A New Hybrid Aerogel Approach to Modification of Bioderived Polymers for Materials Applications
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A New Hybrid Aerogel Approach to Modification of Bioderived Polymers for Materials Applications Mingzhe Wang, Xipeng Liu, Shuang Ji and William M. Risen, Jr. Department of Chemistry, Brown University, Providence, RI 02912-9108, USA ABSTRACT Novel hybrid aerogels consisting of the bioderived polymers chitosan, pectic acid or alginic acid and silica have been synthesized as clear monoliths from clear gels by supercritical fluid extraction. These renewable resource polymers, which are soluble and typically unassociated in aqueous systems, are separately arranged in aerogel scaffolding. This makes them exposed and available for reactions that in some cases would not be possible in aqueous systems. Thus, it is possible to react the amine groups of chitosan, a polymer derived from chitin, with isocyanateterminated prepolymers to form urea linkages and to form new linked chitosan materials. Following the reactions, the scaffold can be dissolved and the new material isolated. Variations lead to hydrophobic or hydrophilic materials. The reactions of aerogel scaffolded bioderived polymers to form urethane- or amide-group linked materials also will be presented and the properties of newly formed materials will be described. Further reactions enabled by the approach, including ones leading to radiation curable polymeric systems will be presented. INTRODUCTION Aerogels containing bio-derived polymers and silica have been reported in the case of chitosan-silica [1-6], pectic acid-silica [2-4], alginic acid-silica [2-4] compositions and several of their metal ion-containing forms [1-5]. These polymers themselves typically are soluble in aqueous solution but not in the non-aqueous media in which many chemical reactions must be carried out. Chitosan is an important example. It is soluble in aqueous acetic acid and similar acidic hydrogen bonding solutions. Clearly, it would be difficult to carry out certain chemical modifications of chitosan when water must be excluded. Chitosan is the deacetylated form of the biopolymer chitin, which is contained in the skin or shell of arthropods in a composite with other compounds. Chitin is an essentially linear polysaccharide consisting of β- (1-4)-linked 2-deoxy-2-acetamido-D-glucose repeating units. It typically is deacetylated to a degree of about 80%, and at that degree chitosan is a copolymer containing about 20% β- (1-4)-2-acetamido-2-deoxy-D-glucose units and 80% β- (1-4)-2-amino2-deoxy-D-glucose units. Thus, it contains both amine groups and hydroxyl groups with which reactions could proceed to produce useful derivatives. Chitosan has a wide range of applications [7], from water purification and waste management to novel polymer composites, cosmetics and pharmaceuticals, and this range could be expanded if new ways to modify it chemically could be found. Some modifications have been successful [8-10]. For example, Lee, et al. grafted deoxycholic acid [11], and Yalpani, et al. [12] have grafted poly (3-hydroxybutyrate) onto chitosan. Nonetheless, it would be advantageous to be able to ca
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