the chitosan–glutaraldehyde interaction. These data suggest that free pendant amine groups of chitosan polymer interact with the aldehydic group of the glutaraldehyde to form stable imine bonds, due to the resonance established with adjacent double ethylenic bonds. The crosslinking is formed by the nonuniform length of chains and by terminal unities. The crosslinking formation can involve two chitosan unities belonging, or not, to the same polymeric chain. The sequence of reactions was established for a chitosan:glutaraldehyde molar proportion of 1:20. The degree of crystallinity and particle size decreased as the amount of glutaraldehyde was increased in the polymer. Physical and chemical properties are not just affected for the chitosan–glutaraldehyde reaction, but are also affected strongly by the dissolution of the natural chitosan.

Introduction

The reaction of glutaraldehyde with primary amine groups to produce covalent glutaraldehyde crosslinking is explored in many circumstances in order to detect the presence of free amine organic functions in simple or complex inorganic and organic compounds [1], [2], [3], [4], [5]. A large and clear application of this procedure can be illustrated with many polymeric matrices [6], [7]. However, the precise mechanism of reaction and the structure of the chemical compounds formed have not been studied in detail. Normally, three distinct structures are suggested. In order to interpret this behaviour, three propositions are considered: (a) there is formation of only one Schiff base, with one aldehyde group of the glutaraldehyde, the other aldehyde group remains free, and is commonly used for a subsequent reaction [2], [8]; (b) the crosslinking is formed with only one glutaraldehyde molecule and two chitosan unities, resulting in formation of two Schiff bases involving both aldehyde groups of the glutaraldehyde molecule [9]; (c) the crosslinking is formed with not only one glutaraldehyde molecule, but polymerization of glutaraldehyde, consequently forming a greater crosslinking chain [10], [11].

In general, the mechanism of this reaction is not discussed in many works. Nevertheless, Monsan et al. [4] studied the mechanism of interaction of glutaraldehyde with protein and Navarro and Manson [3] studied the mechanism of interaction of glutaraldehyde with microorganisms; they concluded that in a normal occurrence of glutaraldehyde reaction with a primary amine group, an imine bond is immediately formed. This bond is stabilized by resonance with the adjacent ethylenic double bond.

Chitosan is the product obtained from the deacetylation of the natural biopolymer chitin. Both biopolymers are chemically similar to cellulose, differing only in the functional group situated at carbon-2 of the monomeric unit. The presence of free amine groups in chitosan enhances the greater solubility and reactivity of this polymer than that of chitin and cellulose. Some products obtained by chemical modification of the chitosan have found multiple applications in various fields [12], [13], [14].

The interest in modifying chitosan by use of the glutaraldehyde has recently increased. The polymers obtained have been employed for many applications, mainly for the immobilization of the protein [15], [16], [17], [18], [19], [20], [21], [22], [23]. However, the mechanism of the chitosan and glutaraldehyde reaction and, much less, the structure of the chemical compounds formed were little studied.

Few investigations reported that the concentration of chitosan, glutaraldehyde and acetic acid, pH and temperature of the reaction medium are relevant features to be considered in the chitosan–glutaraldehyde reaction, mainly when these determinations are related to physical and chemical properties of the polymers obtained. The properties considered include dynamic storage modulus [11], the efficacy of metallic cation adsorption [24], [25], resistance to crush, solubility [9], enzyme immobilization, internal area [26], rate of gelation and colour formation [10]. On the other hand, the different degree of deacetylation of the chitosan seems to affect the formation of the covalent glutaraldehyde crosslinking on chitosans, an increased dynamic storage modulus was found within gels high in minor degree of deacetylation, suggesting that hydrophobic interactions are involved [11]. Recently, Crescenzi et al [27] noticed a counterproposal to the chitosan–monofunctional aldehyde interaction; the stoichiometry cannot be easily controlled with other aldehydes. Therefore, since the beginning this was a well-known complexity of this reaction [23]. The aim of this publication is to report some results connected to homogeneous chitosan–glutaraldehyde system reactions, in which the biopolymer chitosan is dissolved in acetic acid and reacted with glutaraldehyde in variable concentrations. From the collected data, a contribution to improve the understanding of the physical–chemical properties, the mechanism of this global reaction and the polymeric structure formed is proposed.

Section snippets

Chemicals

Chitin from shrimps shells was acquired from Fine Chemical Kito (Palhoça-SC-Brasil). Glutaraldehyde 25% solution in water (Aldrich), glacial acetic acid (Ecibra), hydrochloric acid (Nuclear), sodium borohydrate (Aldrich), EDTA (Nuclear) and copper nitrate (Vetec) or bidistilled water were used in the experiments.

Preparation of chitosan

Chitosan was prepared by deacetylation of chitin in alkali solution, NaOH 50%, during 1 h at 110°C. The solid was filtered, washed intensively with bidistilled water until a nearly

Degree of deacetylation of the chitosan

The degree of deacetylation was verified by infrared spectroscopy. The absorbance values found for amide I at 1655 cm−1 and hydroxyl groups at 3450 cm−1, gave 0.343 and 0.481, respectively. Applying these values in the correspondent equation gave 63.8%, which is an accepted good degree of deacetylation for this procedure [11]. This transformation changes the physical and chemical properties related to solubility of this natural organic polymer chitin, mainly in dilute acetic acid solutions [31]

Conclusions

Based on the obtained results, the reaction of chitosan dissolved in acetic acid at pH 3–4 with glutaraldehyde performed in a short time, less than 1 h. In this connection, the expected protonation of the amine groups of the chitosan does not affect the reaction, as observed by a series of the compounds obtained in this chitosan–glutaraldehyde system. However, the concentration of glutaraldehyde strongly affects the physical and chemical properties of the general CGX compounds formed. In the

Acknowledgements

The authors thank CNPq for fellowships and gratefully acknowledge FAPESP for financial support.

References (41)

  • C. Giacomini et al.

    J Mol Catal B

    (1998)
  • J. Rogalski et al.

    J Mol Catal B Enzymatic

    (1997)
  • H. Ishii et al.

    Int J Biol Macromol

    (1995)
  • K.I. Draget

    Polym Gels Networks

    (1996)
  • B. Krajewska et al.

    J Mol Catal B Enzymatic

    (1997)
  • K. Kurita et al.

    Carbohydr Polym

    (1997)
  • G.F. Payne et al.

    Polymer

    (1996)
  • R. Agarwal et al.

    Anal Chim Acta

    (1995)
  • R.A.A. Muzzarelli

    Enzyme Microbiol Technol

    (1980)
  • V. Crescenzi et al.

    Polym Gels Networks

    (1997)
  • G.H. Moore et al.

    Int J Biol Macromol

    (1980)
  • M. Hasegawa et al.

    Carbohydr Polym

    (1993)
  • J. Hajdu et al.

    Anal Biochem

    (1975)
  • R.A.A. Muzzarelli et al.

    Carbohydr Polym

    (1989)
  • A.R. Cestari et al.

    Langmuir

    (1997)
  • J.M. Navarro et al.

    Ann Microbiol

    (1976)
  • P. Monsan et al.

    Biochimie

    (1975)
  • P.M. Hardy et al.

    J Chem Soc Perkin Trans

    (1972)
  • G. Wuff

    Angew Chem Int Ed Engl

    (1995)
  • T.-Y. Hsien et al.

    Sep Sci Technol

    (1995)
  • Cited by (0)

    View full text