Macmmol. Rapid Commun. 18,163-167 (1997) 163

Synthesis of polymeric organic-inorganic hybrid materials. Partially deacetylated chitin-silica hybrid

Jaime Retuert*, Alicia Nuiiez, Francisco Martinez

Departamento de Quimica, Facultad de Ciencias Fisicas y Matemhticas, Universidad de Chile, Casilla 2777, Santiago 1, Chile

Mehrdad Yazdani-Pedram

Departamento de Quimica Orginica, Facultad de Ciencias Quimicas y FarmacCuticas, Universidad de Chile, Casilla 233, Santiago 1, Chile

(Received: February 12, 1996; revised manuscript of November 5, 1996)

SUMMARY This work reports the synthesis of a novel polymeric organic-inorganic hybrid. The

inorganic component is a silica network obtained by controlled hydrolysis of tetraethyl orthosilicate via sol-gel process and the organic counterpart is partially deacetylated chitin (CHI). The resulting polymer hybrids were homogeneous transparent film forming glassy materials being compatible through a wide composition range. Simultaneous ther- mal analysis of a CHI/silica 1 : 1 mixture confirms the intermolecular complex formation between organic and inorganic polymers.

Introduction Using the sol-gel method, it is possible to obtain novel organic-inorganic poly-

meric hybrid materials by incorporating organic molecules or polymers at molecular level into an inorganic oxide matrix, such as silica. This can be done by hydrolyzing tetraethyl orthosilicate (TEOS) in the presence of a suitable organic polymer. Poly- mers containing amide groups such as poly(2-methyl-2-oxazoline)') or poly(viny1- pyrrolidone)') has been found to form molecular hybrids with silica gel through strong hydrogen bonding. One of the principal applications of these hybrid materials is to improve the mechanical properties of ceramic materials, to produce porous ceramic products with complicated shapes and in some cases with controlled poros- ity.

In this work partially deacetylated chitin was used as organic polymer for obtain- ing silica-bhsed hybrid compounds. Partially deacetylated chitin is a biopolymer structurally similar to cellulose, but containing predominantly amino groups at C-2 position, being the remainder acetamido moieties. This functionality of this biopoly- mer can be used as hydrogen-bond forming group with silica.

Experimental part A solution containing linear polysiloxanes was used as silica source. This was obtained

by partial hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol as solvent and HC1 as catalyst with molar ratio of H,O/TEOS/HCl= 2.1/1/0.01.

0 1997, Hiithig & Wepf Verlag, Zug CCC 1022- 1336/97/$02.50

164 J. Retuert, A. Nuiiez, F. Martinez, M. Yazdani-Pedram

This solution was prepared by adding slowly a solution containing 11.1 mL (0.05 mol) of TEOS in 10 mL of absolute ethanol to a solution of 1.9 mL of HCl(0.27 N). Only low molecular weight siloxane polymers (& 3 000-8 OOO) with essentially linear structure are produced under these reaction condition^^-^) through a sequence of reactions as shown in Scheme I. The solution can be converted to a rigid silica xerogel on standing in an open container.

Scheme 1:

a) Hydrolysis:

Si(OEt), + H,O + Si(OEt),(OH) + EtOH

b) Condensation:

Si(OEt),(OH) + Si(OEt), + (OEt),Si+Si(OEt), + EtOH, etc.

The partially deacetylated chitin was received from Bioquimica Austral S.A., Punta Arenas, Chile. Its deacetylation degree was 86% and mw was 1,2 x lo6.

CHI solutions were prepared by dissolving 10 g of CHI in 1 OOO mL of 2% acetic acid at 25°C with agitation during 24 h. Hybrid samples were prepared by slow addition, under stirring, of a partially hydrolyzed TEOS solution to an appropriate volume of CHI solution. In this manner, it was possible to obtain solutions containing CHI and siloxane polymer with different CHI/SiO, mass relations. For the preparation of the hybrid with CHVSiO, mass relation 1 : 1 the solution of partially hydrolyzed TEOS prepared as described above was added to 300 mL of 1% CHI solution. After 14-18 h stirring at 3 0 T , the solution was concentrated in a Rotary evaporator to half of its volume. The films were prepared by casting on a stretched poly(propy1ene) film and letting the solvent to evaporate slowly at room temperature. The hybrid films were then dried to constant weight under vacuum (lo-' Tom) at 50 "C.

Measurements

The FTIR spectra were taken using a Bruker Vector 22 spectrophotometer. The spectra of the hybrid sample and that of the partially deacetylated chitin (Fig. 1 b and c, respec- tively) were taken using thin films. The spectrum of the solid silicate sample (Fig. 1 a) was taken from a IU3r pellet (due to the fragility of the silica film).

The DTAK'GA analyses were taken in a Netzch STA 409 apparatus under air and at a heating rate of lO"C/min (see text). In order to prepare the samples, the corresponding films were previously finely cut.

Results and discussion

Samples with a CHUSi02 mass relation in the range from 0.05 to 4.0 were pre- pared by mixing solutions containing polysiloxanes and CHI with predetermined proportions. The resulting polymer hybrids were homogeneous and transparent glassy materials for the whole range studied. The homogeneity of the hybrids could be due to the dispersion of CHI at molecular level in the silica network. The forma- tion of strong hydrogen bonding between the functional groups of the partially dea- cetylated chitin and the silanol groups could be attributed to some degree to the compatibility between CHI and the inorganic polymer network.

Synthesis of polymeric organic-inorganic hybrid materials ... 165

In this work it should be noted that solutions containing polysiloxanes were mixed with solutions of the organic polymer. This is somewhat different from the already published methods for obtaining this kind of hybrid, where TEOS is hydrolyzed in the presence of the organic polymer. Therefore, in this case the compatibilization between the organic and inorganic polymeric systems takes place between relatively short polysiloxane chains and the very large CHI macromolecules. Thereafter cross- linking of the polysiloxane chains and subsequent gelation occurs followed by the evolution towards the formation of silica network.

FTIR analysis

Fig. la shows the IR spectrum of a solid silicate sample obtained from a partially hydrolyzed TEOS solution (silica xerogel). This spectrum can be compared with that of the hybrid sample with CHI/Si02 = 1, presented in Fig. lb, where character- istic bands due to both organic and inorganic polymers are present.

1552

0 1750 1700 1650 1600 1550 1500 1450 1400 1350 130C

Wavelength in cm-'

Fig. 1. FTIR spectrum of (a) solid silicate obtained from partially hydrolyzed TEOS, (b) hybrid sample with CHUSiO, = 1 and (c) partially deacetylated chitin

As can be seen, the spectrum corresponding to the hybrid sample is similar to the sum of both spectra of CHI (Fig. lc) and that of silica xerogel (Fig. la) with only minor displacements of some absorption bands, the most noticeable being the shift of the amide I and 11 absorption bands to lower frequencies: from 1649 to 1639 cm-' and from 1 575 to 1552 cm-', respectively. This is probably due to the interac- tion between silanol and the amido-carbonyl groups through hydrogen bonding. In the case of the silica xerogel some weak absorption bands are present. These bands are absent in the spectrum of silica and they are probably due to ethoxy groups that remain as consequence of an uncomplete hydrolysislpolycondensation process.

166 J. Retuert, A. Nuiiez, F. Martinez, M. Yazdani-Pedram

Simultaneous thermal analysis ( D T m G A )

All thermal analysis was carried out under air at a heating rate of lO"C/min. The DTASTGA curve of CHI is shown in Fig. 2a. The initial weight loss of about 4% up to 160 "C corresponds to endothermic volatilization of adsorbed water followed by an exothermic event without appreciable mass loss. At 270°C a rapid weight loss begins accompanied with a marked exothermic degradation process culminating at 305 "C. At this point CHI has lost around 40% of its original weight. As the sample was further heated to 570°C a slow degradation pattern without significant thermal effect was observed conducting to a global mass loss of 62%.

TG/ 'I. OTA / UV

-70 --no 0 100 200 300 LOO 500 600

lomporature / * C

Fig. 2. DTMGA curves of (a) partially deacetylated chitin, (b) silica xerogel, (c) hybrid sample with CHVSiO2 = 1 and (d) a 1 : 1 physical mix- ture of partially deacetylated chitin and silica

S i 0 2 xerogel

In the case of the xerogel (Fig. 2b) an endothermic event is observed up to 150°C with about 8% mass loss reaching around 12% at 200°C. This process could be

Synthesis of polymeric organic-inorganic hybrid materials ... 167

attributed to the loss of retained solvent by xerogel. Thereafter a mild exothermic process associated with a much slower mass loss is observed. This event should be caused by further condensation of silanol groups with loss of water.

CHI/Si02 I : I hybrid

The curve corresponding to a hybrid sample of CHI/Si02 1 : 1 (by weight) shown in Fig. 2c is considerably different from that of a physical mixture of CHI/Si02 1 : 1 presented in Fig. 2d. For the physical mixture two completely separated weight loss events corresponding to both components are observed. On the other hand, it is clearly seen in Fig. 2c that the TGA curve of the hybrid sample, shows a single weight loss step, confirming the existence of strong interaction, at molecular level, between the silica network and the CHI macromolecules. In this case the weight loss observed between 50 and 230°C ( ~ 1 3 % ) is associated with a mild endothermic pro- cess as was already explained for the case of xerogel and CHI.

The results obtained in this study show the possibility of preparing new hybrid materials with a wide range of organic/inorganic proportions maintaining their com- patibility.

The synthesis of this type of polymeric hybrid materials, with additional compo- nents, are presently under study in our laboratories.

Acknowledgements: The authors are indebted to DTI, Universidad de Chile (Project No 43671) for financial support.

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