Chitosan Gel Systems as Novel Host Materials for Optical Limiters
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Materials Directorate, Wright Laboratory, WL/MLPJ, WPAFB, Ohio 45433-7702 *Anteon Inc., Dayton, OH 45431. #Science Applications International Corporation, Dayton, OH 45431. "**TechnicalManagement Concepts, Inc., Beavercreek, OH 45434.
Abstract Two chitosan gel systems, chitosan/acetic anhydride and chitosan/glutaraldehyde, were laser studied as host materials for optical limiters. Both gels are transparent and have a very high damage threshold. The chitosan/acetic anhydride gel has a damage threshold > 540 J/cm 2 while the chitosan/glutaraldehyde gel, which is slightly yellow in color, has a damage threshold > 600 J/cm 2 (measurements made with 6.8 ns laser pulses at 532 nm). Different chromophore dopants, including porphyrin and CuPc, were tested. The optical limiting behavior of the guest/host gel systems was similar to their corresponding solution systems. The morphological structure of the gel systems was studied and the gelation process is discussed. Our current research explores the effect of gel morphology on the optical limiting properties of the chromophores and studies the relation between chromophores, cross-linking agents and the host materials. We have also investigated the relationship between optical properties and chemical structure of the gel/chromophore systems in order to optimize the optical behavior.
Introduction Since the mid 1960s, there has been growing interest in using polymer materials in laser applications [1,2,3]. However, laser devices made with plastics are readily damaged because of the low laser damage threshold of the polymer materials [4]. Recently, we have used polymer gels instead of polymer solids in guest-host systems for optical limiting. The two chitosan gel systems show very high laser damage thresholds measured with 6.8 ns laser pulses at 532 nm. The chitosan/acetic anhyd-ride gel has a damage threshold > 540 J/cm 2 and the chitosan/glutaraldehyde gel, which is slightly yellow in color, has a damage threshold > 600 J/cm 2 . In this paper, we will discuss the processing, morphology and the laser damage mechanism of the polymer gels.
Experimental Procedures Materials and Gel Processing Two kinds of chitosan gels have been prepared: (1) chitosan/acetic anhydride gel and (2) chitosan/glutaraldehyde gel. Low molecular weight chitosan (MW approximately 70,000) was purchased from Fluka and used without further purification. Fig. la shows the unit molecular structure of chitosan. Figs. lb and Ic are the chemical structures of the cross-linking agents, acetic anhydride (Aldrich) and glutaric dialdehyde (Polysciences). Three chromophores were used for this study: tetramethyl pyridinyl porphyrin (porphyrin), Zn-octabromotetraphyenyl porphyrin (ZnOBP), and copper (1I) phthalocyaninetetrasulfonic acid, tetrasodium salt (CuPc). The general procedure for the preparation of chitosan/acetic anhydride gels is as follows: chitosan was dissolved with stirring into a 10% acetic acid (HAc) aqueous solution to form a 2 wt% chitosan/HAc solution at room temperature. The chitosan solution was s
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