versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.49 n.4 Concepción dic. 2004
| J. Chil. Chem. Soc., 49, N 4 (2004): págs: 291-295 |
FTIR AND TGA STUDIES OF CHITOSAN COMPOSITE FILMS
GALO CARDENAS*1 AND S. PATRICIA MIRANDA1
1Lab. Quitina/Quitosano, Departamento de Polímeros, Facultad de Ciencias Químicas, Edmundo Larenas 129, Universidad de Concepción, Concepción, CHILE, e-mail : firstname.lastname@example.org
The synthesis of chitosan and resulting chitosan composite films was carried out using glycerol, tween, and beeswax as additives to prepare the composites films, after which a complete study by FTIR was carried out for all the chitosan composite films. Several differences in the absorption bands were seen for each composite, confirming the presence of the material incorporated therein. Thermal stability was studied by thermogravimetric analysis, which further verified the differences between pure chitosan films and the series of composite films. The composite films are good alternatives to existing food storage materials due to important similarities with commercial polypropylenes, not to mention their environmental advantages as regards improved degradability.
Crustacean skeletons (prawn, shrimps, krill and crabs) are the raw materials used to obtain chitin, which is generally obtained from fisheries, often at environmental risk. In general, chitin can display a variety of chemical characteristics, such as varying molecular weight and acetylation degree, depending on its source of origin and the method used for its separation.1 The most important derivative of chitin is the biopolymer chitosan, which is found in nature as a component of the cell walls of fungi and can be obtained via the deacetylation of chitin with concentrated alkali.2,3 Usually, this deacetylation is not complete, and several treatments are required which can alter the molecular weight of the resulting product. Generally, chitosan is formed by a mixture of b-(1,4)-D-N-acetylglucosamine and b-(1,4)-D-glucosamine. The relation between these units along the polymeric chain depends on the conditions used in the deacetylation process. In fact, the term chitosan is not used only to refer to a chemical structure. In practical terms, chitosan is a polymer with a sufficient degree of deacetylation to be soluble in weak acids, otherwise the presence of chitin can be detected4.
There is considerable research on the synthesis of films and membranes using synthetic polymers to improve their biocompatibility and expanded use as biomaterials. For example, Kim and coworkers prepared some similar membranes using 76% deacetylated chitosan. They showed that the crosslinking of this blend strongly correlates with the increase in the strength the membrane and the thermal stability of the blend. These membranes show characteristic swelling depending on pH, and they were used as materials for the slow release of riboflavine and insulin5,6.
Some authors have studied membranes prepared from blends of chitosan and polyvinyl alcohol (PVA). Several initial studies have shown that chitosan forms a clear and homogeneous blend with PVA whose resistance is greater than that of the pure components.7,8
Moreover, transport of halogenated ions and diffusion of cattle serum albumin in membranes prepared from these cross-linked blends have been observed9.
Nakatsuka studied the permeability and diffusion of vitamin B12 in chitosan membranes and in cross-linked chitosan/PVA blends. While the cross-linked chitosan/PVA decreases the swelling equilibrium of the hydrogel in the blend composition increases in proportion to the PVA content of the sample. In both membranes the diffusion coefficient of vitamin B-12 was affected because it depends only on the degree of hydration.10
Hasegawa and coworkers prepared some films from a chitosan-cellulose blend in trifluoroacetic acid (TFA) as solvent and using the casting technique. After the films had dried, they were treated with sodium hydroxide. The films were transparent, quite strong and flexible without the addition of tensioactive products; moreover, when polymer blends were prepared in a composition range of 0 to 100% of each polymer, an increase in the tension force of the film with 30% chitosan was observed.11 Based on these analyses, it is postulated that the increase in plasticity is probably due to the presence of intermolecular interactions between both polysacharides and water molecules. Subsequent thermal treatment of the films suggests a destruction of those interactions in the interphase region, leading to a decrease in mechanical force and in Young's modulus.
By investigating the thermal behavior of polymers as a function of weight loss with heat, it is possible to obtain information about their stability. Thermogravimetric analysis (TGA) has been widely used to study the thermal stability and characteristics of the thermal decomposition of polymers. TGA and DSC studies of other chitosan-based polymers (Mv =100,000 g/mol) have reported a decomposition temperature of 310 C.12 These results are similar to values reported previously by our group.13 In this work we report the characterization by FTIR and TGA of a series of chitosan composite films.
Synthesis of chitosan
The chitin was obtained by treatment with HCl 1N and NaOH 2% at 100 °C for 2 h. The chitin was placed in a 10 L glass reactor with mechanical stirrer. The solid/liquid was added in a 1:15 ratio. The mixture was stirred at 500 rpm and heated at 90 °C. A 10% (w/v) solution of NaBH4 was added to avoid oxidation and depolymerization. Reaction time was 90 min. The final product was washed with hot water to eliminate sodium hydroxide. The product was dried at room temperature.
Chitosan molecular weight
The molecular weight is a critical component and is highly dependent on the polymerization method used. In our case, the method reported by Rinaudo and coworkers14 was used. For chitosan the K and a values are strongly dependent on the degree of deacetylation.
To obtain the highest. Mv, the viscosity molecular weight was calculated by the Mark-Houwink equation, [h] = KMa, where K = 0.076 and a= 0.76. The curve was adjusted and the intercept afforded the reduced viscosity. Two different chitosans were used: one of Mv = 263,600 g/mol and degree of deacetylation of 92%, and another of Mv = 134,300 g/mol and deacetylation degree of 86%.
Chitosan composite films.
From a solution of 2 % chitosan in 2 % acetic acid using magnetic stirring, chitosan composite films were obtained after casting. The films were produced by adding a platicizer such as glycerol, an emulsifier such as Tween, and beeswax in several ratios to obtain a homogeneous mixture. The films were cast at room temperature in Petri dishes previously washed with ethanol.
Infrared spectra were measured using an FT-IR Nicolet Magna 5PC spectrophotometer coupled to a PC with OMNIC analysis software. The films were placed in the holder directly in the IR laser beam. Spectra were recorded at a resolution of 4 cm-1 and 64 scans were accumulated.
A Perkin-Elmer Model TGA-7 thermogravimetric system with a microprocessor driven temperature control unit and a TA data station, was used. The mass of the samples was generally in the range of 2-3 mg. The sample pan was placed in the balance system equipment and the temperature was raised from 25 to 550 °C at a heating rate of 10 °C per minute. The mass of the sample pan was continuously recorded as a function of temperature.
RESULTS AND DISCUSSION
Polysacharides usually have a strong affinity for water, and in the solid state these macromolecules may have disordered structures which can be easily hydrated. The hydration properties of these polysacharides depend on primary and supra macromolecular structure.15 The chitosan composite films that were prepared showed a macro morphology similar to cellophane and commercial polypropylenes. Since the stability of the films is critical for a variety of applications, a comprehensive study of the chitosan and chitosan composite films was carried out by TGA. Their thermal stability and thermal behavior was obtained. The composition of the chitosan composite films was obtained by IR studies.
Table 1 summarizes the TGA and FTIR data of the low molecular weight chitosan films (Mv = 134,300 g/mol). In the series of films the decomposition temperature (TD) is similar to that of chitosan (TD = 313 C) (Fig. 1). The Q-8 composite chitosan film was shown to be the most stable (TD =317 C) (Fig. 2). In this film (Q-8) the TD = 460 C can be attributed to the presence of the emulsifier, while the (Q-11) film shows a TD = 165 C due to the plasticizer and a TD = 457 C from the emulsifier. The Q-9 and Q-10 films had a similar weight loss of around 28% with lower thermal stability than chitosan.
It is interesting to note that with regard to the Q-11 composite films, the films containing Tween plus a plasticizer and beeswax have the lowest weight loss (22%), which means that the higher stability films were produced at 36% weight loss at 550 C. Similar behavior was observed for commercial polypropylenes (Fig. 3). This stability is probably due to the increased hydrogen bonding interaction between glycerol and chitosan and between sorbitan oleic esters from Tween16 and the esters and fatty acids from beeswax and glycerol with chitosan. This good thermal behavior has potential for a variety of industrial applications.
From these data, chitosan with similar degree of deacetylation exhibit a decomposition temperature of TD = 313 C, the same as the 313 °C reported by Kittur12, who concludes that decomposition temperatures decrease with decreasing acetyl units. This reasoning was shown using chitosan with the same number of acetyl groups but different molecular weight. It is generally agreed that a decrease in the degree of polymerization results in a reduction in thermal stability.17 In chitosan, the decomposition process of the N-acetylated compound is overlapped by the N-deacetylated unit, thereby increasing the widening process seen at temperatures up to 400 C. In our case, even though the molecular weight and degree of deacetylation were different, both chitosans had almost the same TD and percentage weight loss at 550 C.
Table 1 evaluates low molecular weight chitosan composite films (Mv = 134,300 g/mol). These films show the typical bands already reported for solids prepared in KBr pellets18 (see Fig. 4). The Q-7 film neutralized via casting shows a very small difference with the Q-6 film, where the nNH appears at 1583 cm-1, most probably due to the unprotonated amino group. The Q-8 film shows a band at 1570 cm-1 corresponding to nCOO- from the esters of the Tween emulsifier (See Fig. 5) which was not seen in the film without neutralization. The Q-9 film showed a chitosan band corresponding to nNH from amide II and nNH, in addition to the OH band at 3363 cm-1 from glycerol and the different nC-O at 1062 cm-1.
The Q-10 film with chitosan and beeswax is a complex mixtures of saturated long chain aliphatic compounds (acid, alcohols, esters and hydrocarbons). The band at 1637 cm-1 is a mixture of amide II from chitosan and nC=C from fatty acids.19,20. The nCH2 corresponding to the acids at 1420 cm-1 is different from that of chitosan at 1407 cm-1. The propylene spectrum shows the typical bands of nC-H at 2886 cm-1 and nCH2 at 1429 cm-1. The OH band is due to humidity in the film (See Fig. 6).
Table 2 summarizes data pertinent to films prepared with high molecular weight chitosan (Mv = 263.000 g/mol). The TGA of these chitosan films shows a TD = 313.5 C, similar to that of the lower Mv, but also higher than the composite chitosan films, which had values around 295 C. The total weight loss at 550 C is almost identical to the 36% displayed by the low Mv composites. Due to the small amount of emulsifier and of beeswax in some cases, it was not possible to observe the decomposition temperature assigned either to the emulsifier or the beeswax. For the P-3 films, however, we can attribute the TD of 450 C to the emulsifier (polyoxyethylene sorbitan monooleate), while in the P-5 film the TD = 475 C corresponds to the beeswax .
The main difference in the low and high molecular weights is that the low molecular weight films possessed a degree of acetylation of about 86 % with a higher chitin percentage remaining. This results in greater thermal stability. Conversely, the higher Mv films with 92% deacetylation shwed lower thermal stability. In the Q-10 and P-5 composite films with fatty acid the thermal peak shown is most probably due to the presence of the acid. In the Q-11 and P-6 composite films, the observed thermal behavior was similar, but in the P-6 the plastifier peak is absent, probably due to the small amount of acid used.
FTIR analysis shows that the P-2 films exhibited resolved NH and OH bands when the films are neutralized. The chitosan film with emulsifier shows three bands: nNH and nOH from chitosan, and nOH from the fatty acid in the beeswax. This also can be attributed to the band at 1659 cm-1 (Esto no calza con lo anterior) (P-3 film). The bands at 1029 and 1149 cm-1 are probably due to the nC-O signal from the glycerol (P-4 film). In the P-5 film the absence of a nC-O band was seen, only the bands from chitosan similar to P-1 film at 1155 and 1090 cm-1 appear. The P-6 film showed nNH, nOH and nNH bands corresponding to the presence of the plastifier and emulsifier in the films. It is interesting to see that the film containing the three additives is quite similar to pure QS. Only the NH and OH bands appear separately. This is a consequence of the small amount of components.
It should be noted that the chitosan showed good antifungal activity21 when applied directly to microorganisms, indicating its potential as a natural fungicide.22,23 Thus, chitosan films have significant potential as food storage materials, particularly when synthesized with selected additives to improve their mechanical properties 24.
1. 00Only a slight difference in the composition of the low and high molecular weight chitosan composite films was observed via IR studies due to the minimal amount of incorporated chitin.
2. 00TGA results suggest that the films with low MW are more stable due to the lower deacetylation degree and higher chitin content.
3. 00The amounts of incorporated emulsifier and lipid acid are as low as 1 % and thus their concentration in the resulting composite films were too low to be detected by FTIR.
4. 00Thermal stability measurements indicate that the Q-11 composite films are quite similar to existing commercial available polypropylene films used for wrapping fruits and vegetables. Given our results, the chitosan composite materials have the potential to replace current polypropylene films, especially when taking into account their advantageous fungicide and bactericide properties.
The authors would like to thanks the financial support of Cátedras (S. P .Miranda) and Innova Bío Bío 03-BI-212-L1 (G. Cárdenas). We also thank the Laboratories of the Facultad de Ciencias Químicas, Universidad de Concepción .
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(Received: June 30, 2004 - Accepted: August 31, 2004)