Thermophysical properties of chitosan, chitosan–starch and chitosan–pullulan films near the glass transition

Abstract

The thermo-mechanical properties of aqueous solution-casted films of chitosan (C), starch–chitosan (SC) and pullulan–chitosan (PC) blends were examined by Dynamic Mechanical Thermal Analysis (DMTA) and large deformation tensile testing. Incorporation of sorbitol (10 and 30% d.b.) and/or adsoprtion of moisture by the films resulted in substantial depression of the glass transition ( T g) of the polysaccharide matrix due to plasticization. For the composite films there was no clear evidence of separate phase transitions of the individual polymeric constituents or a separate polyol phase; a rather broad but single drop of elastic modulus, E′, and a single peak tan δ were observed. The relationship between the T g and moisture for all films could be modeled with the empirical Gordon–Taylor equation. Apparent activation ( E a) energies for the α-relaxation process, estimated from multifrequency DMTA measurements, were within 225–544 kJ mol −1 depending on film composition; the E a and ‘fragility’ parameters decreased with increasing moisture content. Analysis of visoelasticity data using the time–temperature superposition (TTS) principle with the Williams–Landel–Ferry (WLF) equation was successful, provided that the coefficients C 1 and C 2 are optimized and not allowed to assume their ‘universal’ values. Tensile testing of films adjusted at various levels of moisture indicated large drops in Young's modulus and tensile strength ( σ max) with increasing level of polyol and moisture; the sensitivity of the films to placticization was in the order of SC>PC>C. Modeling of the modulus data with the Fermi's equation allowed comparison among samples for the fall in modulus around the glass transition zone as a function of moisture content under isothermal conditions. Relationships between σ max and water content showed an increase in stiffness of the PCs films from 7–11% moisture, and a strong softening effect at higher water contents. The observed range of σ max values (20–80 MPa) for most films is comparable to many medium-strength commercial films.