The role played by the interactions of small molecules with chitosan and their transition temperatures.: Glass-forming liquids:: 1,2,3-propantriol (glycerol)

Dynamic behaviour of chitosan and chitosan blended with 1,2,3-propantriol (glycerol) has been studied by dynamic mechanical analysis (DMTA), solid-state NMR and differential scanning calorimetry (DSC). Two relaxation processes appear in pure chitosan, one at high temperature, the alpha-relaxation which has been attributed to its glass transition temperature and interpreted as torsional oscillations between two glucopyranose rings across a glucosidic oxygen and a cooperative hydrogen bonds reordering. And the other, a low temperature relaxation, beta, which is associated with local motions of lateral groups, namely, -NH2 or as -NH3+-OOC-CH3 (quitosonium acetate), -CH2OH and in some extension -NH-OC-CH3, mainly. The incorporation of glycerol in the blends results, as detected by DMTA analysis, in a substantial depression of the glass transition of the chitosan matrix due to a plasticization process. The beta-relaxation in the blends, which also depends on glycerol concentration, has been interpreted as motions of the side chains of chitosan linked to glycerol by hydrogen bonding. The interaction between both components was also investigated by solid-state NMR, which revealed a decreasing of the glycerol mobility with increasing chitosan content in the blends and an increase of chitosan mobility for enriched glycerol blends. The measurements of the proton spin-lattice relaxation time in the rotating frame, T-1 rho(H) for 50% mol chitosan blend revealed that the blend is homogeneous in a length scale of few nanometers. As the glycerol concentration is increased, two relaxation times could be distinguished, corresponding to glycerol and chitosan domains. For high concentration glycerol blends, we have assumed a clustering model for the glycerol molecules sorbed on chitosan macromolecular chains consisting of a two-step mechanism. A first step, in which the solvent is sorbed by means of hydrogen bonds on polymer-specific sites such as -OH and -NH2 or as -NH3+-OOC-CH3 (quitosonium acetate), and a second one, a solvent clustering around the first sorbed solvent molecules forming a hydrogen bonded network of glycerol molecules. (c) 2006 Elsevier Ltd. All rights reserved.