Deacetylation of chitosan films

Wang Yong, He Yang, Liu Qun, Li Jinyun, Ma Xiaojun
(Biomedical Materials Engineering Laboratory, Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, China)

Received May 29, 2000; Supported by the National Natural Science Foundation (29576264) and the Natural Science Foundation of Liaoning Province (962070).

Abstract In this paper, a novel method was established for preparing 100% deacetylated chitosan products. The facile and effective deacetylation was obtained by only one-step alkali treatment after the low deacetylated chitosan sample was dissolved in acid solution and regenerated with the form of film from the solution by solvent evaporation.
Keywords Chitosan, deacetylation

Deacetylation of chitin with aqueous alkali is the most commonly used method to obtain the chitosan products. In general, alkaline deacetylation of chitin proceeds rapidly before the polymer is about 75-85% deacetylated and further treatment has only a very limited effect on the extent of deacetylation unless drastic conditions are used. So the degree of nearly 100% deacetylation is rarely achieved with the relatively mild and simple method.
    There were several reports^[1,2] in the literature dealing with the preparation of highly deacetylated chitosan. However, the emphasis was concentrated on carrying out the multistage treatment interspersed with washing and drying or using water-miscible organic solvents as reaction medium. Their chitosan samples pretreated were prepared from acid solution by coagulation on immersion in alkali. In this paper, a novel method was established for preparing 100% deacetylated chitosan products. The facile and effective deacetylation was obtained by only one-step alkali treatment after the low deacetylated chitosan sample was dissolved in acid solution and regenerated with the form of film from the solution by solvent evaporation.

1 MATERIALS AND METHODS
Low deacetylated chitosan sample was obtained from Chengdu Zhida Inorganic Factory (China). The degree of deacetylation of this sample was 83.5% and the limiting viscosity number (LVN) was 759 ml/g. The degree of deacetylation was determined by solid state CP/MAS ¹³C NMR spectra (400 MHz). The LVN measurements were performed at 30±0.1°C in 0.2mol/L CH₃COOH / 0.1mol/L CH₃COONa aqueous solution with an Ubbelohde dilution capillary viscometer.
    The low deacetylated chitosan was first dissolved in dilute 2 vol.-% acetic acid, then regenerated in the form of film by solvent evaporation at room temperature. The chitosan films were put into 50 wt-% NaOH solution in a flask at 110 or 90°C under a nitrogen atmosphere for 1 or 0.5 hour. Then, the product was washed with deionized water to neutral, washed with alcohol and acetone several times and dried in vacuum. The processes of deacetylation are schematically summarized in Table 1.

2 RESULTS AND DISCUSSION
2.1 Increase of the degree of deacetylation during deacetylation
It was obviously reflected in Fig.1 that the degree of deacetylation of chitosan films increased from 83.5% to 100% in only one-step alkali treatment in all of the four treatment processes, De1, De2, De3 and De4. In fact, the velocity of deacetylation depends on whether or not it is easy for NaOH to contact the C (2)-acetamido group in chitosan, which is mainly determined by the crystal structure and the aggregated state of chitosan. Chitosan with different physical forms possess different aggregated states and crystal structures. The physical form of chitosan relies on the specific route for preparing chitosan sample. There are two routes for regenerating chitosan from solution: one is by solvent evaporation, and the other is by coagulation on immersion in alkali.

Table 1. Experimental conditions of treating low deacetylated chitosan sample

Fig 1. Solid state CP/MAS ¹³C NMR spectra of chitosan before and after the different processes of alkali treatment.

    Theoretically, the infinitely thin film of chitosan can be developed after the infinitely dilute solution spread on the glass plate is dried at room temperature. Thus, the molecular chains of chitosan will be well distributed. However, when chitosan is prepared from solution by coagulation, the chains are deprotonated before packing together. It is reasonable to expect that uncharged chains would aggregate more closely together than charged chains of chitosan films developed by regeneration from solution by evaporation. So it will be easy for alkali to get near and react with the amides of chitosan when chitosan films prepared as thin as possible are immersed in the concentrated alkaline solution, which will lead to the quite effective deacetylation in a short time.
    In addition, the crystal structures affect the efficiency of deacetylation. It was considered reasonable that heterogeneous deacetylation occurs preferentially in the amorphous regions then continues more slowly from surface to center of the crystalline region ^[3]. It was found that the X-ray diffraction diagrams of chitosan films cast from solution by evaporation showed the characteristics of amorphous materials whether chitosan was in the salt form or the free amino form. However, those of chitosan samples precipitated from solution by coagulation on immersion in alkali showed a high degree of crystallininty ^[4]. Therefore, it will be easy and desirable to obtain the effective deacetylation of chitosan films with the amorphous form and loosely aggregated state.

2.2 Decrease of the LVN value during deacetylation
It is unavoidable that chitosan is cleaved in the deacetylation reaction. Many procedures using an inert atmosphere, an oxygen scavenger and a reducing agent have been proposed for reducing the deleterious effects of alkaline degradation. Factually the best method to prevent the chain scission is to shorten the reaction time and decrease the deacetylation temperature. As mentioned above, chitosan with the low degree of deacetylation was deacetylated to 100% in one-step reaction at the low temperature. Under the same conditions, the LVN values did not decrease drastically, as shown in Fig.2. The decrease of LVN value in the processes of De3 and De4 was less than that in the processes of De1 and De2, respectively. Compared with the De1 process, the De2 process was carried out with the higher LVN value of chitosan. These phenomena illustrated that the deacetylation temperature and time had a strong influence on the degradation of chitosan chains.

Fig 2. Variation of the LVN value of chitosan before and after the different processes of alkali treatment.

    In summary, completely deacetylated chitosan with high viscosity can be obtained by treating chitosan films in alkaline solution at low temperature and in a short time.

Acknowledgement
The authors wish to thank Prof. Lanna Yang for her assifanal.

REFERENCES
[1] Mima S, Miya M, Iwamoto R et al. J. Appl. Poly. Sci., 1983, 28: 1909.
[2] Bastista I, Roberts G A F. Makromol. Chem., 1990, 191: 429.
[3] Roberts G A F. Chitin Chemistry, London: Macmillan, 1992, 77.
[4] Nud'ga L A, Plisko E A, Danilov S N. Zhur. Obsh. Khim., 1971, 41: 2555.

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