Synthesis and mechanical properties of quaternary salts of chitosan-based films for food application

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A novel method is described to synthesize quaternary salts of chitosan with dimethylsulfate and subsequently cast films. In an attempt to improve both mechanical and hydrophobic characteristics, the chitosan was previously modified by N-alkylation, introducing 4, 8 and 12 carbons moieties into the polymeric chain. Analysis by FTIR and solid-state CP-MAS 13C NMR spectroscopy confirmed the success of both alkylation and quaternization processes. The average degree of quaternization of these N-methylated derivatives was calculated to be 35%. DMA measurements indicated that chitosan and its derivative films are typically brittle materials, exhibiting similar non-linear viscoelastic behaviors. The films of unmodified chitosan have a very small strain (∼2.8%), though they were the most resistant films (Young's modulus = 2283 MPa; tensile strength >44.0 MPa). In general, the alkyl-chitosan derivatives appear to be more plastic than chitosan films but less resistant, e.g., for butyl chitosan: maximum strain = 13.1%; tensile strength = 13.4 MPa and Young's modulus = 171 MPa. Conversely the quaternization reaction increased the hardness of the parent sample, viz. for quaternary salt of dodecyl chitosan: maximum strain = 2.6%; tensile strength = 38.3 MPa and Young's modulus = 1792 MPa.