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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.2 Concepción jun. 2000

http://dx.doi.org/10.4067/S0366-16442000000200021 

COMUNICACION PREVIA

THERMAL EFFECT ON THE MICROHARDNESS OF
CHITOSAN FILMS.

JAIME RETUERT1*, SANDRA FUENTES2, GUILLERMO GONZÁLEZ2 y ROSARIO BENAVENTE3

1Departamento de Química, Facultad de Ciencias Físicas y Matemáticas,
Universidad de Chile, Casilla 2777, Santiago, Chile.
2 Departamento de Química. Facultad de Ciencias, Universidad de Chile,
Casilla 653, Santiago, Chile.
3 Instituto de Ciencia y Tecnología de Polímeros, Juan de la Cierva 3,
28006-Madrid, España
(Received: December 02, 1999 - Accepted: March 03, 2000)

In memoriam of Doctor Guido S. Canessa C.

RESUMEN

En esta comunicación presentamos un estudio sobre los efectos de tratamientos térmicos sobre la microdureza en películas de quitosano. Se utilizaron dos tipos de Quitosano, de diferentes pesos moleculares pero con similar grado de acetilación. Se encontró que la dureza de las películas preparadas con ambos tipos de quitosano es similar, alrededor de 190 mPa, lo que sugiere que esta propiedad no depende del peso molecular en un amplio rango. Los valores de microdureza aumentan notablemente con un calentamiento moderado (60° C) alcanzando valores constantes de aproximadamente 450 mPa tras 60 minutos a esa temperatura. Esto se atribuye a la pérdida de agua y a la formación de nuevos enlaces intermoleculares que consolidan una red más compacta. El aumento de dureza está acompañada por un aumento en la fragilidad de las películas.

PALABRAS CLAVES: Microdureza, quitosano, películas.

ABSTRACT

In this communication we present the results obtained from a study on the effect of thermal treatments of chitosan films with different molecular weights, but with a similar acetilation degree, carried out by microhardness. It was found that the hardness of films prepared from both type of chitosan is similar, of approximately 190 mPa. This suggest that this property is almost independent from the molecular weight in a wide range. The hardness increases notably with a moderate heating (60° C) reaching a constant value of about 450 Mpa after 60 minutes at this temperature. This is attributed to the loss of water and the formation of new intermacromolecular bonds leading to a more compact network. The increase of hardness is accompanied by an increase in the fragility of the films.

KEY WORDS: Chitosan, films, microhardness.

INTRODUCTION

By means of solvent evaporation of chitosan solutions it is possible to obtain films of this polymer. Chitosan is a polysaccharide that is obtained by means of partial alkaline N-deacetylation of chitin, a biopolymer present in the shell of crustaceans and insects. Chitosan obtained by these method results to be a copolymer composed of both glucosamine and N-acetylglucosamine units (Fig.1). Due to their excellent properties and numerous applications like biocompatible coatings and membranes1,2), these films have been studied in morphological aspects as well as in properties such as crystallinity, porosity, and capacity of ion exchanger, etc3-5). Recently the formation of hybrid films of chitosan with inorganic networks has been also studied.6,7)

FIG. 1. Structure of chitin and chitosan. Chitin is composed predominantly of
(y) units. Chitosan is composed predominantly of (x) units.

In this communication we present the results of a study on the effect of thermal treatments of chitosan films obtained from samples of high molecular weight Chitosan from Aldrich, (QO1), and Chitosan of low molecular weight from Fluka (QO2), by means of microhardness determinations.

The microhardness technique8) is based on generating an impress in the surface of a film by means of an indentor subjected to a predetermined load. In this work a microindentor of Vickers, with pyramidal geometry, was used. The relationship among the microhardness (MH) with the applied load (P) and the size of the impress for this indentor is MH= 2 sen 68 P/ d2, where d represents the indentation diagonals. If P is expressed in Newton and d in meters, then MH is in MPa. In the case of polymers, due to the movement of chain segments the effect of the measurement temperature and contact time are relevant and thereafter should be maintained constant.

Several studies relating the microhardness of polyolefins with their mechanical properties9), the orientation in stretched polymers10-11) and the analysis of deformation processes activated thermally12) have been published. Recently, studies of MH related with thermal treatment effects on gelatin films 13-14) has been also published.

EXPERIMENTAL

Preparation of chitosan films: 1% solutions of Chitosan in diluted formic acid (5%) were casted in Petri dishes and evaporated at ambient temperature until the formation of a film. The used chitosan samples were characterized in their molecular weight by means of viscosity measurements and by a combined Size Exclusion Chromatography (SEC)/ Light Scattering method15). The degree of acetylation was obtained by 1H-NMR spectroscopy16) . The results are shown in the Table 1.

Table I. Characterization of the chitosan samples .


Chitosan (QO) % Acetylation [h] ml/g Molecular Weights
  Degree      

Aldrich (QO1) 17 837* Mv=1.0*106 *Mw= 3.5*105
Fluka (QO2) 16.5 222* Mr@ 70.000 *Mw= 1.0*105

Solvent*: 0.2M AcONa/ 0.3M AcOH
Mr= Molecular Weight determined by rotational Viscometry as indicated by Fluka.
*= Molecular Weight determined by Size Exclution Cromatography combined with Light Scattering.

The studied films have thickness of 36,4 m (QO1) and 32,2 m (QO2). By means of thermogravimetric analysis it was determined that they contain approximately 10% of water at 21C and 25% relative humidity. The weight loss after 120 min at 60C reaches about 7%.

Thermal treatment and microhardness determination: A Vickers indentor attached to a Leitz microhardness tester was used for microindentation measurements. The films were fixed to glass slides and then placed in an oven at 60C for the predetermined times. Once cooled to ambient temperature (25 °C) their microhardness was immediately measured. The contact time was 10". The reported MH values are the average of five or more measurements. It has been determined that the rehydratation of the films requires about 40 min17) No noticeable change in MH values due to atmospheric water absorption was observed during the five or six measurements carried out in each case.

RESULTS AND DISCUSSION

The microhardness values of 191 mPa and 182 mPa, determined for the films of QO1 and QO2 respectively, are considered to be high as compared with that of films made of synthetic polymers such as polyethylene and soft metals as Pb and Sn (MH< 100 mPa). These values are much higher that MH values for those of semicrystalline polymers such as poly(oxymethylene) or poly(ethylene terephtalate) or of metals such as Au, Cu and Ag; these materials present MH values between 100 and 300 mPa.

Fig. 2 shows the variation of the MH with the time of thermal treatment for films of QO1 and QO2. It can be observed that after about 15 minutes a very important increase in the value of MH has taken place. Then the MH value increases slightly after 30 minutes of thermal treatment and then it remains practically constant for longer heating times, reaching values > 450 mPa. As was already mentioned, the chitosan films contains about 10% by weight. Water provides a strong plasticizing effect originating a considerable softening of the material. The observed increase in the microhardness after thermal treatment can be then attributed principally to the water loss from the films. Thereby the macromolecular chains are able to interact closely achieving a denser packing. A similar phenomenon, with values of MH also in the same range, has been reported recently by Vassileva et al.12-14) for gelatin. These authors have explained the effect of the thermal conditions on MH on the basis of chemical interactions between carboxyl, hydroxyl, and amino type side-chain groups of the polypeptide chains resulting in the formation of a more or less dense three-dimensional network. The crosslinked chains induces the formation of denser packing leading to an enhanced MH. In the case of chitosan, although it is a polysaccharide, it contains hydroxyl, amino and amide groups that can produce similar intermacromolecular interactions, although in this case only through hydrogen bonds, leading to a reversible process.

FIG. 2. Variation of the microhardness (MH) with the time of thermal treatment (60 C) for films of chitosan with high molecular weight (QO1 ) and low molecular weight (QO2).

It can be also appreciated in Fig. 2 that the values of MH are practically the same for both samples of chitosan indicating that the hardness of the films is independent of the molecular weight of the chitosan in a wide range. This fact is not surprising, since the most important parameter that determines the thermal stability and mechanical properties of the chitosan films is the acetylation degree. In fact, the amount of acetyl groups determines not only the flexibility of macromolecular chains, but also the intermolecular bonds forming the supramolecular structure of the film18-20). In this respect it should be considered that both chitosan types has almost the same acetylation degree (Table 1) and for this reason the corresponding films can present similar mechanical properties. A study on the behavior of chitosan films obtained from chitosan samples with different acetylation degrees will be undertaken in order to clarify this aspect.

An unusual fact found in this work is the form of the indentation in films after the thermal treatment. Fig.3 shows micrographs of indentations carried out before and after the thermal treatment (120 min at 60C) in films of QO1.

It can be observed in Fig. 3a that the indentation diagonals of the impression before the thermal treatment have practically the same length, as is normally expected in MH measurements. On the other hand, the impress of an indentation carried out after the thermal treatment (Fig. 3b) has the form of a cross in that both diagonals have, apparently, very different longitudes; in this case 1.5:1. This form was observed in films of both types of chitosan in indentations carried out after treatment for 60 min at 60C, which is coincident with the conditions when the MH reaches a constant value (Fig.2). However, a careful observation allows to note that, in this case, the indentation impression has the normal regular form, as it is indicated with a dotted line in Fig 3b.

Fig. 3. Micrographs of indentations carried out before (a) and after (b) the thermal treatment (120 min at 60C) in films of high molecular weight chitosan (QO1).

The apparent lengthening of one of the diagonals would correspond to a fracture following its direction. However the shape of the impress suggests that the "fracture" form a part of the impress. It was observed that after the rehydration of the film, by exposing it to atmospheric humidity, the form of this impression remains unchanged; however new indentations appear now again with a regular form. This fracture can be due to an instrumental factor, together with an increase of the rigidity and fragility of the film. If the film is not leaning completely on the glass support forming a light wave, a sector of the indentor diagonal applies a larger pressure on a rigid film. This produces a fracture in that direction, generating an impress with elongated shape indicating an enhanced crystallinity and the existence of preferred orientation in the film. The influence of preferred chain axis orientation (and crystalline phase orientation) on the shape of the indentation in plastic materials has been discussed in detail21). On the other hand, if the film is sufficiently elastic (hydrated form), it will lean completely on the support and the impression will have a regular form as the result of indentor contact with the film surface.

The increase of the hardness and fragility of the films after the thermal treatment imply that more crystalline regions are formed. The fibrillate nature of the chitosan films22) can also contribute to this process. By losing water molecules which were associated to either hydroxyl, amino and/or amide groups of the carbohydrate units17), the chains are able to come closer and to associate by means of Hydrogen bonds in more compact and orderly structures.

By studying hybrid films made from chitosan and inorganic networks such as silica or amino propylsiloxanes a similar increase in the microhardness and also the formation of irregular indentation impress after thermal treatment has been observed 23).

ACKNOWLEDGMENTS

Work partially financed by DID (University of Chile) project with CSIC (Spain), and projects Fondecyt 197-0730 and 297-0004 .

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