Synthesis and properties of chitosan–silica hybrid aerogels

doi:10.1016/S0022-3093(01)00442-2

Journal of Non-Crystalline Solids

Volume 285, Issues 1–3, June 2001, Pages 123-127

https://doi.org/10.1016/S0022-3093(01)00442-2 Get rights and content

Abstract

Chitosan, a polymer that is soluble in dilute aqueous acid, is derived from chitin, a natural polyglucosamide. Aquagels, where the solid phase consists of both chitosan and silica, can be easily prepared by using an acidic solution of chitosan to catalyze the hydrolysis and condensation of tetraethylorthosilicate. Gels with chitosan/tetraethoxysilane (TEOS) mass ratios of 0.1–1.1 have been prepared by this method. Standard drying processes using CO₂ give the corresponding aerogels. The amount of chitosan in the gel plays a role in the shrinkage of the aerogel during drying. Gels with the lowest chitosan/silica ratios show the most linear shrinkage, up to 24%, while those with the highest ratios show only a 7% linear shrinkage. Pyrolysis at 700°C under nitrogen produces a darkened aerogel due to the thermal decomposition of the chitosan, however, the aerogel retains its monolithic form. The pyrolyzed aerogels absorb slightly more infrared radiation in the 2–5 μm region than the original aerogels. B.E.T. surface areas of these aerogels range from 470 to 750 m²/g. Biocompatibility screening of this material shows a very high value for hemolysis, but a low value for cytotoxicity.

Introduction

Natural materials are attractive candidates for aerogel precursors due to their potentially low cost and environmentally benign nature. Additionally, such materials may provide new application opportunities for aerogel materials. One of the most abundant organic compounds on earth is chitin, a polyglucosamide, which is a component of the shells of crustaceans, the exoskeltons of insects, and the cell walls of fungi and certain yeasts, among other sources [1]. However, chitin in its native state is not amenable to sol–gel processing due to its poor solubility. Alternatively, chitosan which is a polymer derived by the alkaline deacetylation of chitin is soluble in dilute acid.

Chitosan is a commercially available material, whose stability, chemical properties, and biocompatibility have led to many current and potential applications. These include pharmacological, biomedical, agricultural, food, and waste treatment products [2].

Chitosan can be easily crosslinked by reagents such as glutaraldehyde, forming rigid aquagels [3]. However, all attempts to dry these gels to their corresponding aerogels were unsuccessful due to severe shrinkage and deformation of the gel. This is not surprising, as the polymer backbone of chitosan is highly polar, and capable of forming hydrogen bonds with adjacent chains, thus collapsing any low-density structure. To provide a stronger network support that would still allow a chitosan material with a high surface area to be prepared, we have investigated chitosan–silica hybrid aerogels.

Section snippets

Experimental procedures

The chitosan used in this work was obtained from NaturalBiopolymer (Raymond, WA, USA). This material (Poly + L grade), gives a viscosity of ∼20 cp in a 1% aqueous solution of acetic acid, and is 75–80% deacetylated. Tetraethoxysilane (TEOS) was reagent grade, obtained from EM Science.

Surface areas of the dried aerogels were measured with a Quantasorb (Quantachrome) BET analyzer using 30% N₂/He for single-point BET analyses. Infrared spectra were recorded with a Perkin Elmer FTIR from KBr pellets.

Results

Surface area and dimensional shrinkage values for aerogels with various chitosan–silica ratios appear in Table 1. Soaking shrinkage refers to the linear dimensional change in the gel after water has been replaced with ethanol, while total shrinkage is similarly obtained after CO₂ drying. Surface areas ranged from approximately 470–750 m²/g, for samples with low to high chitosan contents, respectively. Total shrinkage values ranged from ∼23% for the samples with the lowest chitosan–silica ratio,

Discussion

The sol–gel process used to form chitosan–silica aerogels differs from standard silica aerogel preparations in that the sol is primarily aqueous. The well-known immiscibility of TEOS with water is overcome as the monomer becomes partially hydrolyzed, and a single, clear phase is formed [4]. In this respect, the acidic solution that is required to solubilize chitosan aids in homogenization, as acidic media favor hydrolysis of TEOS over condensation. However, to achieve reasonable gel times mixed

Conclusions

Though limited to acid-catalyzed preparations, chitosan–silica composite aerogels can be easily synthesized using standard methods. The ratio of chitosan to silica leads to significant changes in the physical properties of these aerogels. These materials may find use in diverse applications such as drug delivery, carbon silica composites via pyrolysis, and wastewater treatment using the chelating effect of the included chitosan.

Acknowledgments

The authors wish to thank X.Y. Song for generous assistance in obtaining the TEM images reported here.

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