Preparation and Chemical/Microstructural Characterization of Azacrown Ether-Crosslinked Chitosan Films

Abstract

Chemically stable porous azacrown ether-crosslinked chitosan films were prepared by reacting varying molar amounts of N,N-diallyl-7,16-diaza-1,4,10,13-tetraoxa-dibenzo-18-crown-6 (molar equivalents ranging from 0, 0.125, 0.167, 0.25 and 0.5) with chitosan. Their chemical and structural properties were characterized by solid state-nuclear magnetic resonance (NMR), elemental, Fourier transform infrared (FTIR), microscopy, and X-ray analyses, as well as gel content. NMR and FTIR analyses of the reaction products suggested that new –CH2– crosslink bridges were produced between the amine groups of chitosan (Ch) and the allyl groups of the azacrown (DAC). The crosslinking chemistry between allyl and amine groups of the reactants was further evidenced with solution NMR studies on model compound of glucosamine with the azacrown. X-ray diffraction analysis of the Ch/azacrown films using wide angle X-ray scattering (WAXS), including synchrotron-WAXS, revealed that the crystalline arrangement of chitosan (Ch) was partially destroyed with increasing grafting of azacrown ether proportion on the Ch polymer chain. Solubility and gel content determination confirmed network formation with a gel content as high as 84–95 wt %. Microstructural analysis revealed microporous morphology with high surface area. The morphology and structure of the azacrown ether-crosslinked chitosan films could be tailored by stoichiometry of the reacting species.

Effects of Grafting Azacrown Ether on Thermal and Swelling Properties of Chitosan Films

“…This starting chitosan polymer (Ch) from shrimp shell chitin was supplied by Sigma-Aldrich (Germany) and previously characterized by Toeri et al [29]. It displayed number-average (Mn) and molecular weights (Mw) of 8.202 × 10 4 (±0.57%) g/mol and 1.810 × 10 5 (±0.30%) g/mol, respectively.…”

“…It had a polydispersity index (PDI) of 2.21. The degree of acetylation and crystallinity index were determined as 23% and 75.7% respectively by Toeri et al [29]. The azacrown ether crosslinking agent, N,N-diallyl-7,16-diaza-1,4,10,13-tetraoxa-dibenzo-18-crown-6 (DAC) was synthesized and characterized by Toeri and Laborie [32].…”

“…The procedure of preparing the chitosan-azacrown ether films (Ch-DAC) was previously reported [29]. Briefly, purified Ch (1.0 g) was dissolved in deionized water containing 2% (v/v) acetic acid and heated to 40 • C. 5 mL of iron (III) chloride hexahydrate solution was added and the mixture was stirred for 15 min under nitrogen flow.…”

“…Five formulations designated as Ch-DAC (0), Ch-DAC (0.125), Ch-DAC (0.167), Ch-DAC (0.25), and Ch-DAC (0.5) based on increasing DAC mole ratio to one mole of Ch deacetylated repeat units were manufactured. Their chemical structure, gel content, and microstructural characterization were determined as previously reported [29].…”

“…Following on the work of Ding et al [25], we reported in a previous study on the preparation, chemical and microstructural characterization of azacrown ether crosslinked chitosan films [29]. Stoichiometric control allowed designing a series of modified Ch with N,N-diallyl-7,13-diaza-1,7,10,16-tetraoxa-dibenzo-18-crown-6 ether (DAC) in controlled molar ratios from Ch:DAC = 1:0, 1:0.125, 1:0.167, 1:0.25, and 1:0.5.…”